HDRenderPipeline: Add LOD cross fade (need C++ update)

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/LayeredLit/LayeredLit.shader

     #pragma multi_compile LIGHTMAP_OFF LIGHTMAP_ON
     #pragma multi_compile DIRLIGHTMAP_OFF DIRLIGHTMAP_COMBINED
     #pragma multi_compile DYNAMICLIGHTMAP_OFF DYNAMICLIGHTMAP_ON
+    // enable dithering LOD crossfade
+    #pragma multi_compile _ LOD_FADE_CROSSFADE
     // TODO: We should have this keyword only if VelocityInGBuffer is enable, how to do that ?
     //#pragma multi_compile VELOCITYOUTPUT_OFF VELOCITYOUTPUT_ON 
 

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/LayeredLit/LayeredLitTessellation.shader

     #pragma multi_compile LIGHTMAP_OFF LIGHTMAP_ON
     #pragma multi_compile DIRLIGHTMAP_OFF DIRLIGHTMAP_COMBINED
     #pragma multi_compile DYNAMICLIGHTMAP_OFF DYNAMICLIGHTMAP_ON
+    // enable dithering LOD crossfade
+    #pragma multi_compile _ LOD_FADE_CROSSFADE
     // TODO: We should have this keyword only if VelocityInGBuffer is enable, how to do that ?
     //#pragma multi_compile VELOCITYOUTPUT_OFF VELOCITYOUTPUT_ON
 

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.shader

     #pragma multi_compile LIGHTMAP_OFF LIGHTMAP_ON
     #pragma multi_compile DIRLIGHTMAP_OFF DIRLIGHTMAP_COMBINED
     #pragma multi_compile DYNAMICLIGHTMAP_OFF DYNAMICLIGHTMAP_ON
+    // enable dithering LOD crossfade
+    #pragma multi_compile _ LOD_FADE_CROSSFADE
     // TODO: We should have this keyword only if VelocityInGBuffer is enable, how to do that ?
     //#pragma multi_compile VELOCITYOUTPUT_OFF VELOCITYOUTPUT_ON
 

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/LitData.hlsl

 
 void GetSurfaceAndBuiltinData(FragInputs input, float3 V, inout PositionInputs posInput, out SurfaceData surfaceData, out BuiltinData builtinData)
 {
+#ifdef LOD_FADE_CROSSFADE // enable dithering LOD transition if user select CrossFade transition in LOD group
+    LODDitheringTransition(posInput.unPositionSS, unity_LODFade.y); // Note that we pass the quantized value of LOD fade
+#endif
+
     ApplyDoubleSidedFlipOrMirror(input); // Apply double sided flip on the vertex normal
 
     LayerTexCoord layerTexCoord;
 
 void GetSurfaceAndBuiltinData(FragInputs input, float3 V, inout PositionInputs posInput, out SurfaceData surfaceData, out BuiltinData builtinData)
 {
+#ifdef LOD_FADE_CROSSFADE // enable dithering LOD transition if user select CrossFade transition in LOD group
+    LODDitheringTransition(posInput.unPositionSS, unity_LODFade.y); // Note that we pass the quantized value of LOD fade
+#endif
+
     ApplyDoubleSidedFlipOrMirror(input); // Apply double sided flip on the vertex normal
 
     LayerTexCoord layerTexCoord;

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/LitTessellation.shader

     #pragma multi_compile LIGHTMAP_OFF LIGHTMAP_ON
     #pragma multi_compile DIRLIGHTMAP_OFF DIRLIGHTMAP_COMBINED
     #pragma multi_compile DYNAMICLIGHTMAP_OFF DYNAMICLIGHTMAP_ON
+    // enable dithering LOD crossfade
+    #pragma multi_compile _ LOD_FADE_CROSSFADE
     // TODO: We should have this keyword only if VelocityInGBuffer is enable, how to do that ?
     //#pragma multi_compile VELOCITYOUTPUT_OFF VELOCITYOUTPUT_ON
 

Assets/ScriptableRenderPipeline/ShaderLibrary/Common.hlsl

     return float4(uv * 2.0 - 1.0, 1.0, 1.0);
 }
 
+// LOD dithering transition helper
+// ditherFactor should be a quantized value between 0..15/16, i.e the one provide by Unity
+// LOD0 must use this function with ditherFactor 1..0
+// LOD1 must use this functoin with ditherFactor 0..1
+void LODDitheringTransition(uint2 unPositionSS, float ditherFactor)
+{
+    // Generate a fixed pattern
+    float p = cos(dot(unPositionSS, float2(443.8975, 397.2973)));
+    p = frac(p * 491.1871);
+
+    // We want to have a symmetry between 0..0.5 ditherFactor and 0.5..1 so no pixels are transparent during the transition
+    // this is handled by this test which reverse the pattern
+    p = (ditherFactor >= 0.5) ? (15.0 / 16.0) - p : p;
+    clip(ditherFactor - t);
+}
+
 #endif // UNITY_COMMON_INCLUDED

Assets/TestScenes/HDTest/Rock/rcgRock012/Materials/rcgRock012Material.mat

     - _HorizonFade: 1
     - _MaterialID: 0
     - _Metalic: 0
-    - _Metallic: 0.583
+    - _Metallic: 0
     - _Mettalic: 0
     - _Mode: 0
     - _NormalMapSpace: 0
     - _PPDMaxSamples: 15
     - _PPDMinSamples: 5
     - _Parallax: 0.02
-    - _Smoothness: 0
+    - _Smoothness: 0.524
     - _SmoothnessTextureChannel: 0
     - _SpecularHighlights: 1
     - _SrcBlend: 1