/ gridhopping Public
Switch branches/tags
Nothing to show

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?

Cannot retrieve contributors at this time
430 lines (344 sloc) 11.2 KB

The LambdaCAD tool comes with a relatively large pool of built-in funcitonality for generating and combining primitives, and basic operations on shapes, such as scaling and rotation. The following examples demonstrate some of these capabilities.

## Basic operations

An ellipse can be obtained be elongating (stretching) a sphere:

``````return scale(sphere(0.3), 1, 0.5, 1);
``````

A caped cone can be obtained by intersecting a cone and a half-space:

``````const c = cone(0.2, 0.4);
const p = plane([0, 0, 0.2], [0, 0, 0.2]);
return intersection(p, c);
``````

You can also combine several capsules (3D lines) with a union operation to obtain a custom shape:

``````// a line-based tetrahedron
const t = 0.02; // line thickness
const c = 0.4;  // coordinate helper
const l1 = capsule([+c, +c, +c], [+c, -c, -c], t);
const l2 = capsule([+c, +c, +c], [-c, +c, -c], t);
const l3 = capsule([+c, +c, +c], [-c, -c, +c], t);
const l4 = capsule([-c, -c, +c], [-c, +c, -c], t);
const l5 = capsule([-c, -c, +c], [+c, -c, -c], t);
const l6 = capsule([+c, -c, -c], [-c, +c, -c], t);
return union(l1, l2, l3, l4, l5, l6);
``````
``````const a=0.45, t=0.025;
const p = [
[-a, -a, -a], [+a, -a, -a], [+a, +a, -a], [-a, +a, -a],
[-a, -a, +a], [+a, -a, +a], [+a, +a, +a], [-a, +a, +a]
];
const lines = [
capsule(p[0], p[1], t), capsule(p[1], p[2], t), capsule(p[2], p[3], t), capsule(p[3], p[0], t),
capsule(p[4], p[5], t), capsule(p[5], p[6], t), capsule(p[6], p[7], t), capsule(p[7], p[4], t),
capsule(p[0], p[4], t), capsule(p[2], p[6], t), capsule(p[3], p[7], t), capsule(p[1], p[5], t)
];
const skeleton = union(lines);
return skeleton;
``````

To carve the interior of a shape, use the `shell` operation (we also subtract a half-space to see the interior):

``````a = shell(torus(0.3, 0.05), 0.01);
b = shell(shell(sphere(0.15), 0.02), 0.01);
p = plane([0, 0, 0], [1, 0, -1]);
r = difference(union(a, b), p);
return r;
``````

This is a Steinmetz tricylinder:

``````const r=0.25, h=1.0;
const c1 = cylinder(r, h);
const c2 = rotate(c1, [1, 0, 0], 3.14156/2.0);
const c3 = rotate(c1, [0, 1, 0], 3.14156/2.0);
return intersection(c1, c2, c3);
``````

And here is a sphericon:

``````const a=0.5, pi = 3.14156;
const bicone = union(cone(a, a), rotate(cone(a, a), [1, 0, 0], pi));
const slice = intersection(bicone, plane([0, 0, 0], [1, 0, 0]));
return union(slice, rotate(rotate(slice, [0, 1, 0], pi), [1, 0, 0], pi/2.0));
``````

## Writing custom signed distance bounds

While we can make interesting shapes by combining the basic operations and primitives from the previous section, sometimes it is more desirable to write a custom distance bound. The reason could be speed as it can be faster to evaluate a custom bound than a combined one. The other reason is that some shapes are difficult to obtain by combining simpler shapes.

Under the hood, LambdaCAD requires a function `f` that takes a 3D position of a point as an argument and computes a distance bound to its associated shape. The basic primitives and operations do just that in a functional way. However, you can explicitly write the distance bounds yourself. Some examples follow.

A kind of a cone with a parabolic mantle:

``````return function (x, y, z) {
const r0 = 0.5;
const h0 = 0.5;
const r = Math.sqrt(x*x + y*y);
const d = h0*(r-r0)*(r-r0)/(r0*r0) - z;

return Math.max(Math.max(-z, -d/(4*h0/r0)), r-r0);
};
``````

A kind of a cone with a parabolic mantle:

``````return function (x, y, z) {
const r = Math.sqrt(x*x + y*y);
const hyp = (r*r/0.05 - z*z/0.05 - 1.0)/20; // a rotated hyperbola
const cyl = Math.max(Math.max(r - 0.3, -(-z + 0.2)), -(+z + 0.2)); // a cylinder

return Math.max(cyl, hyp);
};
``````

