v3.2.0 (2020-07-18)

We're still chasing that elusive stable project state where we're mostly done with large changes,
yet we keep finding more and more to tweak and improve. Besides the usual batch of corrections and
small improvements, for this change we plodded through the complete code progression for both books
one and two (In One Weekend and The Next Week). This caught a lot of issues (to our dismay),
and allowed us to generate a complete set of new render images for both books, to catch up with all
of the changes we've been making. The end result is that readers should find a significantly better
agreement between the book and their code as they progress, and their renders should also generally
match.

Besides the new rendered images, we also much improved the image parameters, which were frequently
missing from the previous version, leaving readers to guess at their values, or go to the code to
try to figure out how we created some of the images. In general, our working renders are now 400
pixels wide, usually 16:9 aspect ratio. We now use an explicit aspect ratio and deduce the image
height and other camera values, so you can tweak your render size just by changing the image width
(instead of updating a bunch of dependent parameters).

One interesting late change we made was adding explicit C++ override labels to subclass methods.
We did this mostly to aid code readers, but were surprised to find that it actually caught a pretty
significant bug hiding in our code (see entry in common changes below).

You'll also see a new citation section at the end of the books, to encourage uniform citations out
in the world, making it easier for people to refer to and track these books.

As is typical, though we roughly follow semantic versioning, we're
considering this release a minor change instead of a major one. It's a common reflex, because people
generally have a (misguided) aversion to bumping the major version a lot. We consider it minor
because most of the changes are quite local, some classes get new constructors and any variances
should be quite simple and easy to fix up. Still, one might consider this more properly a major
version bump.

For our next larger-than-patch release, we're beginning a large revisit of book 3,
Ray Tracing: The Rest of Your Life. There's a lot of work to do, and this will likely be a
significant change and improvement. We're hoping that changes to books one and two will be small,
but that's never worked out for us before. Ah, dreams.

Common

• Bug: Found a bug in book 3 source isotropic::scatter() method. Commented out, using default
  (as it was previously). (#669)
• Delete: vestigial vec3::write_color() method (now in color.h)
• Change: All images and figures renamed to follow more logical convention, using the following
  pattern: {fig,img}-<book>.<sequence>-<title>.<filetype> (#495)
• Change: main() function gets organized into image, world, camera, and render chunks
• Change: Added header guards to the text of all three books whenever a new header file was
  introduced, consistent with source code (#645)
• New: Added constructors that take color arguments in addition to the constructors
  taking shared_ptr<texture> arguments, simplifying calling code. Applies to checker_texture,
  constant_medium, diffuse_light, lambertian, and isotropic (#516, #644)
• Change: Added override keywords throughout. This keyword marks a subclass method as one that
  is intended to override a superclass method. It makes the code a bit easier to understand, and
  ensures that your function is actually overriding the method you think it is. Which is good,
  because it already caught an existing bug in The Rest of Your Life source. This change
  includes commenting out the book 3 isotropic::scatter() method, which was accidentally ignored
  anyway. (#639, #669)
• New: each book gets a section of recommended citation examples (#500)

In One Weekend

• Change: Updated all rendered images except for 1.13, 1.14 (#179, #547, #548, #549, #550, #551,
  #552, #553, #554, #555, #556, #557, #560, #561, #562, #563, #564, #565, #566)
• Change: Standard working render width changed to 400 pixels
• Change: Image 6 is now a before-and-after pair to illustrate antialiasing
• Change: Listing 48: Refactored material and geometry declarations
• Change: Listing 52: Refactored assignment of etai_over_etat
• Change: Listing 56: Refactored material declarations
• Change: Listing 61: Refactored material and geometry declarations
• Fix: Corrected various missed change highlights in code listings
• Fix: Listing 7: Added missing color.h, vec3.h includes
• Fix: Listing 18: Add missing double t member of struct hit_record (#428)
• Fix: Listing 24: Add missing color.h include
• Fix: Listing 30: Add missing camera.h include
• Fix: Listing 42: Don't need to include ray.h when using rtweekend.h
• Fix: Listing 48: Add missing material.h include
• Fix: Listing 51: refract() function was missing fabs() on sqrt() argument (#559)
• Fix: Listing 61: Include updated cam declaration, show context w/highlighting
• Fix: Listing 62: Highlight rename of camera::get_ray() parameters to s, t (#616)
• Fix: Listing 63: Show reverted scene declarations
• Fix: Listing 68: Show final scene render parameters with highlighting
• Fix: Rewrote refracted ray perpendicular and parallel components for correctness (#526)
• New: Listing 50: Show the updated material definitions

The Next Week

• Delete: Deleted the section covering the old flip_face class, renumbered images as this
  eliminated the rendering with missing Cornell box faces (#270, #482, #661)
• Delete: scenes 7 & 9 from the original (cornell_balls and cornell_final), as these were not
  covered in the book. Made the source and book consistent with each other. There are now a total
  of eight scenes for the second book (#653, #620)
• Change: Listing 10: Separate out world & camera definitions in main (#646)
• Change: Updated most rendered images for book 2: 2.01-2.03, 2.07-2.13, 2.15-2.22.
• Change: Scenes get custom image parameters (#650)
• Fix: Reduced code duplication in dielectric::scatter() function
• Fix: "Intance" typo in Chapter 8.1 to "Instance" (#629)
• Fix: Listing 7: Show reverted viewing parameters from book 1 final scene
• Fix: Typo in listing caption for filename moving-sphere.h

The Rest of Your Life

• Change: use vup for camera, as in other two books
• Fix: world and camera setup in main(), and include full body in book listing (#646)
• New: flip_face moved to book 3, where it's needed for the light source (#661)