Add various comment on filtering + change from _variance to _NA for suffix

com.unity.render-pipelines.core/CoreRP/ShaderLibrary/CommonMaterial.hlsl

     roughnessB = ClampRoughnessForAnalyticalLights(roughnessB);
 }
 
-// Use with stack BRDF (clear coat / coat)
+// Use with stack BRDF (clear coat / coat) - This only used same equation to convert from Blinn-Phong spec power to Beckmann roughness
 real RoughnessToVariance(real roughness)
 {
     return 2.0 / Sq(roughness) - 2.0;
 float FilterPerceptualSmoothness(float perceptualSmoothness, float variance, float threshold)
 {
     float roughness = PerceptualSmoothnessToRoughness(perceptualSmoothness);
+    // Ref: Geometry into Shading - http://graphics.pixar.com/library/BumpRoughness/paper.pdf - equation (3)
     float squaredRoughness = saturate(roughness * roughness + min(2.0 * variance, threshold * threshold)); // threshold can be really low, square the value for easier control
 
     return 1.0 - RoughnessToPerceptualRoughness(sqrt(squaredRoughness));
         float avgNormLen2 = avgNormalLength * avgNormalLength;
         float kappa = (3.0 * avgNormalLength - avgNormalLength * avgNormLen2) / (1.0 - avgNormLen2);
 
+        // Ref: Frequency Domain Normal Map Filtering - http://www.cs.columbia.edu/cg/normalmap/normalmap.pdf (equation 21)
+        // Relationship between between the standard deviation of a Gaussian distribution and the roughness parameter of a Beckmann distribution.
+        // is roughness^2 = 2 variance    (note: variance is sigma^2)
+        // (Ref: Filtering Distributions of Normals for Shading Antialiasing - Equation just after (14))
-        // so to get variance we must use variance = 1 / (4 * kappa)
+        // (Equation 36 of  Normal map filtering based on The Order : 1886 SIGGRAPH course notes implementation).
+        // So to get variance we must use variance = 1 / (4 * kappa)
         return 0.25 * kappa;
     }
 

com.unity.render-pipelines.high-definition/HDRP/Editor/AssetPostProcessors/NormalMapVarianceTexturePostprocessor.cs

 
 public class NormalMapVarianceTexturePostprocessor : AssetPostprocessor
 {
+    static string s_Extension = "_NA";
-        if (assetPath.IndexOf("_variance", StringComparison.InvariantCultureIgnoreCase) != -1)
+        // Any texture with _NA suffix will store average normal lenght in alpha
+        if (assetPath.IndexOf("_NA", StringComparison.InvariantCultureIgnoreCase) != -1)
         {
             // Make sure we don't convert as a normal map.
             TextureImporter textureImporter = (TextureImporter)assetImporter;
 
     void OnPostprocessTexture(Texture2D texture)
     {
-        if (assetPath.IndexOf("_variance", StringComparison.InvariantCultureIgnoreCase) != -1)
+        if (assetPath.IndexOf("_NA", StringComparison.InvariantCultureIgnoreCase) != -1)
         {
             // For mip 0, set the normal length to 1.
             {