Improve comments

Assets/ScriptableRenderLoop/HDRenderPipeline/Material/Lit/Lit.hlsl

 
     // We need to take into account the modified normal for faking anisotropic here.
     float3 iblR = reflect(-V, iblNormalWS);
-    preLightData.iblDirWS = GetSpecularDominantDir(bsdfData.normalWS, iblR, bsdfData.roughness);
+    preLightData.iblDirWS = GetSpecularDominantDir(bsdfData.normalWS, iblR, bsdfData.roughness, preLightData.NdotV);
 
     // Area light specific
     // UVs for sampling the LUTs

Assets/ScriptableRenderLoop/ShaderLibrary/ImageBasedLighting.hlsl

 // Util image based lighting
 //-----------------------------------------------------------------------------
 
-// The *approximated* version of the non-linear remapping which improves
-// the perceptual roughness distribution and adds reflection (contact) hardening.
+// The *approximated* version of the non-linear remapping. It works by
+// approximating the cone of the specular lobe, and then computing the MIP map level
+// which (approximately) covers the footprint of the lobe with a single texel.
+// Improves the perceptual roughness distribution.
 float perceptualRoughnessToMipmapLevel(float perceptualRoughness)
 {
     perceptualRoughness = perceptualRoughness * (1.7 - 0.7 * perceptualRoughness);
 
-// The *accurate* version of the non-linear remapping which improves
-// the perceptual roughness distribution and adds reflection (contact) hardening.
+// The *accurate* version of the non-linear remapping. It works by
+// approximating the cone of the specular lobe, and then computing the MIP map level
+// which (approximately) covers the footprint of the lobe with a single texel.
+// Improves the perceptual roughness distribution and adds reflection (contact) hardening.
 // TODO: optimize!
 float perceptualRoughnessToMipmapLevel(float perceptualRoughness, float NdotR)
 {
     return perceptualRoughness * UNITY_SPECCUBE_LOD_STEPS;
 }
 
-// The *approximated* version of the non-linear remapping which improves
-// the perceptual roughness distribution and adds reflection (contact) hardening.
+// The inverse of perceptualRoughnessToMipmapLevel().
 float mipmapLevelToPerceptualRoughness(float mipmapLevel)
 {
     float perceptualRoughness = saturate(mipmapLevel / UNITY_SPECCUBE_LOD_STEPS);
 
 // Ref: See "Moving Frostbite to PBR" Listing 22
 // This formulation is for GGX only (with smith joint visibility or regular)
-float3 GetSpecularDominantDir(float3 N, float3 R, float roughness)
+float3 GetSpecularDominantDir(float3 N, float3 R, float roughness, float NdotV)
+    // TODO: we could bias this further (and use 'NdotV') to better match the reference.
     float lerpFactor = a * (sqrt(a) + roughness);
     // The result is not normalized as we fetch in a cubemap
     return lerp(N, R, lerpFactor);
             float omegaS    = rcp(sampleCount) * invPdf;
             // invOmegaP is precomputed on CPU and provide as a parameter of the function
             // float omegaP = FOUR_PI / (6.0f * cubemapWidth * cubemapWidth);
-            mipLevel        = 0.5 * log2(omegaS * invOmegaP);
+            mipLevel        = 0.5 * log2(omegaS * invOmegaP + 1.0);
 
             // Bias the MIP map level to compensate for the importance sampling bias.
             // This will blur the reflection.