Light shafts

Light shafts are seen in nature when a light soure like the sun is occluded by an object like a tree. We can then observe some rays spreading from the occluder which are in fact volumetric shadows in the atmosphere. This effect can be implemented in games as a post-process, which makes it easy to add into a 3D engine. Here is a screenshot of my implementation:

I am still working on this one, there is still minor tweaks required to enhance both quality and performances, and I will post it here as soon as I am done with it. I will then post a bit more explanation on how it is done. 

Preliminary version of the code:

[Vertex_Shader]

varying vec3 lightPos;

void main(void)

{

   vec4 v = (gl_ProjectionMatrix * gl_ModelViewMatrix * 

      gl_LightSource[0].position);

   lightPos = v.xyz / v.w;

   gl_Position = ftransform();

   gl_TexCoord[0] = gl_MultiTexCoord0;

}

[Pixel_Shader]

uniform sampler2D sceneTex;

varying vec3 lightPos;

void main()

{ 

   int NUM_SAMPLES = 100;

   float density = 0.9;

   float illuminationDecay = 1.0;

   float weight = 0.010;

   float decay = 0.99;

 

   vec2 uv = gl_TexCoord[0].xy;

   lightPos.xy = (lightPos.xy + vec2(1.0, 1.0)) / 2.0;

   vec2 deltaTexCoord = uv - lightPos.xy;

 

   deltaTexCoord = deltaTexCoord / (float(NUM_SAMPLES) * density);

     

   vec3 color = texture2D(sceneTex, uv).xyz;

 

   for(int i=0; i<NUM_SAMPLES; ++i)

   {

      uv -= deltaTexCoord;

      vec3 sample = texture2D(sceneTex, uv).xyz;

      sample = sample * illuminationDecay * weight;

      color += sample;

      illuminationDecay = illuminationDecay * decay;

   }

 

   gl_FragColor = vec4(color, 1.0);

}

Don't hesitate to post any comment, advice or improvement ideas!