A revolved vesica:

``````function vesica3d(r, d) {
const b = Math.sqrt(r*r - d*d);
return function (x, y, z) {
x = Math.sqrt(x*x + z*z);
y = Math.abs(y);
if ((y-b)*d > x*b)
return Math.sqrt(x*x + (y-b)*(y-b));
else
return Math.sqrt((x+d)*(x+d) + y*y) - r;
};
}

return vesica3d(0.6, 0.4);
``````

A solid of constant width obtained by revolving a Reuleaux triangle around an axis of symmetry:

``````return function (x, y, z) {
z = z + 0.25;
const c = 0.4; // coordinate helper
const r = Math.sqrt(x*x + y*y);
const s1 = Math.sqrt((r-c)*(r-c) + z*z) - 2*c;
const s2 = Math.sqrt((r+c)*(r+c) + z*z) - 2*c;
const s3 = Math.sqrt(r*r + (z-Math.sqrt(3)*c)*(z-Math.sqrt(3)*c)) - 2*c;

return Math.max(s1, Math.max(s2, s3));
};
``````
``````function rot2d(x, y, a)
{
const newx = Math.cos(a)*x + Math.sin(a)*y;
const newy = -Math.sin(a)*x + Math.cos(a)*y;

return [newx, newy];
}
return function (x, y, z)
{
const a = Math.atan2(z, x);
[x, z] = rot2d(x, z, a);
x = x - 0.45;
[x, y] = rot2d(x, y, 0.5*a);

return (Math.abs(x) + Math.abs(y) - 0.05)/2;
};
``````

A surface of genus 2:

``````return function (x, y, z)
{
x=x*4; y=y*4; z=z*4; // rescale to fit 1x1x1 box
const r = Math.max(Math.abs(x), Math.abs(y), Math.abs(z));
const L = 10 + r*50; // a dynamic Lipschitz constant

return (2*y*(y*y-3*x*x)*(1.0-z*z) + (x*x + y*y)*(x*x + y*y) - (9*z*z - 1.0)*(1.0-z*z))/L/4;
};
``````

A surface obtained by rotating a teardrop curve:

``````return function (x, y, z) {
x=3*x; y=3*y; z=3*z; // rescale to fit 1x1x1 box
const r = Math.sqrt(x*x + y*y);
const L = 1.0 + 75*Math.abs(r);

return ( 4*r*r - (1-z)*(1-z)*(1-z)*(1+z) )/L;
}
``````

A gyroid.

``````return function (x, y, z) {
x = x*10; y=y*10; z=z*10; // rescale to fit box
const L=2;

return (Math.sin(x)*Math.cos(y) + Math.sin(y)*Math.cos(z) + Math.sin(z)*Math.cos(x))/L/10;
}
``````

A heart, see this video for explanation:

``````return function (x, y, z) {
const r = 0.4;
y = y - 3.5;
z = z*(2 - y/15);
y = 4 + 1.2*y + Math.abs(x)*Math.sqrt((20 - Math.abs(x))/15)
const L = 3;
return (Math.sqrt(x*x + y*y + z*z) - r)/L;
};
``````
``````return function (x, y, z) {
let n = 0;
const scale=2.0;

while (n < 9)
{
let tmp;
if(x + y < 0) {tmp=x; x=-y; y=-tmp;}
if(x + z < 0) {tmp=x; x=-z; z=-tmp;}
if(y + z < 0) {tmp=y; y=-z; z=-tmp;}
x = scale*x - 0.4*(scale-1.0);
y = scale*y - 0.4*(scale-1.0);
z = scale*z - 0.4*(scale-1.0);
n++;
}

return Math.sqrt(x*x + y*y + z*z)*Math.pow(scale, -n) - 0.0075;
};
``````

Some kind of a knot. See here and here for details:

``````return function (x, y, z) {
const scale = 6;
x *= scale;
y *= scale;
z *= scale;

const r = Math.sqrt(x*x + y*y + z*z);
const phi = Math.atan2(y, x);
const r1 = Math.sin(1.5 * phi) + 1.5;
const z1 = Math.cos(1.5 * phi);
const r2 = Math.sin(1.5 * phi + 3.14156) + 1.5;
const z2 = Math.cos(1.5 * phi + 3.14156);

const x1=r1*Math.cos(phi), y1=r1*Math.sin(phi);
const x2=r2*Math.cos(phi), y2=r2*Math.sin(phi);

return (Math.min(
Math.sqrt((x-x1)*(x-x1)+(y-y1)*(y-y1)+(z-z1)*(z-z1)),
Math.sqrt((x-x2)*(x-x2)+(y-y2)*(y-y2)+(z-z2)*(z-z2))) - 0.3)/3.0/scale;
};
``````

