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                                   **Ray Tracing: The Next Week**
                                         [Peter Shirley][]
                      edited by [Steve Hollasch][] and [Trevor David Black][]
                                                <br>
                                     Version 3.2.3, 2020-12-07
                                                <br>
                      Copyright 2018-2020 Peter Shirley. All rights reserved.



Overview
====================================================================================================

In Ray Tracing in One Weekend, you built a simple brute force path tracer. In this installment we’ll
add textures, volumes (like fog), rectangles, instances, lights, and support for lots of objects
using a BVH. When done, you’ll have a “real” ray tracer.

A heuristic in ray tracing that many people--including me--believe, is that most optimizations
complicate the code without delivering much speedup. What I will do in this mini-book is go with the
simplest approach in each design decision I make. Check https://in1weekend.blogspot.com/ for
readings and references to a more sophisticated approach. However, I strongly encourage you to do no
premature optimization; if it doesn’t show up high in the execution time profile, it doesn’t need
optimization until all the features are supported!

The two hardest parts of this book are the BVH and the Perlin textures. This is why the title
suggests you take a week rather than a weekend for this endeavor. But you can save those for last if
you want a weekend project. Order is not very important for the concepts presented in this book, and
without BVH and Perlin texture you will still get a Cornell Box!

Thanks to everyone who lent a hand on this project. You can find them in the acknowledgments section
at the end of this book.




Motion Blur
====================================================================================================

When you decided to ray trace, you decided that visual quality was worth more than run-time. In your
fuzzy reflection and defocus blur you needed multiple samples per pixel. Once you have taken a step
down that road, the good news is that almost all effects can be brute-forced. Motion blur is
certainly one of those. In a real camera, the shutter opens and stays open for a time interval, and
the camera and objects may move during that time. Its really an average of what the camera sees over
that interval that we want.


Introduction of SpaceTime Ray Tracing
--------------------------------------
We can get a random estimate by sending each ray at some random time when the shutter is open. As
long as the objects are where they should be at that time, we can get the right average answer with
a ray that is at exactly a single time. This is fundamentally why random ray tracing tends to be
simple.

The basic idea is to generate rays at random times while the shutter is open and intersect the model
at that one time. The way it is usually done is to have the camera move and the objects move, but
have each ray exist at exactly one time. This way the “engine” of the ray tracer can just make sure
the objects are where they need to be for the ray, and the intersection guts don’t change much.

<div class='together'>
For this we will first need to have a ray store the time it exists at:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class ray {
        public:
            ray() {}
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            ray(const point3& origin, const vec3& direction, double time = 0.0)
                : orig(origin), dir(direction), tm(time)
            {}
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

            point3 origin() const  { return orig; }
            vec3 direction() const { return dir; }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            double time() const    { return tm; }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

            point3 at(double t) const {
                return orig + t*dir;
            }

        public:
            point3 orig;
            vec3 dir;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            double tm;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [time-ray]: <kbd>[ray.h]</kbd> Ray with time information]
</div>


Updating the Camera to Simulate Motion Blur
--------------------------------------------
Now we need to modify the camera to generate rays at a random time between `time1` and `time2`.
Should the camera keep track of `time1` and `time2` or should that be up to the user of camera when
a ray is created? When in doubt, I like to make constructors complicated if it makes calls simple,
so I will make the camera keep track, but that’s a personal preference. Not many changes are needed
to camera because for now it is not allowed to move; it just sends out rays over a time period.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class camera {
        public:
            camera(
                point3 lookfrom,
                point3 lookat,
                vec3   vup,
                double vfov, // vertical field-of-view in degrees
                double aspect_ratio,
                double aperture,
                double focus_dist,
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                double _time0 = 0,
                double _time1 = 0
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            ) {
                auto theta = degrees_to_radians(vfov);
                auto h = tan(theta/2);
                auto viewport_height = 2.0 * h;
                auto viewport_width = aspect_ratio * viewport_height;

                w = unit_vector(lookfrom - lookat);
                u = unit_vector(cross(vup, w));
                v = cross(w, u);

                origin = lookfrom;
                horizontal = focus_dist * viewport_width * u;
                vertical = focus_dist * viewport_height * v;
                lower_left_corner = origin - horizontal/2 - vertical/2 - focus_dist*w;

                lens_radius = aperture / 2;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                time0 = _time0;
                time1 = _time1;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            }

            ray get_ray(double s, double t) const {
                vec3 rd = lens_radius * random_in_unit_disk();
                vec3 offset = u * rd.x() + v * rd.y();

                return ray(
                    origin + offset,
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                    lower_left_corner + s*horizontal + t*vertical - origin - offset,
                    random_double(time0, time1)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                );
            }

        private:
            point3 origin;
            point3 lower_left_corner;
            vec3 horizontal;
            vec3 vertical;
            vec3 u, v, w;
            double lens_radius;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            double time0, time1;  // shutter open/close times
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    };

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [time-camera]: <kbd>[camera.h]</kbd> Camera with time information]


Adding Moving Spheres
----------------------
We also need a moving object. I’ll create a sphere class that has its center move linearly from
`center0` at `time0` to `center1` at `time1`. Outside that time interval it continues on, so those
times need not match up with the camera aperture open and close.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #ifndef MOVING_SPHERE_H
    #define MOVING_SPHERE_H

    #include "rtweekend.h"

    #include "hittable.h"


    class moving_sphere : public hittable {
        public:
            moving_sphere() {}
            moving_sphere(
                point3 cen0, point3 cen1, double _time0, double _time1, double r, shared_ptr<material> m)
                : center0(cen0), center1(cen1), time0(_time0), time1(_time1), radius(r), mat_ptr(m)
            {};

            virtual bool hit(
                const ray& r, double t_min, double t_max, hit_record& rec) const override;

            point3 center(double time) const;

        public:
            point3 center0, center1;
            double time0, time1;
            double radius;
            shared_ptr<material> mat_ptr;
    };

    point3 moving_sphere::center(double time) const {
        return center0 + ((time - time0) / (time1 - time0))*(center1 - center0);
    }

    #endif
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [moving-sphere]: <kbd>[moving_sphere.h]</kbd> A moving sphere]

<div class='together'>
An alternative to making a new moving sphere class is to just make them all move, while stationary
spheres have the same begin and end position. I’m on the fence about that trade-off between fewer
classes and more efficient stationary spheres, so let your design taste guide you.

The `moving_sphere::hit()` function is almost identical to the `sphere::hit()` function: `center`
just needs to become a function `center(time)`:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    bool moving_sphere::hit(const ray& r, double t_min, double t_max, hit_record& rec) const {
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
        vec3 oc = r.origin() - center(r.time());
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        auto a = r.direction().length_squared();
        auto half_b = dot(oc, r.direction());
        auto c = oc.length_squared() - radius*radius;

        auto discriminant = half_b*half_b - a*c;
        if (discriminant < 0) return false;
        auto sqrtd = sqrt(discriminant);

        // Find the nearest root that lies in the acceptable range.
        auto root = (-half_b - sqrtd) / a;
        if (root < t_min || t_max < root) {
            root = (-half_b + sqrtd) / a;
            if (root < t_min || t_max < root)
                return false;
        }

        rec.t = root;
        rec.p = r.at(rec.t);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
        auto outward_normal = (rec.p - center(r.time())) / radius;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        rec.set_face_normal(r, outward_normal);
        rec.mat_ptr = mat_ptr;

        return true;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [moving-sphere-hit]: <kbd>[moving_sphere.h]</kbd> Moving sphere hit function]
</div>


Tracking the Time of Ray Intersection
--------------------------------------
<div class='together'>
Now that rays have a time property, we need to update the `material::scatter()` methods to account
for the time of intersection:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class lambertian : public material {
        ...
            virtual bool scatter(
                const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
            ) const override {
                auto scatter_direction = rec.normal + random_unit_vector();

                // Catch degenerate scatter direction
                if (scatter_direction.near_zero())
                    scatter_direction = rec.normal;


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                scattered = ray(rec.p, scatter_direction, r_in.time());
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                attenuation = albedo;
                return true;
            }
            ...
    };

    class metal : public material {
        ...
            virtual bool scatter(
                const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
            ) const override {
                vec3 reflected = reflect(unit_vector(r_in.direction()), rec.normal);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                scattered = ray(rec.p, reflected + fuzz*random_in_unit_sphere(), r_in.time());
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                attenuation = albedo;
                return (dot(scattered.direction(), rec.normal) > 0);
            }
            ...
    };

    class dielectric : public material {
        ...
            virtual bool scatter(
                ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                scattered = ray(rec.p, direction, r_in.time());
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                return true;
            }
            ...
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [material-time]: <kbd>[material.h]</kbd>
        Handle ray time in the material::scatter() methods
    ]
</div>


Putting Everything Together
----------------------------
<div class='together'>
The code below takes the example diffuse spheres from the scene at the end of the last book, and
makes them move during the image render. (Think of a camera with shutter opening at time 0 and
closing at time 1.) Each sphere moves from its center $\mathbf{C}$ at time $t=0$ to $\mathbf{C} +
(0, r/2, 0)$ at time $t=1$, where $r$ is a random number in $[0,1)$:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
    #include "moving_sphere.h"
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

    ...
    hittable_list random_scene() {
        hittable_list world;

        auto ground_material = make_shared<lambertian>(color(0.5, 0.5, 0.5));
        world.add(make_shared<sphere>(point3(0,-1000,0), 1000, ground_material));

        for (int a = -11; a < 11; a++) {
            for (int b = -11; b < 11; b++) {
                auto choose_mat = random_double();
                point3 center(a + 0.9*random_double(), 0.2, b + 0.9*random_double());

                if ((center - vec3(4, 0.2, 0)).length() > 0.9) {
                    shared_ptr<material> sphere_material;

                    if (choose_mat < 0.8) {
                        // diffuse
                        auto albedo = color::random() * color::random();
                        sphere_material = make_shared<lambertian>(albedo);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                        auto center2 = center + vec3(0, random_double(0,.5), 0);
                        world.add(make_shared<moving_sphere>(
                            center, center2, 0.0, 1.0, 0.2, sphere_material));
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                    } else if (choose_mat < 0.95) {
                        // metal
                        auto albedo = color::random(0.5, 1);
                        auto fuzz = random_double(0, 0.5);
                        sphere_material = make_shared<metal>(albedo, fuzz);
                        world.add(make_shared<sphere>(center, 0.2, sphere_material));
                    } else {
                        // glass
                        sphere_material = make_shared<dielectric>(1.5);
                        world.add(make_shared<sphere>(center, 0.2, sphere_material));
                    }
                }
            }
        }

        auto material1 = make_shared<dielectric>(1.5);
        world.add(make_shared<sphere>(point3(0, 1, 0), 1.0, material1));

        auto material2 = make_shared<lambertian>(color(0.4, 0.2, 0.1));
        world.add(make_shared<sphere>(point3(-4, 1, 0), 1.0, material2));

        auto material3 = make_shared<metal>(color(0.7, 0.6, 0.5), 0.0);
        world.add(make_shared<sphere>(point3(4, 1, 0), 1.0, material3));

        return world;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [scene-spheres-moving]:
    <kbd>[main.cc]</kbd> Last book's final scene, but with moving spheres]
</div>

<div class='together'>
And with these viewing parameters:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    int main() {

        // Image


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
        auto aspect_ratio = 16.0 / 9.0;
        int image_width = 400;
        int samples_per_pixel = 100;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        const int max_depth = 50;

        ...

        // Camera

        point3 lookfrom(13,2,3);
        point3 lookat(0,0,0);
        vec3 vup(0,1,0);
        auto dist_to_focus = 10.0;
        auto aperture = 0.1;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
        int image_height = static_cast<int>(image_width / aspect_ratio);


        camera cam(lookfrom, lookat, vup, 20, aspect_ratio, aperture, dist_to_focus, 0.0, 1.0);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [scene-spheres-moving-camera]: <kbd>[main.cc]</kbd> Viewing parameters]

gives the following result:

  ![Image 1: Bouncing spheres](../images/img-2.01-bouncing-spheres.png class=pixel)

</div>



Bounding Volume Hierarchies
====================================================================================================

This part is by far the most difficult and involved part of the ray tracer we are working on. I am
sticking it in this chapter so the code can run faster, and because it refactors `hittable` a
little, and when I add rectangles and boxes we won't have to go back and refactor them.

The ray-object intersection is the main time-bottleneck in a ray tracer, and the time is linear with
the number of objects. But it’s a repeated search on the same model, so we ought to be able to make
it a logarithmic search in the spirit of binary search. Because we are sending millions to billions
of rays on the same model, we can do an analog of sorting the model, and then each ray intersection
can be a sublinear search. The two most common families of sorting are to 1) divide the space, and
2) divide the objects. The latter is usually much easier to code up and just as fast to run for most
models.


