art with code

2013-12-26

Opus 2, GLSL ray tracing tutorial

A bit older shader on fhtr.net this time, from the time I was getting started with ray tracing. This one's path tracing spheres and triangles with motion blur. Might even work on Windows this time, but YMMV. I should get a test machine for that.

"So", I hear you ask, "how does that work?" I'm glad you asked! It's a common Shadertoy-style WebGL shader where two triangles and a trivial vertex shader are coupled with a delightfully insane fragment shader. In this case, the fragment shader takes the current time as its input and uses it to compute the positions of a few objects and the color of the background. The shader then shoots a couple of rays through the scene, bouncing them off the objects to figure out the color of the current pixel. And yes, it does all this on every single pixel.

Path tracing

The path tracing part is pretty simple. The shader shoots out a ray and tests it against the scene. On detecting a hit, the ray is reflected and tested again. If the ray doesn't hit anything, I set its color to the background color. If the ray does hit something and gets reflected into the background, I multiply the object color with the background color. If the reflected ray hits another object, I give up and leave the color black. To make this one-bounce lighting look less harsh, I mix the ray color with the fog color based on the distance traveled by the ray. For the fog color, I'm using the background color so that the objects blend in well with the bg.

Digression: The nice thing about ray tracing -based rendering is that a lot of the things that are difficult and hacky to do with rasterization suddenly become simple. Reflections, refractions, transparent meshes, shadows, focus blur, motion blur, adaptive anti-aliasing schemes, all are pretty much "shoot an extra ray, add up the light from the two rays, done!" It just gets slowwwer the more rays you trace.

And if you're doing path tracing, you're going to need a whole lot of rays. The idea behind path tracing is to shoot a ray into the scene and randomly bounce it around until it becomes opaque enough that further hits with light sources wouldn't contribute to the pixel color. By summing up enough of these random ray paths, you arrive at a decent approximation of the light arriving to the pixel through all the different paths in the scene.

For specular materials, you can get away with a relatively small amount of rays, as the variance between ray paths is very small. A mirror-like surface is going to reflect the ray along its normal, so any two rays are going to behave pretty much the same. Throw in diffuse materials, and you're in a whole new world of hurt. A fully diffuse surface can reflect a ray into any direction visible from it, so you're going to need to trace a whole lot of paths to approximate the light hitting a diffuse surface.

Motion blur

The motion blur in the shader is very simple. Grab a random number from a texture, multiply the frame exposure time with that, add this time delta to the current time and trace! Now every ray is jittered in time between the current frame and the next frame. That alone gets you motion blur, albeit in a very noisy fashion.

I'm using two time-jittered rays per pixel in the shader, first one at a random time in the first half of the exposure time, the second in the second half. Then I add them together and divide by two to get the final motion blurred color. It looks quite a bit better without totally crashing the frame rate. For high-quality motion blur, you can bump the ray count to a hundred or so and revel in your 0.4 fps.

Background color

I made the background by adding up a bunch of gradients based on the sun position and the plane of the horizon. The gist of the technique is to take the dot product between two directions and multiply the gradient color with that. Quick explanation: the dot of two directions (or unit vectors) is the cosine of the angle between them, its value varies from 1 at zero angle to 0 at a 90 degree angle and -1 at 180 degrees. To make a nice diffuse sun, take the dot between the sun direction and the ray direction, clamp it to 0..1-range, raise it to some power to tighten up the highlight and multiply with the sun color. Done! In code: bgCol += pow(max(0.0, dot(sunDir, rayDir)), 256.0)*sunColor;

You can also use this technique to make gradients that go from the horizon to the zenith. Instead of using the sun direction, you use the up vector. Again, super simple: bgCol += pow(max(0.0, dot(vec3(0.0, 1.0, 0.0), rayDir)), 2.0)*skyColor;

By mixing a couple of these gradients you can get very nice results. Say, use low-pow sun gradient to make haze, high-pow for the sun disk, a horizon-up gradient for the skydome, horizon-down gradient for the ground and a reversed high-pow horizon gradient to add horizon glow (like in the shader in the previous post).

Let's write a path tracer!

Here's a small walk-through of a path tracer like this: First, normalize the coordinates of the current pixel to -1..1 range and scale them by the aspect ratio of the canvas so that we get square pixels. Second, set up the current ray position and direction. Third, shoot out the ray and test for intersections. If we have a hit, multiply the ray color with the hit color, reflect the ray off the surface normal and shoot it out again.

vec2 uv = (-1.0 + 2.0*gl_FragCoord.xy / iResolution.xy) * vec2(iResolution.x/iResolution.y, 1.0);
vec3 ro = vec3(0.0, 0.0, -6.0);     // Ray origin.
vec3 rd = normalize(vec3(uv, 1.0)); // Ray direction.
vec3 transmit = vec3(1.0);          // How much light the ray lets through.
vec3 light = vec3(0.0);             // How much light hits the eye through the ray.

float epsilon = 0.001;

float bounce_count = 2.0; // How many rays we trace.

for (int i=0; i<bounce_count; i++) {
  float dist = intersect(ro, rd);
  if (dist > 0.0) { // Object hit.
    transmit *= material(ro, rd); // Make the ray more opaque.
    vec3 nml = normal(ro, rd);    // Get surface normal for reflecting the ray.
    ro += rd*dist;                // Move the ray to the hit point.
    rd = reflect(rd, nml);        // Reflect the ray.
    ro += rd*epsilon;             // Move the ray off the surface to avoid hitting the same point twice.
  } else { // Background hit.
    light += transmit * background(rd); // Put the background light through the ray and add it to the light seen by the eye.
    break;                              // Don't bounce off the background.
  }
}

gl_FragColor = vec4(light, 1.0); // Set pixel color to the amount of light seen.

