godrays.glsl

vec4 godrays(float density, float weight, float decay, float exposure,
  int numSamples, sampler2D occlusionTexture, vec2 sunPositionScreenSpace,
  vec2 uv)
{
  vec4 color = vec4(0.0);
  vec2 delta = (sunPositionScreenSpace - uv) / float(numSamples) * density;
  float intensity = 1.0;
  for (int i = 0; i < 1337; ++i) {
    if (i >= numSamples) break;
    uv += delta;
    color += texture2D(occlusionTexture, uv) * intensity * weight;
    intensity *= decay;
  }
  color *= exposure;
  return color;
}

varying vec2 vertexUv;
uniform sampler2D textureSampler;
uniform vec2 sunPositionScreenSpace;

void main() {
  gl_FragColor = godrays(1.0, 0.005, 1.0, 1.0, 256, textureSampler,
    sunPositionScreenSpace, vertexUv);
}