The Key Idea
-------------
<div class='together'>
The key idea of a bounding volume over a set of primitives is to find a volume that fully encloses
(bounds) all the objects. For example, suppose you computed a bounding sphere of 10 objects. Any ray
that misses the bounding sphere definitely misses all ten objects. If the ray hits the bounding
sphere, then it might hit one of the ten objects. So the bounding code is always of the form:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    if (ray hits bounding object)
        return whether ray hits bounded objects
    else
        return false
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
</div>

A key thing is we are dividing objects into subsets. We are not dividing the screen or the volume.
Any object is in just one bounding volume, but bounding volumes can overlap.


Hierarchies of Bounding Volumes
--------------------------------
<div class='together'>
To make things sub-linear we need to make the bounding volumes hierarchical. For example, if we
divided a set of objects into two groups, red and blue, and used rectangular bounding volumes, we’d
have:

  ![Figure [bvol-hierarchy]: Bounding volume hierarchy](../images/fig-2.01-bvol-hierarchy.jpg)

</div>

<div class='together'>
Note that the blue and red bounding volumes are contained in the purple one, but they might overlap,
and they are not ordered -- they are just both inside. So the tree shown on the right has no concept
of ordering in the left and right children; they are simply inside. The code would be:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    if (hits purple)
        hit0 = hits blue enclosed objects
        hit1 = hits red enclosed objects
        if (hit0 or hit1)
            return true and info of closer hit
    return false
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
</div>


Axis-Aligned Bounding Boxes (AABBs)
------------------------------------
To get that all to work we need a way to make good divisions, rather than bad ones, and a way to
intersect a ray with a bounding volume. A ray bounding volume intersection needs to be fast, and
bounding volumes need to be pretty compact. In practice for most models, axis-aligned boxes work
better than the alternatives, but this design choice is always something to keep in mind if you
encounter unusual types of models.

From now on we will call axis-aligned bounding rectangular parallelepiped (really, that is what they
need to be called if precise) axis-aligned bounding boxes, or AABBs. Any method you want to use to
intersect a ray with an AABB is fine. And all we need to know is whether or not we hit it; we don’t
need hit points or normals or any of that stuff that we need for an object we want to display.

<div class='together'>
Most people use the “slab” method. This is based on the observation that an n-dimensional AABB is
just the intersection of n axis-aligned intervals, often called “slabs” An interval is just the
points between two endpoints, _e.g._, $x$ such that $3 < x < 5$, or more succinctly $x$ in $(3,5)$.
In 2D, two intervals overlapping makes a 2D AABB (a rectangle):

  ![Figure [2d-aabb]: 2D axis-aligned bounding box](../images/fig-2.02-2d-aabb.jpg)

</div>

<div class='together'>
For a ray to hit one interval we first need to figure out whether the ray hits the boundaries. For
example, again in 2D, this is the ray parameters $t_0$ and $t_1$. (If the ray is parallel to the
plane those will be undefined.)

  ![Figure [ray-slab]: Ray-slab intersection](../images/fig-2.03-ray-slab.jpg)

</div>

<div class='together'>
In 3D, those boundaries are planes. The equations for the planes are $x = x_0$, and $x = x_1$. Where
does the ray hit that plane? Recall that the ray can be thought of as just a function that given a
$t$ returns a location $\mathbf{P}(t)$:

  $$ \mathbf{P}(t) = \mathbf{A} + t \mathbf{b} $$
</div>

<div class='together'>
That equation applies to all three of the x/y/z coordinates. For example, $x(t) = A_x + t b_x$. This
ray hits the plane $x = x_0$ at the $t$ that satisfies this equation:

  $$ x_0 = A_x + t_0 b_x $$
</div>

<div class='together'>
Thus $t$ at that hitpoint is:

  $$ t_0 = \frac{x_0 - A_x}{b_x} $$
</div>

<div class='together'>
We get the similar expression for $x_1$:

  $$ t_1 = \frac{x_1 - A_x}{b_x} $$
</div>

<div class='together'>
The key observation to turn that 1D math into a hit test is that for a hit, the $t$-intervals need
to overlap. For example, in 2D the green and blue overlapping only happens if there is a hit:

  ![Figure [ray-slab-interval]: Ray-slab $t$-interval overlap
  ](../images/fig-2.04-ray-slab-interval.jpg)

</div>


Ray Intersection with an AABB
------------------------------
<div class='together'>
The following pseudocode determines whether the $t$ intervals in the slab overlap:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    compute (tx0, tx1)
    compute (ty0, ty1)
    return overlap?( (tx0, tx1), (ty0, ty1))
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

That is awesomely simple, and the fact that the 3D version also works is why people love the
slab method:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    compute (tx0, tx1)
    compute (ty0, ty1)
    compute (tz0, tz1)
    return overlap?( (tx0, tx1), (ty0, ty1), (tz0, tz1))
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
</div>

<div class='together'>
There are some caveats that make this less pretty than it first appears. First, suppose the ray is
travelling in the negative $x$ direction. The interval $(t_{x0}, t_{x1})$ as computed above might be
reversed, _e.g._ something like $(7, 3)$. Second, the divide in there could give us infinities. And
if the ray origin is on one of the slab boundaries, we can get a `NaN`. There are many ways these
issues are dealt with in various ray tracers’ AABB. (There are also vectorization issues like SIMD
which we will not discuss here. Ingo Wald’s papers are a great place to start if you want to go the
extra mile in vectorization for speed.) For our purposes, this is unlikely to be a major bottleneck
as long as we make it reasonably fast, so let’s go for simplest, which is often fastest anyway!
First let’s look at computing the intervals:

  $$ t_{x0} = \frac{x_0 - A_x}{b_x} $$
  $$ t_{x1} = \frac{x_1 - A_x}{b_x} $$
</div>

<div class='together'>
One troublesome thing is that perfectly valid rays will have $b_x = 0$, causing division by zero.
Some of those rays are inside the slab, and some are not. Also, the zero will have a ± sign under
IEEE floating point. The good news for $b_x = 0$ is that $t_{x0}$ and $t_{x1}$ will both be +∞ or
both be -∞ if not between $x_0$ and $x_1$. So, using min and max should get us the right answers:

  $$ t_{x0} = \min(
     \frac{x_0 - A_x}{b_x},
     \frac{x_1 - A_x}{b_x})
  $$

  $$ t_{x1} = \max(
     \frac{x_0 - A_x}{b_x},
     \frac{x_1 - A_x}{b_x})
  $$
</div>

The remaining troublesome case if we do that is if $b_x = 0$ and either $x_0 - A_x = 0$ or $x_1 -
A_x = 0$ so we get a `NaN`. In that case we can probably accept either hit or no hit answer, but
we’ll revisit that later.

<div class='together'>
Now, let’s look at that overlap function. Suppose we can assume the intervals are not reversed (so
the first value is less than the second value in the interval) and we want to return true in that
case. The boolean overlap that also computes the overlap interval $(f, F)$ of intervals $(d, D)$ and
$(e, E)$ would be:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    bool overlap(d, D, e, E, f, F)
        f = max(d, e)
        F = min(D, E)
        return (f < F)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
</div>

<div class='together'>
If there are any `NaN`s running around there, the compare will return false so we need to be sure
our bounding boxes have a little padding if we care about grazing cases (and we probably should
because in a ray tracer all cases come up eventually). With all three dimensions in a loop, and
passing in the interval $[t_{min}$, $t_{max}]$, we get:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #ifndef AABB_H
    #define AABB_H

    #include "rtweekend.h"

    class aabb {
        public:
            aabb() {}
            aabb(const point3& a, const point3& b) { minimum = a; maximum = b;}

            point3 min() const {return minimum; }
            point3 max() const {return maximum; }

            bool hit(const ray& r, double t_min, double t_max) const {
                for (int a = 0; a < 3; a++) {
                    auto t0 = fmin((minimum[a] - r.origin()[a]) / r.direction()[a],
                                   (maximum[a] - r.origin()[a]) / r.direction()[a]);
                    auto t1 = fmax((minimum[a] - r.origin()[a]) / r.direction()[a],
                                   (maximum[a] - r.origin()[a]) / r.direction()[a]);
                    t_min = fmax(t0, t_min);
                    t_max = fmin(t1, t_max);
                    if (t_max <= t_min)
                        return false;
                }
                return true;
            }

            point3 minimum;
            point3 maximum;
    };

    #endif
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [aabb]: <kbd>[aabb.h]</kbd> Axis-aligned bounding box class]
</div>


An Optimized AABB Hit Method
-----------------------------
<div class='together'>
In reviewing this intersection method, Andrew Kensler at Pixar tried some experiments and proposed
the following version of the code. It works extremely well on many compilers, and I have adopted it
as my go-to method:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    inline bool aabb::hit(const ray& r, double t_min, double t_max) const {
        for (int a = 0; a < 3; a++) {
            auto invD = 1.0f / r.direction()[a];
            auto t0 = (min()[a] - r.origin()[a]) * invD;
            auto t1 = (max()[a] - r.origin()[a]) * invD;
            if (invD < 0.0f)
                std::swap(t0, t1);
            t_min = t0 > t_min ? t0 : t_min;
            t_max = t1 < t_max ? t1 : t_max;
            if (t_max <= t_min)
                return false;
        }
        return true;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [aabb-hit]: <kbd>[aabb.h]</kbd> Axis-aligned bounding box hit function]
</div>


Constructing Bounding Boxes for Hittables
------------------------------------------
<div class='together'>
We now need to add a function to compute the bounding boxes of all the hittables. Then we will make
a hierarchy of boxes over all the primitives, and the individual primitives--like spheres--will live
at the leaves. That function returns a bool because not all primitives have bounding boxes (_e.g._,
infinite planes). In addition, moving objects will have a bounding box that encloses the object for
the entire time interval [`time0`,`time1`].

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
    #include "aabb.h"
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    ...

    class hittable {
        public:
            ...
            virtual bool hit(const ray& r, double t_min, double t_max, hit_record& rec) const = 0;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            virtual bool bounding_box(double time0, double time1, aabb& output_box) const = 0;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            ...
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [hittable-bbox]: <kbd>[hittable.h]</kbd> Hittable class with bounding-box]
</div>

<div class='together'>
For a sphere, that `bounding_box` function is easy:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class sphere : public hittable {
        public:
            ...
            virtual bool hit(
                const ray& r, double t_min, double t_max, hit_record& rec) const override;


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            virtual bool bounding_box(double time0, double time1, aabb& output_box) const override;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            ...
    };

    ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
    bool sphere::bounding_box(double time0, double time1, aabb& output_box) const {
        output_box = aabb(
            center - vec3(radius, radius, radius),
            center + vec3(radius, radius, radius));
        return true;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [sphere-bbox]: <kbd>[sphere.h]</kbd> Sphere with bounding box]
</div>

<div class='together'>
For `moving sphere`, we can take the box of the sphere at $t_0$, and the box of the sphere at $t_1$,
and compute the box of those two boxes:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
    #include "aabb.h"
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    ...

    class moving_sphere : public hittable {
        public:
            ...
            virtual bool hit(
                const ray& r, double t_min, double t_max, hit_record& rec) const override;


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            virtual bool bounding_box(
                double _time0, double _time1, aabb& output_box) const override;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            ...
    };

    ...

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
    bool moving_sphere::bounding_box(double _time0, double _time1, aabb& output_box) const {
        aabb box0(
            center(_time0) - vec3(radius, radius, radius),
            center(_time0) + vec3(radius, radius, radius));
        aabb box1(
            center(_time1) - vec3(radius, radius, radius),
            center(_time1) + vec3(radius, radius, radius));
        output_box = surrounding_box(box0, box1);
        return true;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [moving-sphere-bbox]: <kbd>[moving_sphere.h]</kbd> Moving sphere with bounding box]
</div>


Creating Bounding Boxes of Lists of Objects
--------------------------------------------
<div class='together'>
For lists you can store the bounding box at construction, or compute it on the fly. I like doing it
the fly because it is only usually called at BVH construction.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
    #include "aabb.h"
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    ...

    class hittable_list : public hittable {
        public:
            ...
            virtual bool hit(
                const ray& r, double t_min, double t_max, hit_record& rec) const override;


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            virtual bool bounding_box(
                double time0, double time1, aabb& output_box) const override;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        ...
    };

    ...

