Pixel Shader
float  distance;
float3 offset   = normalize(v2f.tanEyeVec);
float3 texCoord = v2f.texCoord;
// March the ray
for (int i = 0; i < NUM_ITERATIONS; i++)
{
   distance  = tex3D(distanceTex, texCoord).x;
   texCoord += distance * offset;
}