Add energy conservation specular color

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/Material/Lit/LitUI.cs

             public static GUIContent coatMaskText = new GUIContent("Coat Mask", "Attenuate the coating effect (similar to change to IOR of 1");
 
             // Specular color
+            public static GUIContent energyConservingSpecularColorText = new GUIContent("Energy Conserving Specular Color", "Enable energy conservation when using Specular Color mode (i.e high Specular Color mean lower Diffuse Color");
             public static GUIContent specularColorText = new GUIContent("Specular Color", "Specular color (RGB)");
 
             // Specular occlusion
         protected MaterialProperty[] detailSmoothnessScale = new MaterialProperty[kMaxLayerCount];
         protected const string kDetailSmoothnessScale = "_DetailSmoothnessScale";
 
+        protected MaterialProperty energyConservingSpecularColor = null;
+        protected const string kEnergyConservingSpecularColor = "_EnergyConservingSpecularColor";
         protected MaterialProperty specularColor = null;
         protected const string kSpecularColor = "_SpecularColor";
         protected MaterialProperty specularColorMap = null;
             // The next properties are only supported for regular Lit shader (not layered ones) because it's complicated to blend those parameters if they are different on a per layer basis.
 
             // Specular Color
+            energyConservingSpecularColor = FindProperty(kEnergyConservingSpecularColor, props);
             specularColor = FindProperty(kSpecularColor, props);
             specularColorMap = FindProperty(kSpecularColorMap, props);
 
             // We reuse thickness from SSS
         }
 
+        protected void ShaderSpecularColorInputGUI(Material material)
+        {
+            m_MaterialEditor.TexturePropertySingleLine(Styles.specularColorText, specularColorMap, specularColor);
+            EditorGUI.indentLevel++;
+            m_MaterialEditor.ShaderProperty(energyConservingSpecularColor, Styles.energyConservingSpecularColorText);
+            EditorGUI.indentLevel--;
+        }
+
         protected void ShaderSSSInputGUI(Material material, int layerIndex)
         {
             var hdPipeline = RenderPipelineManager.currentPipeline as HDRenderPipeline;
                     ShaderAnisoInputGUI();
                     break;
                 case BaseLitGUI.MaterialId.LitSpecular:
-                    m_MaterialEditor.TexturePropertySingleLine(Styles.specularColorText, specularColorMap, specularColor);
+                    ShaderSpecularColorInputGUI(material);
                     break;
                 case BaseLitGUI.MaterialId.LitClearCoat:
                     ShaderClearCoatInputGUI();

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/Lit.shader

 
         _CoatMask("Coat Mask", Range(0.0, 1.0)) = 1.0
 
+        [ToggleUI] _EnergyConservingSpecularColor("_EnergyConservingSpecularColor", Float) = 1.0
         _SpecularColor("SpecularColor", Color) = (1, 1, 1, 1)
         _SpecularColorMap("SpecularColorMap", 2D) = "white" {}
 

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/LitDataIndividualLayer.hlsl

 #ifdef _SPECULARCOLORMAP
     surfaceData.specularColor *= SAMPLE_UVMAPPING_TEXTURE2D(_SpecularColorMap, sampler_SpecularColorMap, layerTexCoord.base).rgb;
 #endif
+#ifdef _MATERIAL_FEATURE_SPECULAR_COLOR
+    // Require to have setup baseColor
+    // Reproduce the energy conservation done in legacy Unity. Not ideal but better for compatibility and users can unchek it
+    surfaceData.baseColor *= _EnergyConservingSpecularColor > 0.0 ? (1.0 - Max3(surfaceData.specularColor.r, surfaceData.specularColor.g, surfaceData.specularColor.b)) : 1.0;
+#endif
 
 #if HAS_REFRACTION
     surfaceData.ior = _IOR;

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/LitProperties.hlsl

 
 float _CoatMask;
 
-float4 _SpecularColor;
+float4 _SpecularColor;
+float _EnergyConservingSpecularColor;
 
 float _TexWorldScale;
 float _InvTilingScale;

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

 
         _CoatMask("Coat Mask", Range(0.0, 1.0)) = 1.0
 
+        [ToggleUI] _EnergyConservingSpecularColor("_EnergyConservingSpecularColor", Float) = 1.0
         _SpecularColor("SpecularColor", Color) = (1, 1, 1, 1)
         _SpecularColorMap("SpecularColorMap", 2D) = "white" {}