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
    bool hittable_list::bounding_box(double time0, double time1, aabb& output_box) const {
        if (objects.empty()) return false;

        aabb temp_box;
        bool first_box = true;

        for (const auto& object : objects) {
            if (!object->bounding_box(time0, time1, temp_box)) return false;
            output_box = first_box ? temp_box : surrounding_box(output_box, temp_box);
            first_box = false;
        }

        return true;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [hit-list-bbox]: <kbd>[hittable_list.h]</kbd> Hittable list with bounding box]
</div>

<div class='together'>
This requires the `surrounding_box` function for `aabb` which computes the bounding box of two
boxes:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    aabb surrounding_box(aabb box0, aabb box1) {
        point3 small(fmin(box0.min().x(), box1.min().x()),
                     fmin(box0.min().y(), box1.min().y()),
                     fmin(box0.min().z(), box1.min().z()));

        point3 big(fmax(box0.max().x(), box1.max().x()),
                   fmax(box0.max().y(), box1.max().y()),
                   fmax(box0.max().z(), box1.max().z()));

        return aabb(small,big);
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [surrounding-box]: <kbd>[aabb.h]</kbd> Surrounding bounding box]
</div>


The BVH Node Class
-------------------
<div class='together'>
A BVH is also going to be a `hittable` -- just like lists of `hittable`s. It’s really a container,
but it can respond to the query “does this ray hit you?”. One design question is whether we have two
classes, one for the tree, and one for the nodes in the tree; or do we have just one class and have
the root just be a node we point to. I am a fan of the one class design when feasible. Here is such
a class:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #ifndef BVH_H
    #define BVH_H

    #include "rtweekend.h"

    #include "hittable.h"
    #include "hittable_list.h"


    class bvh_node : public hittable {
        public:
            bvh_node();

            bvh_node(const hittable_list& list, double time0, double time1)
                : bvh_node(list.objects, 0, list.objects.size(), time0, time1)
            {}

            bvh_node(
                const std::vector<shared_ptr<hittable>>& src_objects,
                size_t start, size_t end, double time0, double time1);

            virtual bool hit(
                const ray& r, double t_min, double t_max, hit_record& rec) const override;

            virtual bool bounding_box(double time0, double time1, aabb& output_box) const override;

        public:
            shared_ptr<hittable> left;
            shared_ptr<hittable> right;
            aabb box;
    };

    bool bvh_node::bounding_box(double time0, double time1, aabb& output_box) const {
        output_box = box;
        return true;
    }

    #endif
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [bvh]: <kbd>[bvh.h]</kbd> Bounding volume hierarchy]
</div>

Note that the children pointers are to generic hittables. They can be other `bvh_nodes`, or
`spheres`, or any other `hittable`.

<div class='together'>
The `hit` function is pretty straightforward: check whether the box for the node is hit, and if so,
check the children and sort out any details:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    bool bvh_node::hit(const ray& r, double t_min, double t_max, hit_record& rec) const {
        if (!box.hit(r, t_min, t_max))
            return false;

        bool hit_left = left->hit(r, t_min, t_max, rec);
        bool hit_right = right->hit(r, t_min, hit_left ? rec.t : t_max, rec);

        return hit_left || hit_right;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [bvh-hit]: <kbd>[bvh.h]</kbd> Bounding volume hierarchy hit function]
</div>


Splitting BVH Volumes
----------------------
<div class='together'>
The most complicated part of any efficiency structure, including the BVH, is building it. We do this
in the constructor. A cool thing about BVHs is that as long as the list of objects in a `bvh_node`
gets divided into two sub-lists, the hit function will work. It will work best if the division is
done well, so that the two children have smaller bounding boxes than their parent’s bounding box,
but that is for speed not correctness. I’ll choose the middle ground, and at each node split the
list along one axis. I’ll go for simplicity:

  1. randomly choose an axis
  2. sort the primitives (`using std::sort`)
  3. put half in each subtree

</div>

<div class='together'>
When the list coming in is two elements, I put one in each subtree and end the recursion. The
traversal algorithm should be smooth and not have to check for null pointers, so if I just have one
element I duplicate it in each subtree. Checking explicitly for three elements and just following
one recursion would probably help a little, but I figure the whole method will get optimized later.
This yields:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #include <algorithm>
    ...

    bvh_node::bvh_node(
        std::vector<shared_ptr<hittable>>& src_objects,
        size_t start, size_t end, double time0, double time1
    ) {
        auto objects = src_objects; // Create a modifiable array of the source scene objects

        int axis = random_int(0,2);
        auto comparator = (axis == 0) ? box_x_compare
                        : (axis == 1) ? box_y_compare
                                      : box_z_compare;

        size_t object_span = end - start;

        if (object_span == 1) {
            left = right = objects[start];
        } else if (object_span == 2) {
            if (comparator(objects[start], objects[start+1])) {
                left = objects[start];
                right = objects[start+1];
            } else {
                left = objects[start+1];
                right = objects[start];
            }
        } else {
            std::sort(objects.begin() + start, objects.begin() + end, comparator);

            auto mid = start + object_span/2;
            left = make_shared<bvh_node>(objects, start, mid, time0, time1);
            right = make_shared<bvh_node>(objects, mid, end, time0, time1);
        }

        aabb box_left, box_right;

        if (  !left->bounding_box (time0, time1, box_left)
           || !right->bounding_box(time0, time1, box_right)
        )
            std::cerr << "No bounding box in bvh_node constructor.\n";

        box = surrounding_box(box_left, box_right);
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [bvh-node]: <kbd>[bvh.h]</kbd> Bounding volume hierarchy node]
</div>

<div class='together'>
    This uses a new function: `random_int()`:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    inline int random_int(int min, int max) {
        // Returns a random integer in [min,max].
        return static_cast<int>(random_double(min, max+1));
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [random-int]: <kbd>[rtweekend.h]</kbd> A function to return random integers in a range]
</div>

The check for whether there is a bounding box at all is in case you sent in something like an
infinite plane that doesn’t have a bounding box. We don’t have any of those primitives, so it
shouldn’t happen until you add such a thing.


The Box Comparison Functions
-----------------------------
<div class='together'>
Now we need to implement the box comparison functions, used by `std::sort()`. To do this, create a
generic comparator returns true if the first argument is less than the second, given an additional
axis index argument. Then define axis-specific comparison functions that use the generic comparison
function.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    inline bool box_compare(const shared_ptr<hittable> a, const shared_ptr<hittable> b, int axis) {
        aabb box_a;
        aabb box_b;

        if (!a->bounding_box(0,0, box_a) || !b->bounding_box(0,0, box_b))
            std::cerr << "No bounding box in bvh_node constructor.\n";

        return box_a.min().e[axis] < box_b.min().e[axis];
    }


    bool box_x_compare (const shared_ptr<hittable> a, const shared_ptr<hittable> b) {
        return box_compare(a, b, 0);
    }

    bool box_y_compare (const shared_ptr<hittable> a, const shared_ptr<hittable> b) {
        return box_compare(a, b, 1);
    }

    bool box_z_compare (const shared_ptr<hittable> a, const shared_ptr<hittable> b) {
        return box_compare(a, b, 2);
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [bvh-x-comp]: <kbd>[bvh.h]</kbd> BVH comparison function, X-axis]
</div>



Solid Textures
====================================================================================================

A texture in graphics usually means a function that makes the colors on a surface procedural. This
procedure can be synthesis code, or it could be an image lookup, or a combination of both. We will
first make all colors a texture. Most programs keep constant rgb colors and textures in different
classes, so feel free to do something different, but I am a big believer in this architecture
because being able to make any color a texture is great.

The First Texture Class: Constant Texture
------------------------------------------

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #ifndef TEXTURE_H
    #define TEXTURE_H

    #include "rtweekend.h"

    class texture {
        public:
            virtual color value(double u, double v, const point3& p) const = 0;
    };

    class solid_color : public texture {
        public:
            solid_color() {}
            solid_color(color c) : color_value(c) {}

            solid_color(double red, double green, double blue)
              : solid_color(color(red,green,blue)) {}

            virtual color value(double u, double v, const vec3& p) const override {
                return color_value;
            }

        private:
            color color_value;
    };

    #endif
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [texture]: <kbd>[texture.h]</kbd> A texture class]

We'll need to update the `hit_record` structure to store the U,V surface coordinates of the
ray-object hit point.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    struct hit_record {
        vec3 p;
        vec3 normal;
        shared_ptr<material> mat_ptr;
        double t;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
        double u;
        double v;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        bool front_face;
        ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [hit-record-uv]: <kbd>[hittable.h]</kbd> Adding U,V coordinates to the `hit_record`]

We will also need to compute $(u,v)$ texture coordinates for hittables.


Texture Coordinates for Spheres
--------------------------------
<div class='together'>
For spheres, texture coordinates are usually based on some form of longitude and latitude, _i.e._,
spherical coordinates. So we compute $(\theta,\phi)$ in spherical coordinates, where $\theta$ is the
angle up from the bottom pole (that is, up from -Y), and $\phi$ is the angle around the Y-axis (from
-X to +Z to +X to -Z back to -X).

We want to map $\theta$ and $\phi$ to texture coordinates $u$ and $v$ each in $[0,1]$, where
$(u=0,v=0)$ maps to the bottom-left corner of the texture. Thus the normalization from
$(\theta,\phi)$ to $(u,v)$ would be:

  $$ u = \frac{\phi}{2\pi} $$
  $$ v = \frac{\theta}{\pi} $$
</div>

<div class='together'>
To compute $\theta$ and $\phi$ for a given point on the unit sphere centered at the origin, we start
with the equations for the corresponding Cartesian coordinates:

  $$ \begin{align*}
      y &= -\cos(\theta)            \\
      x &= -\cos(\phi) \sin(\theta) \\
      z &= \quad\sin(\phi) \sin(\theta)
     \end{align*}
  $$
</div>

<div class='together'>
We need to invert these equations to solve for $\theta$ and $\phi$. Because of the lovely `<cmath>`
function `atan2()`, which takes any pair of numbers proportional to sine and cosine and returns the
angle, we can pass in $x$ and $z$ (the $\sin(\theta)$ cancel) to solve for $\phi$:

  $$ \phi = \text{atan2}(z, -x) $$
</div>

<div class='together'>
`atan2()` returns values in the range $-\pi$ to $\pi$, but they go from 0 to $\pi$, then flip to
$-\pi$ and proceed back to zero. While this is mathematically correct, we want $u$ to range from $0$
to $1$, not from $0$ to $1/2$ and then from $-1/2$ to $0$. Fortunately,

  $$ \text{atan2}(a,b) = \text{atan2}(-a,-b) + \pi, $$

and the second formulation yields values from $0$ continuously to $2\pi$. Thus, we can compute
$\phi$ as

  $$ \phi = \text{atan2}(-z, x) + \pi $$
</div>

<div>
The derivation for $\theta$ is more straightforward:

  $$ \theta = \text{acos}(-y) $$
</div>

<div class='together'>
So for a sphere, the $(u,v)$ coord computation is accomplished by a utility function that takes
points on the unit sphere centered at the origin, and computes $u$ and $v$:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class sphere : public hittable {
        ...
        private:
            static void get_sphere_uv(const point3& p, double& u, double& v) {
                // p: a given point on the sphere of radius one, centered at the origin.
                // u: returned value [0,1] of angle around the Y axis from X=-1.
                // v: returned value [0,1] of angle from Y=-1 to Y=+1.
                //     <1 0 0> yields <0.50 0.50>       <-1  0  0> yields <0.00 0.50>
                //     <0 1 0> yields <0.50 1.00>       < 0 -1  0> yields <0.50 0.00>
                //     <0 0 1> yields <0.25 0.50>       < 0  0 -1> yields <0.75 0.50>

                auto theta = acos(-p.y());
                auto phi = atan2(-p.z(), p.x()) + pi;

                u = phi / (2*pi);
                v = theta / pi;
            }
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [get-sphere-uv]: <kbd>[sphere.h]</kbd> get_sphere_uv function]
</div>

<div class='together'>
Update the `sphere::hit()` function to use this function to update the hit record UV coordinates.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    bool sphere::hit(...) {
        ...

        rec.t = root;
        rec.p = r.at(rec.t);
        vec3 outward_normal = (rec.p - center) / radius;
        rec.set_face_normal(r, outward_normal);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
        get_sphere_uv(outward_normal, rec.u, rec.v);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        rec.mat_ptr = mat_ptr;

        return true;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [get-sphere-uv-call]: <kbd>[sphere.h]</kbd> Sphere UV coordinates from hit]
</div>

<div class='together'>

<div class='together'>
Now we can make textured materials by replacing the `const color& a` with a texture pointer:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
    #include "texture.h"
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

    ...
    class lambertian : public material {
        public:
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            lambertian(const color& a) : albedo(make_shared<solid_color>(a)) {}
            lambertian(shared_ptr<texture> a) : albedo(a) {}
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

            virtual bool scatter(
                const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
            ) const override {
                auto scatter_direction = rec.normal + random_unit_vector();

                // Catch degenerate scatter direction
                if (scatter_direction.near_zero())
                    scatter_direction = rec.normal;

                scattered = ray(rec.p, scatter_direction, r_in.time());
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                attenuation = albedo->value(rec.u, rec.v, rec.p);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                return true;
            }

        public:
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            shared_ptr<texture> albedo;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [lambertian-textured]: <kbd>[material.h]</kbd> Lambertian material with texture]
</div>


A Checker Texture
------------------
<div class='together'>
We can create a checker texture by noting that the sign of sine and cosine just alternates in a
regular way, and if we multiply trig functions in all three dimensions, the sign of that product
forms a 3D checker pattern.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class checker_texture : public texture {
        public:
            checker_texture() {}

            checker_texture(shared_ptr<texture> _even, shared_ptr<texture> _odd)
                : even(_even), odd(_odd) {}

            checker_texture(color c1, color c2)
                : even(make_shared<solid_color>(c1)) , odd(make_shared<solid_color>(c2)) {}

            virtual color value(double u, double v, const point3& p) const override {
                auto sines = sin(10*p.x())*sin(10*p.y())*sin(10*p.z());
                if (sines < 0)
                    return odd->value(u, v, p);
                else
                    return even->value(u, v, p);
            }

        public:
            shared_ptr<texture> odd;
            shared_ptr<texture> even;
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [checker-texture]: <kbd>[texture.h]</kbd> Checkered texture]
</div>

Those checker odd/even pointers can be to a constant texture or to some other procedural texture.
This is in the spirit of shader networks introduced by Pat Hanrahan back in the 1980s.

