I've got Variance Shadow Maps working in GLSL based on the code in GPUGems3. For example, my code for calculating the moments when generating the shadow map is:
vec2 ComputeMoments(float depth)
moments.x = depth;
moments.y = depth * depth;
float dx = dFdx(depth);
float dy = dFdy(depth);
moments.y += 0.25 * (dx * dx + dy * dy);
It works great on my nVidia card but the shader does not compile on ATI cards (Radeon HD 3670 and 3870) with the latest drivers returning the errors:
"error(#202) No matching overloaded function found dFdx"
"error(#202) No matching overloaded function found dFdy"
Can I get this to compile on these cards (maybe by enabling an extension) or are the derivative functions just not available on older ATI cards? Or is there a way to calculate the derivatives manually?