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);