<div class='together'>
If we add this to our `random_scene()` function’s base sphere:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    hittable_list random_scene() {
        hittable_list world;


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
        auto checker = make_shared<checker_texture>(color(0.2, 0.3, 0.1), color(0.9, 0.9, 0.9));
        world.add(make_shared<sphere>(point3(0,-1000,0), 1000, make_shared<lambertian>(checker)));
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

        for (int a = -11; a < 11; a++) {
            ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [checker-example]: <kbd>[main.cc]</kbd> Checkered texture in use]

We get:

  ![Image 2: Spheres on checkered ground](../images/img-2.02-checker-ground.png class=pixel)

</div>


Rendering a Scene with a Checkered Texture
-------------------------------------------
We're going to add a second scene to our program, and will add more scenes after that as we progress
through this book. To help with this, we'll set up a hard-coded switch statement to select the
desired scene for a given run. Clearly, this is a crude approach, but we're trying to keep things
dead simple and focus on the raytracing. You may want to use a different approach in your own
raytracer.

<div class='together'>
Here's the scene construction function:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    hittable_list two_spheres() {
        hittable_list objects;

        auto checker = make_shared<checker_texture>(color(0.2, 0.3, 0.1), color(0.9, 0.9, 0.9));

        objects.add(make_shared<sphere>(point3(0,-10, 0), 10, make_shared<lambertian>(checker)));
        objects.add(make_shared<sphere>(point3(0, 10, 0), 10, make_shared<lambertian>(checker)));

        return objects;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [scene-two-checker]: <kbd>[main.cc]</kbd> Scene with two checkered spheres]
</div>

<div class='together'>
The following changes set up our main function:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    // World


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
    hittable_list world;

    point3 lookfrom;
    point3 lookat;
    auto vfov = 40.0;
    auto aperture = 0.0;

    switch (0) {
        case 1:
            world = random_scene();
            lookfrom = point3(13,2,3);
            lookat = point3(0,0,0);
            vfov = 20.0;
            aperture = 0.1;
            break;

        default:
        case 2:
            world = two_spheres();
            lookfrom = point3(13,2,3);
            lookat = point3(0,0,0);
            vfov = 20.0;
            break;
    }

    // Camera

    vec3 vup(0,1,0);
    auto dist_to_focus = 10.0;
    int image_height = static_cast<int>(image_width / aspect_ratio);

    camera cam(lookfrom, lookat, vup, vfov, aspect_ratio, aperture, dist_to_focus, 0.0, 1.0);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [scene-two-checker-view]: <kbd>[main.cc]</kbd> Second scene]
</div>

<div class='together'>
We get this result:

  ![Image 3: Checkered spheres](../images/img-2.03-checker-spheres.png class=pixel)

</div>



Perlin Noise
====================================================================================================

<div class='together'>
To get cool looking solid textures most people use some form of Perlin noise. These are named after
their inventor Ken Perlin. Perlin texture doesn’t return white noise like this:

  ![Image 4: White noise](../images/img-2.04-white-noise.jpg class=pixel)

Instead it returns something similar to blurred white noise:

  ![Image 5: White noise, blurred](../images/img-2.05-white-noise-blurred.jpg class=pixel)

</div>

A key part of Perlin noise is that it is repeatable: it takes a 3D point as input and always returns
the same randomish number. Nearby points return similar numbers. Another important part of Perlin
noise is that it be simple and fast, so it’s usually done as a hack. I’ll build that hack up
incrementally based on Andrew Kensler’s description.


Using Blocks of Random Numbers
-------------------------------
<div class='together'>
We could just tile all of space with a 3D array of random numbers and use them in blocks. You get
something blocky where the repeating is clear:

  ![Image 6: Tiled random patterns](../images/img-2.06-tile-random.jpg class=pixel)

</div>

<div class='together'>
Let’s just use some sort of hashing to scramble this, instead of tiling. This has a bit of support
code to make it all happen:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #ifndef PERLIN_H
    #define PERLIN_H

    #include "rtweekend.h"

    class perlin {
        public:
            perlin() {
                ranfloat = new double[point_count];
                for (int i = 0; i < point_count; ++i) {
                    ranfloat[i] = random_double();
                }

                perm_x = perlin_generate_perm();
                perm_y = perlin_generate_perm();
                perm_z = perlin_generate_perm();
            }

            ~perlin() {
                delete[] ranfloat;
                delete[] perm_x;
                delete[] perm_y;
                delete[] perm_z;
            }

            double noise(const point3& p) const {
                auto i = static_cast<int>(4*p.x()) & 255;
                auto j = static_cast<int>(4*p.y()) & 255;
                auto k = static_cast<int>(4*p.z()) & 255;

                return ranfloat[perm_x[i] ^ perm_y[j] ^ perm_z[k]];
            }

        private:
            static const int point_count = 256;
            double* ranfloat;
            int* perm_x;
            int* perm_y;
            int* perm_z;

            static int* perlin_generate_perm() {
                auto p = new int[point_count];

                for (int i = 0; i < perlin::point_count; i++)
                    p[i] = i;

                permute(p, point_count);

                return p;
            }

            static void permute(int* p, int n) {
                for (int i = n-1; i > 0; i--) {
                    int target = random_int(0, i);
                    int tmp = p[i];
                    p[i] = p[target];
                    p[target] = tmp;
                }
            }
    };

    #endif
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [perlin]: <kbd>[perlin.h]</kbd> A Perlin texture class and functions]
</div>

<div class='together'>
Now if we create an actual texture that takes these floats between 0 and 1 and creates grey colors:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #include "perlin.h"

    class noise_texture : public texture {
        public:
            noise_texture() {}

            virtual color value(double u, double v, const point3& p) const override {
                return color(1,1,1) * noise.noise(p);
            }

        public:
            perlin noise;
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [noise-texture]: <kbd>[texture.h]</kbd> Noise texture]
</div>

<div class='together'>
We can use that texture on some spheres:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    hittable_list two_perlin_spheres() {
        hittable_list objects;

        auto pertext = make_shared<noise_texture>();
        objects.add(make_shared<sphere>(point3(0,-1000,0), 1000, make_shared<lambertian>(pertext)));
        objects.add(make_shared<sphere>(point3(0, 2, 0), 2, make_shared<lambertian>(pertext)));

        return objects;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [scene-perlin]: <kbd>[main.cc]</kbd> Scene with two Perlin-textured spheres]
</div>

<div class='together'>
With a similar scene setup to before:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    int main() {
        ...
        switch (0) {
            ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ delete
            default:
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            case 2:
                ...

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            default:
            case 3:
                world = two_perlin_spheres();
                lookfrom = point3(13,2,3);
                lookat = point3(0,0,0);
                vfov = 20.0;
                break;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        }
        ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [scene-perlin-view]: <kbd>[main.cc]</kbd> Viewing parameters]
</div>

<div class='together'>
Add the hashing does scramble as hoped:

  ![Image 7: Hashed random texture](../images/img-2.07-hash-random.png class=pixel)

</div>


Smoothing out the Result
-------------------------
<div class='together'>
To make it smooth, we can linearly interpolate:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

    class perlin {
        public:
            ...
            double noise(point3 vec3& p) const {
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                auto u = p.x() - floor(p.x());
                auto v = p.y() - floor(p.y());
                auto w = p.z() - floor(p.z());

                auto i = static_cast<int>(floor(p.x()));
                auto j = static_cast<int>(floor(p.y()));
                auto k = static_cast<int>(floor(p.z()));
                double c[2][2][2];

                for (int di=0; di < 2; di++)
                    for (int dj=0; dj < 2; dj++)
                        for (int dk=0; dk < 2; dk++)
                            c[di][dj][dk] = ranfloat[
                                perm_x[(i+di) & 255] ^
                                perm_y[(j+dj) & 255] ^
                                perm_z[(k+dk) & 255]
                            ];

                return trilinear_interp(c, u, v, w);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            }
            ...
        private:
            ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            static double trilinear_interp(double c[2][2][2], double u, double v, double w) {
                auto accum = 0.0;
                for (int i=0; i < 2; i++)
                    for (int j=0; j < 2; j++)
                        for (int k=0; k < 2; k++)
                            accum += (i*u + (1-i)*(1-u))*
                                    (j*v + (1-j)*(1-v))*
                                    (k*w + (1-k)*(1-w))*c[i][j][k];

                return accum;
            }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [perlin-trilinear]: <kbd>[perlin.h]</kbd> Perlin with trilienear interpolation]
</div>

<div class='together'>
And we get:

  ![Image 8: Perlin texture with trilinear interpolation
  ](../images/img-2.08-perlin-trilerp.png class=pixel)

</div>


Improvement with Hermitian Smoothing
-------------------------------------
<div class='together'>
Smoothing yields an improved result, but there are obvious grid features in there. Some of it is
Mach bands, a known perceptual artifact of linear interpolation of color. A standard trick is to use
a Hermite cubic to round off the interpolation:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class perlin (
        public:
            ...
            double noise(const point3& p) const {
                auto u = p.x() - floor(p.x());
                auto v = p.y() - floor(p.y());
                auto w = p.z() - floor(p.z());

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                u = u*u*(3-2*u);
                v = v*v*(3-2*v);
                w = w*w*(3-2*w);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

                auto i = static_cast<int>(floor(p.x()));
                auto j = static_cast<int>(floor(p.y()));
                auto k = static_cast<int>(floor(p.z()));
                ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [perlin-smoothed]: <kbd>[perlin.h]</kbd> Perlin smoothed]
</div>

<div class='together'>
This gives a smoother looking image:

  ![Image 9: Perlin texture, trilinearly interpolated, smoothed
  ](../images/img-2.09-perlin-trilerp-smooth.png class=pixel)

</div>


Tweaking The Frequency
-----------------------
<div class='together'>
It is also a bit low frequency. We can scale the input point to make it vary more quickly:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class noise_texture : public texture {
        public:
            noise_texture() {}
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            noise_texture(double sc) : scale(sc) {}
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

            virtual color value(double u, double v, const point3& p) const override {
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                return color(1,1,1) * noise.noise(scale * p);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            }

        public:
            perlin noise;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            double scale;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [perlin-smoothed-2]: <kbd>[texture.h]</kbd> Perlin smoothed, higher frequency]
</div>

<div class='together'>
We then add that scale to the `two_perlin_spheres()` scene description:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    hittable_list two_perlin_spheres() {
        hittable_list objects;

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
        auto pertext = make_shared<noise_texture>(4);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        objects.add(make_shared<sphere>(point3(0,-1000,0), 1000, make_shared<lambertian>(pertext)));
        objects.add(make_shared<sphere>(point3(0, 2, 0), 2, make_shared<lambertian>(pertext)));

        return objects;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [scale-perlin]: <kbd>[main.cc]</kbd> Perlin-textured spheres with a scale to the noise]
</div>

which gives:

  ![Image 10: Perlin texture, higher frequency](../images/img-2.10-perlin-hifreq.png class=pixel)