Another knot:

``````// params

function twist(x, y, z) {
const ra = z*-2/3;
const raz = z*-4/3;
return Math.sqrt(u*u + v*v) - tuberad;
}

function knot(x, y, z) {
y = y - 0.5; // translate to origin
const r = Math.sqrt(x*x + z*z);
const a = Math.atan2(z, x);
let d = 1e6;
for (let i=0; i<3; ++i)
d = Math.min(twist(r-objrad, y, a + 2*3.14156*i), d);

// modulation is necessary to ensure that the estimator does not overestimate distances
// too small value breaks marching cubes, too big and raymarching passes through the surface without detecting it
const m = Math.min(0.5, 2.375*r + 0.025)

return m*d;
}

// the cylinder (capsule) in the middle of the structure masks the singularity happening there
// this area is difficult to handle with marching cubes because the structure there is too thin
return union(knot, capsule([0, -0.1, 0], [0, +0.3, 0], 0.035));
``````

A Menger sponge (takes around 10 seconds to generate):

``````function mod(a, b) {
if (a<0) a=-a;
return a%b;
}

function column(x, y) {
x = Math.abs(x)-0.5;
y = Math.abs(y)-0.5;
return Math.sqrt(Math.max(x, 0)*Math.max(x, 0) + Math.max(y, 0)*Math.max(y, 0)) + Math.min(Math.max(x, y), 0);
}

function lattice(x, y, z) {
const c = 3.0;

x = x - c/2.0;
y = y - c/2.0;
z = z - c/2.0;

// domain repetition
x = mod(x, c) - c/2.0;
y = mod(y, c) - c/2.0;
z = mod(z, c) - c/2.0;

return Math.min(column(x, y), Math.min(column(x, z), column(y, z)));
};

let sponge = cuboid(1, 1, 1);
for (let i=1; i<4; ++i)
sponge = difference(sponge, scale(lattice, Math.pow(3, -i), Math.pow(3, -i), Math.pow(3, -i)));

return scale(sponge, 0.75, 0.75, 0.75);
``````

## Extruding 2D shapes

Interesting 3D objects can be obtained by extruding (adding a 3rd dimension) 2D shapes. You can check https://www.iquilezles.org/www/articles/distfunctions2d/distfunctions2d.htm for more 2D signed distance bounds. Some examples follow.

``````function circle(r) {
return function (x, y) {
return Math.sqrt(x*x + y*y) - r;
};
}

return extrusion(circle(0.25), 0.5);
``````
``````function line2d(p1, p2, r) {
const [xa, ya] = p1;
const [xb, yb] = p2;
return function (x, y) {
const pa = [x - xa, y - ya];
const ba = [xb - xa, yb - ya];
let h = (pa[0]*ba[0] + pa[1]*ba[1])/(ba[0]*ba[0] + ba[1]*ba[1]);
h = Math.max(0.0, Math.min(1.0, h));
const d = [pa[0] - h*ba[0], pa[1] - h*ba[1]];
return Math.sqrt(d[0]*d[0] + d[1]*d[1]) - r;
};
}

const l = line2d(
[-0.45, -0.45],
[0.45, 0.45],
0.01
);

return extrusion(l, 0.5);
``````
``````// https://en.wikipedia.org/wiki/Lemniscate_of_Gerono
function lemn(x, y) {
x*=3; y*=3;
return (x*x*x*x - x*x + y*y)/3;
}

return extrusion(lemn, 0.5);
``````
``````// an equilateral triangle
function triangle(x, y)
{
const s = 3;
[x, y] = [Math.abs(s*x) - 1.0, s*y + 1.0/Math.sqrt(3)];
if(x + Math.sqrt(3)*y > 0)
[x, y] = [
(x - Math.sqrt(3)*y)/2.0,
(-Math.sqrt(3)*x - y)/2.0
]
x -= Math.max(-2.0, Math.min(0.0, x))

if (y > 0)
return -Math.sqrt(x*x + y*y)/s;
else
return +Math.sqrt(x*x + y*y)/s;
}

return extrusion(triangle, 0.5);
``````
``````// https://www.gamedev.net/blogs/entry/2250902-logarithmic-spiral-distance-field/
function logspi(a, b, R)
{
return function (x, y) {
const pi = 3.14156;
const r = Math.sqrt(x*x + y*y);
const t = Math.atan2(y, x) + pi;

const n = (Math.log(r/a)/b - t)/(2*pi);
const n1 = Math.floor(n);
const n2 = Math.ceil(n);

const r1 = a*Math.exp(b*(t + 2*pi*n1));
const r2 = a*Math.exp(b*(t + 2*pi*n2));

return Math.abs(Math.min(r2-r, r-r1)) - R;
}
}

return extrusion(logspi(0.1, 0.1, 0.01), 0.1);
``````