Spheres!

Ok, let's get real! Time to trace two spheres! Spheres are pretty easy to trace. A point is on the surface of a sphere if (point-center)^2 = r^2. A point is on a ray (o, d) if point = o+d*t | t > 0. By combining these two equations, we get ((o+d*t)-center)^2 = r^2 | t > 0, which we then have to solve for t. First, let's write it out and shuffle the terms a bit.

(o + d*t - c) • (o + d*t - c) = r^2

o•o + o•dt - o•c + o•dt + dt•dt - c•dt - c•o - c•dt + c•c = r^2

(d•d)t^2 + (2*o•dt - 2*c•dt) + o•o - 2o•c + c•c - r^2 = 0

(d•d)t^2 + 2*(o-c)•dt + (o-c)•(o-c) - r^2 = 0

Ok, that looks better. Now we can use the formula for solving quadratic equation and go on our merry way: for Ax^2 + Bx + C = 0, x = (-B +- sqrt(B^2-4AC)) / 2A. From the above equation, we get

A = (d•d)t^2   // We can optimize this to t^2 if the direction d is a unit vector.
               // To explain, first remember that a•b = length(a)*length(b)*cos(angleBetween(a,b))
               // Now if we set both vectors to be the same:
               // a•a = length(a)^2 * cos(0) = length(a)^2, as cos(0) = 1
               // And for unit vectors, that simplifies to u•u = 1^2 = 1
B = 2*(o-c)•d
C = (o-c)•(o-c) - r^2

And solve

D = B*B - 4*A*C
if (D < 0) {
  return No solution;
}
t = -B - sqrt(D);
if (t < 0) { // Closest intersection behind the ray
  t += 2*sqrt(D); // t = -B + sqrt(D)
}
if (t < 0) {
  return Sphere is behind the ray.
}
return Distance to sphere is t.

In GLSL, it's five lines of math and a comparison in the end.

float rayIntersectsSphere(vec3 ray, vec3 dir, vec3 center, float radius, float closestHit)
{
  vec3 rc = ray-center;
  float c = dot(rc, rc) - (radius*radius);
  float b = dot(dir, rc);
  float d = b*b - c;
  float t = -b - sqrt(abs(d));
  if (d < 0.0 || t < 0.0 || t > closestHit) {
    return closestHit; // Didn't hit, or wasn't the closest hit.
  } else {
    return t;
  }
}

Demos

Right, enough theory for now, I think. Here's a minimal ray tracer using the ideas above.

float sphere(vec3 ray, vec3 dir, vec3 center, float radius)
{
 vec3 rc = ray-center;
 float c = dot(rc, rc) - (radius*radius);
 float b = dot(dir, rc);
 float d = b*b - c;
 float t = -b - sqrt(abs(d));
 float st = step(0.0, min(t,d));
 return mix(-1.0, t, st);
}

vec3 background(float t, vec3 rd)
{
 vec3 light = normalize(vec3(sin(t), 0.6, cos(t)));
 float sun = max(0.0, dot(rd, light));
 float sky = max(0.0, dot(rd, vec3(0.0, 1.0, 0.0)));
 float ground = max(0.0, -dot(rd, vec3(0.0, 1.0, 0.0)));
 return 
  (pow(sun, 256.0)+0.2*pow(sun, 2.0))*vec3(2.0, 1.6, 1.0) +
  pow(ground, 0.5)*vec3(0.4, 0.3, 0.2) +
  pow(sky, 1.0)*vec3(0.5, 0.6, 0.7);
}

void main(void)
{
 vec2 uv = (-1.0 + 2.0*gl_FragCoord.xy / iResolution.xy) * 
  vec2(iResolution.x/iResolution.y, 1.0);
 vec3 ro = vec3(0.0, 0.0, -3.0);
 vec3 rd = normalize(vec3(uv, 1.0));
 vec3 p = vec3(0.0, 0.0, 0.0);
 float t = sphere(ro, rd, p, 1.0);
 vec3 nml = normalize(p - (ro+rd*t));
 vec3 bgCol = background(iGlobalTime, rd);
 rd = reflect(rd, nml);
 vec3 col = background(iGlobalTime, rd) * vec3(0.9, 0.8, 1.0);
 gl_FragColor = vec4( mix(bgCol, col, step(0.0, t)), 1.0 );
}


Demo of the minimal ray tracer.

I also made a version that traces three spheres with motion blur. You can check out the source on Shadertoy.


Demo of a motion blur tracer.

Conclusion

If you got all the way down here, well done! I hope I managed to shed some light on the mysterious negaworld of writing crazy fragment shaders. Do try and write your own, it's good fun.

Simple ray tracers are fun and easy to write and you can get 60 FPS on simple scenes, even on integrated graphics. Of course, if you want to render something more complex than a couple of spheres, you're probably in a world of pain. I should give it a try...

P.S. I put together a bunch of screenshots from my ray tracing shaders to make a project pitch deck. You can check out the PDF here. The first couple screenies have decals and clouds painted in PS, the rest are straight from real-time demos.

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Built art installations, web sites, graphics libraries, web browsers, mobile apps, desktop apps, media player themes, many nutty prototypes, much bad code, much bad art.

Have freelanced for Verizon, Google, Mozilla, Warner Bros, Sony Pictures, Yahoo!, Microsoft, Valve Software, TDK Electronics.

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