</div>


Using Random Vectors on the Lattice Points
-------------------------------------------
<div class='together'>
This is still a bit blocky looking, probably because the min and max of the pattern always lands
exactly on the integer x/y/z. Ken Perlin’s very clever trick was to instead put random unit vectors
(instead of just floats) on the lattice points, and use a dot product to move the min and max off
the lattice. So, first we need to change the random floats to random vectors. These vectors are any
reasonable set of irregular directions, and I won't bother to make them exactly uniform:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class perlin {
        public:
            perlin() {
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                ranvec = new vec3[point_count];
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                for (int i = 0; i < point_count; ++i) {
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                    ranvec[i] = unit_vector(vec3::random(-1,1));
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                }

                perm_x = perlin_generate_perm();
                perm_y = perlin_generate_perm();
                perm_z = perlin_generate_perm();
            }

            ~perlin() {
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                delete[] ranvec;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                delete[] perm_x;
                delete[] perm_y;
                delete[] perm_z;
            }
        ...
        private:
            static const int point_count = 256;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            vec3* ranvec;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            int* perm_x;
            int* perm_y;
            int* perm_z;
            ...
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [perlin-randunit]: <kbd>[perlin.h]</kbd> Perlin with random unit translations]
</div>

<div class='together'>
The Perlin class `noise()` method is now:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class perlin {
        public:
            ...
            double noise(const point3& p) const {
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                auto u = p.x() - floor(p.x());
                auto v = p.y() - floor(p.y());
                auto w = p.z() - floor(p.z());
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                auto i = static_cast<int>(floor(p.x()));
                auto j = static_cast<int>(floor(p.y()));
                auto k = static_cast<int>(floor(p.z()));
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                vec3 c[2][2][2];
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

                for (int di=0; di < 2; di++)
                    for (int dj=0; dj < 2; dj++)
                        for (int dk=0; dk < 2; dk++)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                            c[di][dj][dk] = ranvec[
                                perm_x[(i+di) & 255] ^
                                perm_y[(j+dj) & 255] ^
                                perm_z[(k+dk) & 255]
                            ];
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                return perlin_interp(c, u, v, w);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            }
            ...
        }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [perlin-2]: <kbd>[perlin.h]</kbd> Perlin class with new noise() method]
</div>

<div class='together'>
And the interpolation becomes a bit more complicated:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class perlin {
        ...
        private:
            ...
            static double perlin_interp(vec3 c[2][2][2], double u, double v, double w) {
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                auto uu = u*u*(3-2*u);
                auto vv = v*v*(3-2*v);
                auto ww = w*w*(3-2*w);
                auto accum = 0.0;

                for (int i=0; i < 2; i++)
                    for (int j=0; j < 2; j++)
                        for (int k=0; k < 2; k++) {
                            vec3 weight_v(u-i, v-j, w-k);
                            accum += (i*uu + (1-i)*(1-uu))
                                   * (j*vv + (1-j)*(1-vv))
                                   * (k*ww + (1-k)*(1-ww))
                                   * dot(c[i][j][k], weight_v);
                        }

                return accum;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            }
            ...
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [perlin-interp]: <kbd>[perlin.h]</kbd> Perlin interpolation function so far]
</div>

<div class='together'>
The output of the perlin interpretation can return negative values. These negative values will be
passed to the `sqrt()` function of our gamma function and get turned into `NaN`s. We will cast the
perlin output back to between 0 and 1.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class noise_texture : public texture {
        public:
            noise_texture() {}
            noise_texture(double sc) : scale(sc) {}

            virtual color value(double u, double v, const point3& p) const override {
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                return color(1,1,1) * 0.5 * (1.0 + noise.noise(scale * p));
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            }

        public:
            perlin noise;
            double scale;
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [perlin-smoothed-2]: <kbd>[texture.h]</kbd> Perlin smoothed, higher frequency]
</div>

<div class='together'>
This finally gives something more reasonable looking:

  ![Image 11: Perlin texture, shifted off integer values
  ](../images/img-2.11-perlin-shift.png class=pixel)

</div>


Introducing Turbulence
-----------------------
<div class='together'>
Very often, a composite noise that has multiple summed frequencies is used. This is usually called
turbulence, and is a sum of repeated calls to noise:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class perlin {
        ...
        public:
            ...
            double turb(const point3& p, int depth=7) const {
                auto accum = 0.0;
                auto temp_p = p;
                auto weight = 1.0;

                for (int i = 0; i < depth; i++) {
                    accum += weight*noise(temp_p);
                    weight *= 0.5;
                    temp_p *= 2;
                }

                return fabs(accum);
            }
            ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [perlin-turb]: <kbd>[perlin.h]</kbd> Turbulence function]
</div>

Here `fabs()` is the absolute value function defined in `<cmath>`.

<div class='together'>
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class noise_texture : public texture {
        public:
            noise_texture() {}
            noise_texture(double sc) : scale(sc) {}

            virtual color value(double u, double v, const point3& p) const override {
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                return color(1,1,1) * noise.turb(scale * p);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            }

        public:
            perlin noise;
            double scale;
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [noise-tex-2]: <kbd>[texture.h]</kbd> Noise texture with turbulence]
</div>

<div class='together'>
Used directly, turbulence gives a sort of camouflage netting appearance:

  ![Image 12: Perlin texture with turbulence](../images/img-2.12-perlin-turb.png class=pixel)

</div>


Adjusting the Phase
--------------------
<div class='together'>
However, usually turbulence is used indirectly. For example, the “hello world” of procedural solid
textures is a simple marble-like texture. The basic idea is to make color proportional to something
like a sine function, and use turbulence to adjust the phase (so it shifts $x$ in $\sin(x)$) which
makes the stripes undulate. Commenting out straight noise and turbulence, and giving a marble-like
effect is:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class noise_texture : public texture {
        public:
            noise_texture() {}
            noise_texture(double sc) : scale(sc) {}

            virtual color value(double u, double v, const point3& p) const override {
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                return color(1,1,1) * 0.5 * (1 + sin(scale*p.z() + 10*noise.turb(p)));
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            }

        public:
            perlin noise;
            double scale;
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [noise-tex-3]: <kbd>[texture.h]</kbd> Noise texture with marbled texture]

Which yields:

  ![Image 13: Perlin noise, marbled texture](../images/img-2.13-perlin-marble.png class=pixel)

</div>



Image Texture Mapping
====================================================================================================

From the hitpoint $\mathbf{P}$, we compute the surface coordinates $(u,v)$. We then use these to
index into our procedural solid texture (like marble). We can also read in an image and use the
2D $(u,v)$ texture coordinate to index into the image.

<div class='together'>
A direct way to use scaled $(u,v)$ in an image is to round the $u$ and $v$ to integers, and use that
as $(i,j)$ pixels. This is awkward, because we don’t want to have to change the code when we change
image resolution. So instead, one of the the most universal unofficial standards in graphics is to
use texture coordinates instead of image pixel coordinates. These are just some form of fractional
position in the image. For example, for pixel $(i,j)$ in an $N_x$ by $N_y$ image, the image texture
position is:

  $$ u = \frac{i}{N_x-1} $$
  $$ v = \frac{j}{N_y-1} $$
</div>

This is just a fractional position.


Storing Texture Image Data
---------------------------
Now we also need to create a texture class that holds an image. I am going to use my favorite image
utility [stb_image][]. It reads in an image into a big array of unsigned char. These are just packed
RGBs with each component in the range [0,255] (black to full white). The `image_texture` class uses
the resulting image data.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #include "rtweekend.h"
    #include "rtw_stb_image.h"
    #include "perlin.h"

    #include <iostream>

    ...

    class image_texture : public texture {
        public:
            const static int bytes_per_pixel = 3;

            image_texture()
              : data(nullptr), width(0), height(0), bytes_per_scanline(0) {}

            image_texture(const char* filename) {
                auto components_per_pixel = bytes_per_pixel;

                data = stbi_load(
                    filename, &width, &height, &components_per_pixel, components_per_pixel);

                if (!data) {
                    std::cerr << "ERROR: Could not load texture image file '" << filename << "'.\n";
                    width = height = 0;
                }

                bytes_per_scanline = bytes_per_pixel * width;
            }

            ~image_texture() {
                delete data;
            }

            virtual color value(double u, double v, const vec3& p) const override {
                // If we have no texture data, then return solid cyan as a debugging aid.
                if (data == nullptr)
                    return color(0,1,1);

                // Clamp input texture coordinates to [0,1] x [1,0]
                u = clamp(u, 0.0, 1.0);
                v = 1.0 - clamp(v, 0.0, 1.0);  // Flip V to image coordinates

                auto i = static_cast<int>(u * width);
                auto j = static_cast<int>(v * height);

                // Clamp integer mapping, since actual coordinates should be less than 1.0
                if (i >= width)  i = width-1;
                if (j >= height) j = height-1;

                const auto color_scale = 1.0 / 255.0;
                auto pixel = data + j*bytes_per_scanline + i*bytes_per_pixel;

                return color(color_scale*pixel[0], color_scale*pixel[1], color_scale*pixel[2]);
            }

        private:
            unsigned char *data;
            int width, height;
            int bytes_per_scanline;
    };

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [img-texture]: <kbd>[texture.h]</kbd> Image texture class]

<div class='together'>
The representation of a packed array in that order is pretty standard. Thankfully, the [stb_image][]
package makes that super simple -- just write a header called `rtw_stb_image.h` that also deals with
some compiler warnings:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #ifndef RTWEEKEND_STB_IMAGE_H
    #define RTWEEKEND_STB_IMAGE_H

    // Disable pedantic warnings for this external library.
    #ifdef _MSC_VER
        // Microsoft Visual C++ Compiler
        #pragma warning (push, 0)
    #endif

    #define STB_IMAGE_IMPLEMENTATION
    #include "external/stb_image.h"

    // Restore warning levels.
    #ifdef _MSC_VER
        // Microsoft Visual C++ Compiler
        #pragma warning (pop)
    #endif

    #endif
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [rtw-stb-image]: <kbd>[rtw_stb_image.h]</kbd> Include the stb_image package]

The above assumes that you have copied the `stb_image.h` into a folder called `external`. Adjust
according to your directory structure.
</div>


Using an Image Texture
-----------------------
<div class='together'>
I just grabbed a random earth map from the web -- any standard projection will do for our purposes.

  ![Image 14: earthmap.jpg](../images/earthmap.jpg class=pixel)

</div>

<div class='together'>
Here's the code to read an image from a file and then assign it to a diffuse material:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    hittable_list earth() {
        auto earth_texture = make_shared<image_texture>("earthmap.jpg");
        auto earth_surface = make_shared<lambertian>(earth_texture);
        auto globe = make_shared<sphere>(point3(0,0,0), 2, earth_surface);

        return hittable_list(globe);
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [stbi-load-use]: <kbd>[main.cc]</kbd> Using stbi_load() to load an image]
</div>

We start to see some of the power of all colors being textures -- we can assign any kind of texture
to the lambertian material, and lambertian doesn’t need to be aware of it.

<div class='together'>
To test this, throw it into main:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    int main() {
        ...
        switch (0) {
            ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ delete
            default:
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            case 3:
                ...

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            default:
            case 4:
                world = earth();
                lookfrom = point3(13,2,3);
                lookat = point3(0,0,0);
                vfov = 20.0;
                break;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        }
        ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [scene-earth-view]: <kbd>[main.cc]</kbd> Viewing parameters]

If the photo comes back with a large cyan sphere in the middle, then stb_image failed to find your
Earth map photo. The program will look for the file in the same directory as the executable. Make
sure to copy the Earth into your build directory, or rewrite `earth()` to point somewhere else.

  ![Image 15: Earth-mapped sphere](../images/img-2.15-earth-sphere.png class=pixel)

</div>



Rectangles and Lights
====================================================================================================
Lighting is a key component of raytracing. Early simple raytracers used abstract light sources, like
points in space, or directions. Modern approaches have more physically based lights, which have
position and size. To create such light sources, we need to be able to take any regular object and
turn it into something that emits light into our scene.


