Using textureGrad for anisotropic integration approximation
- by Amxx
I'm trying to develop a real time rendering method using real time acquired envmap (cubemap) for lightning.
This implies that my envmap can change as often as every frame and I therefore cannot use any method base on precomputation of the envmap (such as convolution with BRDF...)
So far my method worked well with Phong BRDF. For specular contribution I direclty read the value in my sampleCube and I use mipmap levels + linear filter for simulating the roughtness of the material considered:
int size = textureSize(envmap, 0).x;
float specular_level = log2(size * sqrt(3.0)) - 0.5 * log2(ns + 1);
vec3 env_specular = ks * specular_color * textureLod(envmap, l_g, specular_level);
From this method I would like to upgrade to a microfacet based BRDF. I already have algorithm for evaluating the shape (including anisotropic direction) of the reflection but I cannot manage to read the values I want in my sampleCube.
I believe I have to use
textureGrad(envmap, l_g, X, Y);
with l_g being the reflection direction in global space but I cannot manage to find which values to give to X and Y in order to correctly specify the area I want to consider.
What value should I give to X and Y in orther for
textureGrad(envmap, l_g, X, Y);
to give the same result as
textureLod(envmap, l_g, specular_level);