Emissive Materials
-------------------
<div class='together'>
First, let’s make a light emitting material. We need to add an emitted function (we could also add
it to `hit_record` instead -- that’s a matter of design taste). Like the background, it just tells
the ray what color it is and performs no reflection. It’s very simple:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class diffuse_light : public material  {
        public:
            diffuse_light(shared_ptr<texture> a) : emit(a) {}
            diffuse_light(color c) : emit(make_shared<solid_color>(c)) {}

            virtual bool scatter(
                const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
            ) const override {
                return false;
            }

            virtual color emitted(double u, double v, const point3& p) const override {
                return emit->value(u, v, p);
            }

        public:
            shared_ptr<texture> emit;
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [diffuse-light]: <kbd>[material.h]</kbd> A diffuse light class]
</div>

<div class='together'>
So that I don’t have to make all the non-emitting materials implement `emitted()`, I have the base
class return black:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class material {
        public:
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            virtual color emitted(double u, double v, const point3& p) const {
                return color(0,0,0);
            }

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            virtual bool scatter(
                const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
            ) const = 0;
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [matl-emit]: <kbd>[material.h]</kbd> New emitted function in class material]
</div>


Adding Background Color to the Ray Color Function
--------------------------------------------------
<div class='together'>
Next, we want a pure black background so the only light in the scene is coming from the emitters. To
do this, we’ll add a background color parameter to our `ray_color` function, and pay attention to
the new `emitted` value.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
    color ray_color(const ray& r, const color& background, const hittable& world, int depth) {
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        hit_record rec;

        // If we've exceeded the ray bounce limit, no more light is gathered.
        if (depth <= 0)
            return color(0,0,0);


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
        // If the ray hits nothing, return the background color.
        if (!world.hit(r, 0.001, infinity, rec))
            return background;

        ray scattered;
        color attenuation;
        color emitted = rec.mat_ptr->emitted(rec.u, rec.v, rec.p);

        if (!rec.mat_ptr->scatter(r, rec, attenuation, scattered))
            return emitted;

        return emitted + attenuation * ray_color(scattered, background, world, depth-1);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    }
    ...

    int main() {
        ...

        point3 lookfrom;
        point3 lookat;
        auto vfov = 40.0;
        auto aperture = 0.0;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
        color background(0,0,0);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

        switch (0) {
            case 1:
                world = random_scene();
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                background = color(0.70, 0.80, 1.00);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                lookfrom = point3(13,2,3);
                lookat = point3(0,0,0);
                vfov = 20.0;
                aperture = 0.1;
                break;

            case 2:
                world = two_spheres();
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                background = color(0.70, 0.80, 1.00);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                lookfrom = point3(13,2,3);
                lookat = point3(0,0,0);
                vfov = 20.0;
                break;

            case 3:
                world = two_perlin_spheres();
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                background = color(0.70, 0.80, 1.00);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                lookfrom = point3(13,2,3);
                lookat = point3(0,0,0);
                vfov = 20.0;
                break;


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ delete
            default:
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            case 4:
                world = earth();
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                background = color(0.70, 0.80, 1.00);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
                lookfrom = point3(13,2,3);
                lookat = point3(0,0,0);
                vfov = 20.0;
                break;


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            default:
            case 5:
                background = color(0.0, 0.0, 0.0);
                break;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        }

        ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
                    pixel_color += ray_color(r, background, world, max_depth);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        ...
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [ray-color-emitted]: <kbd>[main.cc]</kbd> ray_color function for emitting materials]

Since we're removing the code that we used to determine the color of the sky when a ray hit it, we
need to pass in a new color value for our old scene renders. We've elected to stick with a flat
bluish-white for the whole sky. You could always pass in a boolean to switch between the previous
skybox code versus the new solid color background. We're keeping it simple here.

</div>


Creating Rectangle Objects
---------------------------
Now, let’s make some rectangles. Rectangles are often convenient for modeling man-made environments.
I’m a fan of doing axis-aligned rectangles because they are easy. (We’ll get to instancing so we can
rotate them later.)

<div class='together'>
First, here is a rectangle in an xy plane. Such a plane is defined by its z value. For example, $z =
k$. An axis-aligned rectangle is defined by the lines $x=x_0$, $x=x_1$, $y=y_0$, and $y=y_1$.

  ![Figure [ray-rect]: Ray-rectangle intersection](../images/fig-2.05-ray-rect.jpg)

</div>

<div class='together'>
To determine whether a ray hits such a rectangle, we first determine where the ray hits the plane.
Recall that a ray $\mathbf{P}(t) = \mathbf{A} + t \mathbf{b}$ has its z component defined by $P_z(t)
= A_z + t b_z$. Rearranging those terms we can solve for what the t is where $z=k$.

  $$ t = \frac{k-A_z}{b_z} $$
</div>

<div class='together'>
Once we have $t$, we can plug that into the equations for $x$ and $y$:

  $$ x = A_x + t b_x $$
  $$ y = A_y + t b_y $$
</div>

It is a hit if $x_0 < x < x_1$ and $y_0 < y < y_1$.

Because our rectangles are axis-aligned, their bounding boxes will have an infinitely-thin side.
This can be a problem when dividing them up with our axis-aligned bounding volume hierarchy. To
counter this, all hittable objects should get a bounding box that has finite width along every
dimension. For our rectangles, we'll just pad the box a bit on the infnitely-thin side.

<div class='together'>
The actual `xy_rect` class is thus:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #ifndef AARECT_H
    #define AARECT_H

    #include "rtweekend.h"

    #include "hittable.h"

    class xy_rect : public hittable {
        public:
            xy_rect() {}

            xy_rect(double _x0, double _x1, double _y0, double _y1, double _k, 
                shared_ptr<material> mat)
                : x0(_x0), x1(_x1), y0(_y0), y1(_y1), k(_k), mp(mat) {};

            virtual bool hit(const ray& r, double t_min, double t_max, hit_record& rec) const override;

            virtual bool bounding_box(double time0, double time1, aabb& output_box) const override {
                // The bounding box must have non-zero width in each dimension, so pad the Z
                // dimension a small amount.
                output_box = aabb(point3(x0,y0, k-0.0001), point3(x1, y1, k+0.0001));
                return true;
            }

        public:
            shared_ptr<material> mp;
            double x0, x1, y0, y1, k;
    };

    #endif
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [xy-rect]: <kbd>[aarect.h]</kbd> XY-plane rectangle objects]
</div>

<div class='together'>
And the hit function is:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    bool xy_rect::hit(const ray& r, double t_min, double t_max, hit_record& rec) const {
        auto t = (k-r.origin().z()) / r.direction().z();
        if (t < t_min || t > t_max)
            return false;
        auto x = r.origin().x() + t*r.direction().x();
        auto y = r.origin().y() + t*r.direction().y();
        if (x < x0 || x > x1 || y < y0 || y > y1)
            return false;
        rec.u = (x-x0)/(x1-x0);
        rec.v = (y-y0)/(y1-y0);
        rec.t = t;
        auto outward_normal = vec3(0, 0, 1);
        rec.set_face_normal(r, outward_normal);
        rec.mat_ptr = mp;
        rec.p = r.at(t);
        return true;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [xy-rect-hit]: <kbd>[aarect.h]</kbd> Hit function for XY rectangle objects]
</div>


Turning Objects into Lights
----------------------------
<div class='together'>
If we set up a rectangle as a light:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    hittable_list simple_light() {
        hittable_list objects;

        auto pertext = make_shared<noise_texture>(4);
        objects.add(make_shared<sphere>(point3(0,-1000,0), 1000, make_shared<lambertian>(pertext)));
        objects.add(make_shared<sphere>(point3(0,2,0), 2, make_shared<lambertian>(pertext)));

        auto difflight = make_shared<diffuse_light>(color(4,4,4));
        objects.add(make_shared<xy_rect>(3, 5, 1, 3, -2, difflight));

        return objects;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [rect-light]: <kbd>[main.cc]</kbd> A simple rectangle light]
</div>

<div class='together'>
And then create a new scene, making careful attention to set the background color black:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #include "rtweekend.h"

    #include "camera.h"
    #include "color.h"
    #include "hittable_list.h"
    #include "material.h"
    #include "moving_sphere.h"
    #include "sphere.h"
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
    #include "aarect.h"
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

    #include <iostream>
    ...
    int main() {
        ...
        switch (0) {
            ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            default:
            case 5:
                world = simple_light();
                samples_per_pixel = 400;
                background = color(0,0,0);
                lookfrom = point3(26,3,6);
                lookat = point3(0,2,0);
                vfov = 20.0;
                break;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        }
        ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [rectangle-light]: <kbd>[main.cc]</kbd> Rectangle light scene with black background]

<div class='together'>
We get:

  ![Image 16: Scene with rectangle light source](../images/img-2.16-rect-light.png class=pixel)

</div>

Note that the light is brighter than $(1,1,1)$. This allows it to be bright enough to light things.

<div class='together'>
Fool around with making some spheres lights too.

  ![Image 17: Scene with rectangle and sphere light sources
  ](../images/img-2.17-rect-sphere-light.png class=pixel)

</div>


More Axis-Aligned Rectangles
-----------------------------
Now let’s add the other two axes and the famous Cornell Box.

<div class='together'>
This is xz and yz:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class xz_rect : public hittable {
        public:
            xz_rect() {}

            xz_rect(double _x0, double _x1, double _z0, double _z1, double _k,
                shared_ptr<material> mat)
                : x0(_x0), x1(_x1), z0(_z0), z1(_z1), k(_k), mp(mat) {};

            virtual bool hit(const ray& r, double t_min, double t_max, hit_record& rec) const override;

            virtual bool bounding_box(double time0, double time1, aabb& output_box) const override {
                // The bounding box must have non-zero width in each dimension, so pad the Y
                // dimension a small amount.
                output_box = aabb(point3(x0,k-0.0001,z0), point3(x1, k+0.0001, z1));
                return true;
            }

        public:
            shared_ptr<material> mp;
            double x0, x1, z0, z1, k;
    };

    class yz_rect : public hittable {
        public:
            yz_rect() {}

            yz_rect(double _y0, double _y1, double _z0, double _z1, double _k,
                shared_ptr<material> mat)
                : y0(_y0), y1(_y1), z0(_z0), z1(_z1), k(_k), mp(mat) {};

            virtual bool hit(const ray& r, double t_min, double t_max, hit_record& rec) const override;

            virtual bool bounding_box(double time0, double time1, aabb& output_box) const override {
                // The bounding box must have non-zero width in each dimension, so pad the X
                // dimension a small amount.
                output_box = aabb(point3(k-0.0001, y0, z0), point3(k+0.0001, y1, z1));
                return true;
            }

        public:
            shared_ptr<material> mp;
            double y0, y1, z0, z1, k;
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [xz-yz-rects]: <kbd>[aarect.h]</kbd> XZ and YZ rectangle objects]
</div>

<div class='together'>
With unsurprising hit functions:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    bool xz_rect::hit(const ray& r, double t_min, double t_max, hit_record& rec) const {
        auto t = (k-r.origin().y()) / r.direction().y();
        if (t < t_min || t > t_max)
            return false;
        auto x = r.origin().x() + t*r.direction().x();
        auto z = r.origin().z() + t*r.direction().z();
        if (x < x0 || x > x1 || z < z0 || z > z1)
            return false;
        rec.u = (x-x0)/(x1-x0);
        rec.v = (z-z0)/(z1-z0);
        rec.t = t;
        auto outward_normal = vec3(0, 1, 0);
        rec.set_face_normal(r, outward_normal);
        rec.mat_ptr = mp;
        rec.p = r.at(t);
        return true;
    }

    bool yz_rect::hit(const ray& r, double t_min, double t_max, hit_record& rec) const {
        auto t = (k-r.origin().x()) / r.direction().x();
        if (t < t_min || t > t_max)
            return false;
        auto y = r.origin().y() + t*r.direction().y();
        auto z = r.origin().z() + t*r.direction().z();
        if (y < y0 || y > y1 || z < z0 || z > z1)
            return false;
        rec.u = (y-y0)/(y1-y0);
        rec.v = (z-z0)/(z1-z0);
        rec.t = t;
        auto outward_normal = vec3(1, 0, 0);
        rec.set_face_normal(r, outward_normal);
        rec.mat_ptr = mp;
        rec.p = r.at(t);
        return true;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [xz-yz]: <kbd>[aarect.h]</kbd> XZ and YZ rectangle object hit functions]
</div>


Creating an Empty “Cornell Box”
--------------------------------
<div class='together'>
The “Cornell Box” was introduced in 1984 to model the interaction of light between diffuse surfaces.
Let’s make the 5 walls and the light of the box:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    hittable_list cornell_box() {
        hittable_list objects;

        auto red   = make_shared<lambertian>(color(.65, .05, .05));
        auto white = make_shared<lambertian>(color(.73, .73, .73));
        auto green = make_shared<lambertian>(color(.12, .45, .15));
        auto light = make_shared<diffuse_light>(color(15, 15, 15));

        objects.add(make_shared<yz_rect>(0, 555, 0, 555, 555, green));
        objects.add(make_shared<yz_rect>(0, 555, 0, 555, 0, red));
        objects.add(make_shared<xz_rect>(213, 343, 227, 332, 554, light));
        objects.add(make_shared<xz_rect>(0, 555, 0, 555, 0, white));
        objects.add(make_shared<xz_rect>(0, 555, 0, 555, 555, white));
        objects.add(make_shared<xy_rect>(0, 555, 0, 555, 555, white));

        return objects;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [cornell-box-empty]: <kbd>[main.cc]</kbd> Cornell box scene, empty]
</div>

<div class='together'>
Add the view and scene info:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

    int main() {
        ...
        switch (0) {
            ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ delete
            default:
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
            case 5:
                ...
                break;


    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
            default:
            case 6:
                world = cornell_box();
                aspect_ratio = 1.0;
                image_width = 600;
                samples_per_pixel = 200;
                background = color(0,0,0);
                lookfrom = point3(278, 278, -800);
                lookat = point3(278, 278, 0);
                vfov = 40.0;
                break;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
        }
        ...
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [cornell-box-view]: <kbd>[main.cc]</kbd> Changing aspect ratio and viewing parameters]
</div>

<div class='together'>
We get:

  ![Image 18: Empty Cornell box](../images/img-2.18-cornell-empty.png class=pixel)

This image is very noisy because the light is small.
</div>



Instances
====================================================================================================

The Cornell Box usually has two blocks in it. These are rotated relative to the walls. First, let’s
make an axis-aligned block primitive that holds 6 rectangles:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #ifndef BOX_H
    #define BOX_H

    #include "rtweekend.h"

    #include "aarect.h"
    #include "hittable_list.h"

    class box : public hittable  {
        public:
            box() {}
            box(const point3& p0, const point3& p1, shared_ptr<material> ptr);

            virtual bool hit(const ray& r, double t_min, double t_max, hit_record& rec) const override;

            virtual bool bounding_box(double time0, double time1, aabb& output_box) const override {
                output_box = aabb(box_min, box_max);
                return true;
            }

        public:
            point3 box_min;
            point3 box_max;
            hittable_list sides;
    };

    box::box(const point3& p0, const point3& p1, shared_ptr<material> ptr) {
        box_min = p0;
        box_max = p1;

        sides.add(make_shared<xy_rect>(p0.x(), p1.x(), p0.y(), p1.y(), p1.z(), ptr));
        sides.add(make_shared<xy_rect>(p0.x(), p1.x(), p0.y(), p1.y(), p0.z(), ptr));

        sides.add(make_shared<xz_rect>(p0.x(), p1.x(), p0.z(), p1.z(), p1.y(), ptr));
        sides.add(make_shared<xz_rect>(p0.x(), p1.x(), p0.z(), p1.z(), p0.y(), ptr));

        sides.add(make_shared<yz_rect>(p0.y(), p1.y(), p0.z(), p1.z(), p1.x(), ptr));
        sides.add(make_shared<yz_rect>(p0.y(), p1.y(), p0.z(), p1.z(), p0.x(), ptr));
    }

    bool box::hit(const ray& r, double t_min, double t_max, hit_record& rec) const {
        return sides.hit(r, t_min, t_max, rec);
    }

    #endif
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [box-class]: <kbd>[box.h]</kbd> A box object]

<div class='together'>
Now we can add two blocks (but not rotated)

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #include "box.h"
    ...
    objects.add(make_shared<box>(point3(130, 0, 65), point3(295, 165, 230), white));
    objects.add(make_shared<box>(point3(265, 0, 295), point3(430, 330, 460), white));
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [add-boxes]: <kbd>[main.cc]</kbd> Adding box objects]

</div>

<div class='together'>
This gives:

  ![Image 19: Cornell box with two blocks](../images/img-2.19-cornell-blocks.png class=pixel)

</div>

<div class='together'>
Now that we have boxes, we need to rotate them a bit to have them match the _real_ Cornell box. In
ray tracing, this is usually done with an _instance_. An instance is a geometric primitive that has
been moved or rotated somehow. This is especially easy in ray tracing because we don’t move
anything; instead we move the rays in the opposite direction. For example, consider a _translation_
(often called a _move_). We could take the pink box at the origin and add 2 to all its x components,
or (as we almost always do in ray tracing) leave the box where it is, but in its hit routine
subtract 2 off the x-component of the ray origin.

  ![Figure [ray-box]: Ray-box intersection with moved ray vs box](../images/fig-2.06-ray-box.jpg)

</div>


Instance Translation
---------------------
<div class='together'>
Whether you think of this as a move or a change of coordinates is up to you. The code for this, to
move any underlying hittable is a _translate_ instance.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class translate : public hittable {
        public:
            translate(shared_ptr<hittable> p, const vec3& displacement)
                : ptr(p), offset(displacement) {}

            virtual bool hit(
                const ray& r, double t_min, double t_max, hit_record& rec) const override;

            virtual bool bounding_box(double time0, double time1, aabb& output_box) const override;

        public:
            shared_ptr<hittable> ptr;
            vec3 offset;
    };

    bool translate::hit(const ray& r, double t_min, double t_max, hit_record& rec) const {
        ray moved_r(r.origin() - offset, r.direction(), r.time());
        if (!ptr->hit(moved_r, t_min, t_max, rec))
            return false;

        rec.p += offset;
        rec.set_face_normal(moved_r, rec.normal);

        return true;
    }

    bool translate::bounding_box(double time0, double time1, aabb& output_box) const {
        if (!ptr->bounding_box(time0, time1, output_box))
            return false;

        output_box = aabb(
            output_box.min() + offset,
            output_box.max() + offset);

        return true;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [translate-class]: <kbd>[hittable.h]</kbd> Hittable translation class]
</div>


Instance Rotation
------------------
<div class='together'>
Rotation isn’t quite as easy to understand or generate the formulas for. A common graphics tactic is
to apply all rotations about the x, y, and z axes. These rotations are in some sense axis-aligned.
First, let’s rotate by theta about the z-axis. That will be changing only x and y, and in ways that
don’t depend on z.

  ![Figure [rot-z]: Rotation about the Z axis](../images/fig-2.07-rot-z.jpg)

</div>

<div class='together'>
This involves some basic trigonometry that uses formulas that I will not cover here. That gives you
the correct impression it’s a little involved, but it is straightforward, and you can find it in any
graphics text and in many lecture notes. The result for rotating counter-clockwise about z is:

  $$ x' = \cos(\theta) \cdot x - \sin(\theta) \cdot y $$
  $$ y' = \sin(\theta) \cdot x + \cos(\theta) \cdot y $$
</div>

The great thing is that it works for any $\theta$ and doesn’t need any cases for quadrants or
anything like that. The inverse transform is the opposite geometric operation: rotate by $-\theta$.
Here, recall that $\cos(\theta) = \cos(-\theta)$ and $\sin(-\theta) = -\sin(\theta)$, so the
formulas are very simple.

<div class='together'>
Similarly, for rotating about y (as we want to do for the blocks in the box) the formulas are:

  $$ x' =  \cos(\theta) \cdot x + \sin(\theta) \cdot z $$
  $$ z' = -\sin(\theta) \cdot x + \cos(\theta) \cdot z $$

And about the x-axis:

  $$ y' = \cos(\theta) \cdot y - \sin(\theta) \cdot z $$
  $$ z' = \sin(\theta) \cdot y + \cos(\theta) \cdot z $$
</div>

Unlike the situation with translations, the surface normal vector also changes, so we need to
transform directions too if we get a hit. Fortunately for rotations, the same formulas apply. If you
add scales, things get more complicated. See the web page https://in1weekend.blogspot.com/ for links
to that.

<div class='together'>
For a y-rotation class we have:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class rotate_y : public hittable {
        public:
            rotate_y(shared_ptr<hittable> p, double angle);

            virtual bool hit(
                const ray& r, double t_min, double t_max, hit_record& rec) const override;

            virtual bool bounding_box(double time0, double time1, aabb& output_box) const override {
                output_box = bbox;
                return hasbox;
            }

        public:
            shared_ptr<hittable> ptr;
            double sin_theta;
            double cos_theta;
            bool hasbox;
            aabb bbox;
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [rot-y]: <kbd>[hittable.h]</kbd> Hittable rotate-Y class]
</div>

<div class='together'>
With constructor:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    rotate_y::rotate_y(shared_ptr<hittable> p, double angle) : ptr(p) {
        auto radians = degrees_to_radians(angle);
        sin_theta = sin(radians);
        cos_theta = cos(radians);
        hasbox = ptr->bounding_box(0, 1, bbox);

        point3 min( infinity,  infinity,  infinity);
        point3 max(-infinity, -infinity, -infinity);

        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 2; j++) {
                for (int k = 0; k < 2; k++) {
                    auto x = i*bbox.max().x() + (1-i)*bbox.min().x();
                    auto y = j*bbox.max().y() + (1-j)*bbox.min().y();
                    auto z = k*bbox.max().z() + (1-k)*bbox.min().z();

                    auto newx =  cos_theta*x + sin_theta*z;
                    auto newz = -sin_theta*x + cos_theta*z;

                    vec3 tester(newx, y, newz);

                    for (int c = 0; c < 3; c++) {
                        min[c] = fmin(min[c], tester[c]);
                        max[c] = fmax(max[c], tester[c]);
                    }
                }
            }
        }

        bbox = aabb(min, max);
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [rot-y-rot]: <kbd>[hittable.h]</kbd> Rotate-Y rotate method]
</div>

<div class='together'>
And the hit function:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    bool rotate_y::hit(const ray& r, double t_min, double t_max, hit_record& rec) const {
        auto origin = r.origin();
        auto direction = r.direction();

        origin[0] = cos_theta*r.origin()[0] - sin_theta*r.origin()[2];
        origin[2] = sin_theta*r.origin()[0] + cos_theta*r.origin()[2];

        direction[0] = cos_theta*r.direction()[0] - sin_theta*r.direction()[2];
        direction[2] = sin_theta*r.direction()[0] + cos_theta*r.direction()[2];

        ray rotated_r(origin, direction, r.time());

        if (!ptr->hit(rotated_r, t_min, t_max, rec))
            return false;

        auto p = rec.p;
        auto normal = rec.normal;

        p[0] =  cos_theta*rec.p[0] + sin_theta*rec.p[2];
        p[2] = -sin_theta*rec.p[0] + cos_theta*rec.p[2];

        normal[0] =  cos_theta*rec.normal[0] + sin_theta*rec.normal[2];
        normal[2] = -sin_theta*rec.normal[0] + cos_theta*rec.normal[2];

        rec.p = p;
        rec.set_face_normal(rotated_r, normal);

        return true;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [rot-y-hit]: <kbd>[hittable.h]</kbd> Hittable Y-rotate hit function]
</div>

<div class='together'>
And the changes to Cornell are:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    shared_ptr<hittable> box1 = make_shared<box>(point3(0, 0, 0), point3(165, 330, 165), white);
    box1 = make_shared<rotate_y>(box1, 15);
    box1 = make_shared<translate>(box1, vec3(265,0,295));
    objects.add(box1);

    shared_ptr<hittable> box2 = make_shared<box>(point3(0,0,0), point3(165,165,165), white);
    box2 = make_shared<rotate_y>(box2, -18);
    box2 = make_shared<translate>(box2, vec3(130,0,65));
    objects.add(box2);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [scene-rot-y]: <kbd>[main.cc]</kbd> Cornell scene with Y-rotated boxes]
</div>

<div class='together'>
Which yields:

  ![Image 20: Standard Cornell box scene](../images/img-2.20-cornell-standard.png class=pixel)

</div>



Volumes
====================================================================================================

One thing it’s nice to add to a ray tracer is smoke/fog/mist. These are sometimes called _volumes_
or _participating media_. Another feature that is nice to add is subsurface scattering, which is
sort of like dense fog inside an object. This usually adds software architectural mayhem because
volumes are a different animal than surfaces, but a cute technique is to make a volume a random
surface. A bunch of smoke can be replaced with a surface that probabilistically might or might not
be there at every point in the volume. This will make more sense when you see the code.


Constant Density Mediums
-------------------------
First, let’s start with a volume of constant density. A ray going through there can either scatter
inside the volume, or it can make it all the way through like the middle ray in the figure. More
thin transparent volumes, like a light fog, are more likely to have rays like the middle one. How
far the ray has to travel through the volume also determines how likely it is for the ray to make it
through.

  ![Figure [ray-vol]: Ray-volume interaction](../images/fig-2.08-ray-vol.jpg)

<div class='together'>
As the ray passes through the volume, it may scatter at any point. The denser the volume, the more
likely that is. The probability that the ray scatters in any small distance $\Delta L$ is:

  $$ \text{probability} = C \cdot \Delta L $$
</div>

<div class='together'>
where $C$ is proportional to the optical density of the volume. If you go through all the
differential equations, for a random number you get a distance where the scattering occurs. If that
distance is outside the volume, then there is no “hit”. For a constant volume we just need the
density $C$ and the boundary. I’ll use another hittable for the boundary. The resulting class is:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #ifndef CONSTANT_MEDIUM_H
    #define CONSTANT_MEDIUM_H

    #include "rtweekend.h"

    #include "hittable.h"
    #include "material.h"
    #include "texture.h"

    class constant_medium : public hittable {
        public:
            constant_medium(shared_ptr<hittable> b, double d, shared_ptr<texture> a)
                : boundary(b),
                  neg_inv_density(-1/d),
                  phase_function(make_shared<isotropic>(a))
                {}

            constant_medium(shared_ptr<hittable> b, double d, color c)
                : boundary(b),
                  neg_inv_density(-1/d),
                  phase_function(make_shared<isotropic>(c))
                {}

            virtual bool hit(
                const ray& r, double t_min, double t_max, hit_record& rec) const override;

            virtual bool bounding_box(double time0, double time1, aabb& output_box) const override {
                return boundary->bounding_box(time0, time1, output_box);
            }

        public:
            shared_ptr<hittable> boundary;
            shared_ptr<material> phase_function;
            double neg_inv_density;
    };

    #endif
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [const-med-class]: <kbd>[constant_medium.h]</kbd> Constant medium class]
</div>

<div class='together'>
The scattering function of isotropic picks a uniform random direction:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    class isotropic : public material {
        public:
            isotropic(color c) : albedo(make_shared<solid_color>(c)) {}
            isotropic(shared_ptr<texture> a) : albedo(a) {}

            virtual bool scatter(
                const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
            ) const override {
                scattered = ray(rec.p, random_in_unit_sphere(), r_in.time());
                attenuation = albedo->value(rec.u, rec.v, rec.p);
                return true;
            }

        public:
            shared_ptr<texture> albedo;
    };
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [isotropic-class]: <kbd>[material.h]</kbd> The isotropic class]
</div>

<div class='together'>
And the hit function is:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    bool constant_medium::hit(const ray& r, double t_min, double t_max, hit_record& rec) const {
        // Print occasional samples when debugging. To enable, set enableDebug true.
        const bool enableDebug = false;
        const bool debugging = enableDebug && random_double() < 0.00001;

        hit_record rec1, rec2;

        if (!boundary->hit(r, -infinity, infinity, rec1))
            return false;

        if (!boundary->hit(r, rec1.t+0.0001, infinity, rec2))
            return false;

        if (debugging) std::cerr << "\nt_min=" << rec1.t << ", t_max=" << rec2.t << '\n';

        if (rec1.t < t_min) rec1.t = t_min;
        if (rec2.t > t_max) rec2.t = t_max;

        if (rec1.t >= rec2.t)
            return false;

        if (rec1.t < 0)
            rec1.t = 0;

        const auto ray_length = r.direction().length();
        const auto distance_inside_boundary = (rec2.t - rec1.t) * ray_length;
        const auto hit_distance = neg_inv_density * log(random_double());

        if (hit_distance > distance_inside_boundary)
            return false;

        rec.t = rec1.t + hit_distance / ray_length;
        rec.p = r.at(rec.t);

        if (debugging) {
            std::cerr << "hit_distance = " <<  hit_distance << '\n'
                      << "rec.t = " <<  rec.t << '\n'
                      << "rec.p = " <<  rec.p << '\n';
        }

        rec.normal = vec3(1,0,0);  // arbitrary
        rec.front_face = true;     // also arbitrary
        rec.mat_ptr = phase_function;

        return true;
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [const-med-hit]: <kbd>[constant_medium.h]</kbd> Constant medium hit method]
</div>

The reason we have to be so careful about the logic around the boundary is we need to make sure this
works for ray origins inside the volume. In clouds, things bounce around a lot so that is a common
case.

In addition, the above code assumes that once a ray exits the constant medium boundary, it will
continue forever outside the boundary. Put another way, it assumes that the boundary shape is
convex. So this particular implementation will work for boundaries like boxes or spheres, but will
not work with toruses or shapes that contain voids. It's possible to write an implementation that
handles arbitrary shapes, but we'll leave that as an exercise for the reader.


Rendering a Cornell Box with Smoke and Fog Boxes
-------------------------------------------------
<div class='together'>
If we replace the two blocks with smoke and fog (dark and light particles), and make the light
bigger (and dimmer so it doesn’t blow out the scene) for faster convergence:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    #include "constant_medium.h"
    ...

    hittable_list cornell_smoke() {
        hittable_list objects;

        auto red   = make_shared<lambertian>(color(.65, .05, .05));
        auto white = make_shared<lambertian>(color(.73, .73, .73));
        auto green = make_shared<lambertian>(color(.12, .45, .15));
        auto light = make_shared<diffuse_light>(color(7, 7, 7));

        objects.add(make_shared<yz_rect>(0, 555, 0, 555, 555, green));
        objects.add(make_shared<yz_rect>(0, 555, 0, 555, 0, red));
        objects.add(make_shared<xz_rect>(113, 443, 127, 432, 554, light));
        objects.add(make_shared<xz_rect>(0, 555, 0, 555, 555, white));
        objects.add(make_shared<xz_rect>(0, 555, 0, 555, 0, white));
        objects.add(make_shared<xy_rect>(0, 555, 0, 555, 555, white));

        shared_ptr<hittable> box1 = make_shared<box>(point3(0,0,0), point3(165,330,165), white);
        box1 = make_shared<rotate_y>(box1, 15);
        box1 = make_shared<translate>(box1, vec3(265,0,295));

        shared_ptr<hittable> box2 = make_shared<box>(point3(0,0,0), point3(165,165,165), white);
        box2 = make_shared<rotate_y>(box2, -18);
        box2 = make_shared<translate>(box2, vec3(130,0,65));

        objects.add(make_shared<constant_medium>(box1, 0.01, color(0,0,0)));
        objects.add(make_shared<constant_medium>(box2, 0.01, color(1,1,1)));

        return objects;
    }

    ...

    int main() {
        ...
        switch (0) {
            ...
            default:
            case 7:
                world = cornell_smoke();
                aspect_ratio = 1.0;
                image_width = 600;
                samples_per_pixel = 200;
                lookfrom = point3(278, 278, -800);
                lookat = point3(278, 278, 0);
                vfov = 40.0;
                break;
        ...
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [cornell-smoke]: <kbd>[main.cc]</kbd> Cornell box, with smoke]
</div>

<div class='together'>
We get:

  ![Image 21: Cornell box with blocks of smoke](../images/img-2.21-cornell-smoke.png class=pixel)

</div>



A Scene Testing All New Features
====================================================================================================

Let’s put it all together, with a big thin mist covering everything, and a blue subsurface
reflection sphere (we didn’t implement that explicitly, but a volume inside a dielectric is what a
subsurface material is). The biggest limitation left in the renderer is no shadow rays, but that is
why we get caustics and subsurface for free. It’s a double-edged design decision.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
    ...
    #include "bvh.h"
    ...

    hittable_list final_scene() {
        hittable_list boxes1;
        auto ground = make_shared<lambertian>(color(0.48, 0.83, 0.53));

        const int boxes_per_side = 20;
        for (int i = 0; i < boxes_per_side; i++) {
            for (int j = 0; j < boxes_per_side; j++) {
                auto w = 100.0;
                auto x0 = -1000.0 + i*w;
                auto z0 = -1000.0 + j*w;
                auto y0 = 0.0;
                auto x1 = x0 + w;
                auto y1 = random_double(1,101);
                auto z1 = z0 + w;

                boxes1.add(make_shared<box>(point3(x0,y0,z0), point3(x1,y1,z1), ground));
            }
        }

        hittable_list objects;

        objects.add(make_shared<bvh_node>(boxes1, 0, 1));

        auto light = make_shared<diffuse_light>(color(7, 7, 7));
        objects.add(make_shared<xz_rect>(123, 423, 147, 412, 554, light));

        auto center1 = point3(400, 400, 200);
        auto center2 = center1 + vec3(30,0,0);
        auto moving_sphere_material = make_shared<lambertian>(color(0.7, 0.3, 0.1));
        objects.add(make_shared<moving_sphere>(center1, center2, 0, 1, 50, moving_sphere_material));

        objects.add(make_shared<sphere>(point3(260, 150, 45), 50, make_shared<dielectric>(1.5)));
        objects.add(make_shared<sphere>(
            point3(0, 150, 145), 50, make_shared<metal>(color(0.8, 0.8, 0.9), 1.0)
        ));

        auto boundary = make_shared<sphere>(point3(360,150,145), 70, make_shared<dielectric>(1.5));
        objects.add(boundary);
        objects.add(make_shared<constant_medium>(boundary, 0.2, color(0.2, 0.4, 0.9)));
        boundary = make_shared<sphere>(point3(0, 0, 0), 5000, make_shared<dielectric>(1.5));
        objects.add(make_shared<constant_medium>(boundary, .0001, color(1,1,1)));

        auto emat = make_shared<lambertian>(make_shared<image_texture>("earthmap.jpg"));
        objects.add(make_shared<sphere>(point3(400,200,400), 100, emat));
        auto pertext = make_shared<noise_texture>(0.1);
        objects.add(make_shared<sphere>(point3(220,280,300), 80, make_shared<lambertian>(pertext)));

        hittable_list boxes2;
        auto white = make_shared<lambertian>(color(.73, .73, .73));
        int ns = 1000;
        for (int j = 0; j < ns; j++) {
            boxes2.add(make_shared<sphere>(point3::random(0,165), 10, white));
        }

        objects.add(make_shared<translate>(
            make_shared<rotate_y>(
                make_shared<bvh_node>(boxes2, 0.0, 1.0), 15),
                vec3(-100,270,395)
            )
        );

        return objects;
    }

    int main() {
        ...
        switch (0) {
            ...
            default:
            case 8:
                world = final_scene();
                aspect_ratio = 1.0;
                image_width = 800;
                samples_per_pixel = 10000;
                background = color(0,0,0);
                lookfrom = point3(478, 278, -600);
                lookat = point3(278, 278, 0);
                vfov = 40.0;
                break;
        ...
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [Listing [scene-final]: <kbd>[main.cc]</kbd> Final scene]

<div class='together'>
Running it with 10,000 rays per pixel yields:

  ![Image 22: Final scene](../images/img-2.22-book2-final.jpg)

</div>

Now go off and make a really cool image of your own! See https://in1weekend.blogspot.com/ for
pointers to further reading and features, and feel free to email questions, comments, and cool
images to me at ptrshrl@gmail.com.



                               (insert acknowledgments.md.html here)



Citing This Book
====================================================================================================
Consistent citations make it easier to identify the source, location and versions of this work. If
you are citing this book, we ask that you try to use one of the following forms if possible.

Basic Data
-----------
  - **Title (series)**: “Ray Tracing in One Weekend Series”
  - **Title (book)**: “Ray Tracing: The Next Week”
  - **Author**: Peter Shirley
  - **Editors**: Steve Hollasch, Trevor David Black
  - **Version/Edition**: v3.2.3
  - **Date**: 2020-12-07
  - **URL (series)**: https://raytracing.github.io/
  - **URL (book)**: https://raytracing.github.io/books/RayTracingTheNextWeek.html

Snippets
---------

  ### Markdown
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [_Ray Tracing: The Next Week_](https://raytracing.github.io/books/RayTracingTheNextWeek.html)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  ### HTML
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    <a href='https://raytracing.github.io/books/RayTracingTheNextWeek.html'>
        <cite>Ray Tracing: The Next Week</cite>
    </a>
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  ### LaTeX and BibTex
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    ~\cite{Shirley2020RTW2}

    @misc{Shirley2020RTW2,
       title = {Ray Tracing: The Next Week},
       author = {Peter Shirley},
       year = {2020},
       month = {December}
       note = {\small \texttt{https://raytracing.github.io/books/RayTracingTheNextWeek.html}},
       url = {https://raytracing.github.io/books/RayTracingTheNextWeek.html}
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  ### BibLaTeX
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    \usepackage{biblatex}

    ~\cite{Shirley2020RTW2}

    @online{Shirley2020RTW2,
       title = {Ray Tracing: The Next Week},
       author = {Peter Shirley},
       year = {2020},
       month = {December}
       url = {https://raytracing.github.io/books/RayTracingTheNextWeek.html}
    }
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  ### IEEE
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    “Ray Tracing: The Next Week.” raytracing.github.io/books/RayTracingTheNextWeek.html
    (accessed MMM. DD, YYYY)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  ### MLA:
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Ray Tracing: The Next Week. raytracing.github.io/books/RayTracingTheNextWeek.html
    Accessed DD MMM. YYYY.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



[stb_image]:          https://github.com/nothings/stb
[Peter Shirley]:      https://github.com/petershirley
[Steve Hollasch]:     https://github.com/hollasch
[Trevor David Black]: https://github.com/trevordblack



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