Pass terrain layer smoothness and metallic to terrainlit shader and stop using _BaseColor properties.

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/LayeredLit/TerrainLit.shader

         [HideInInspector] _Splat2("Layer 2", 2D) = "white" {}
         [HideInInspector] _Splat3("Layer 3", 2D) = "white" {}
 
+        // Since we don't use a mask texture for getting the mask, we'll need the metallic property to be serialized as in sRGB space.
+        [HideInInspector] [Gamma] _Metallic0("Metallic 0", Range(0.0, 1.0)) = 0
+        [HideInInspector] [Gamma] _Metallic1("Metallic 1", Range(0.0, 1.0)) = 0
+        [HideInInspector] [Gamma] _Metallic2("Metallic 2", Range(0.0, 1.0)) = 0
+        [HideInInspector] [Gamma] _Metallic3("Metallic 3", Range(0.0, 1.0)) = 0
+        [HideInInspector] _Smoothness0("Smoothness0", Range(0.0, 1.0)) = 1.0
+        [HideInInspector] _Smoothness1("Smoothness1", Range(0.0, 1.0)) = 1.0
+        [HideInInspector] _Smoothness2("Smoothness2", Range(0.0, 1.0)) = 1.0
+        [HideInInspector] _Smoothness3("Smoothness3", Range(0.0, 1.0)) = 1.0
+
         // TODO: route values from terrain layers. enable _DENSITY_MODE if any of these enabled.
         [HideInInspector] [ToggleUI] _OpacityAsDensity0("_OpacityAsDensity0", Float) = 0.0
         [HideInInspector] [ToggleUI] _OpacityAsDensity1("_OpacityAsDensity1", Float) = 0.0
         // They are use to fill a SurfaceData. With a MaterialGraph this should not exist.
 
         // All the following properties are filled by the referenced lit shader.
-
-        // Reminder. Color here are in linear but the UI (color picker) do the conversion sRGB to linear
-        _BaseColor0("BaseColor0", Color) = (1, 1, 1, 1)
-        _BaseColor1("BaseColor1", Color) = (1, 1, 1, 1)
-        _BaseColor2("BaseColor2", Color) = (1, 1, 1, 1)
-        _BaseColor3("BaseColor3", Color) = (1, 1, 1, 1)
-
-        _Metallic0("Metallic0", Range(0.0, 1.0)) = 0
-        _Metallic1("Metallic1", Range(0.0, 1.0)) = 0
-        _Metallic2("Metallic2", Range(0.0, 1.0)) = 0
-        _Metallic3("Metallic3", Range(0.0, 1.0)) = 0
-
-        _Smoothness0("Smoothness0", Range(0.0, 1.0)) = 1.0
-        _Smoothness1("Smoothness1", Range(0.0, 1.0)) = 1.0
-        _Smoothness2("Smoothness2", Range(0.0, 1.0)) = 1.0
-        _Smoothness3("Smoothness3", Range(0.0, 1.0)) = 1.0
 
         _SmoothnessRemapMin0("SmoothnessRemapMin0", Range(0.0, 1.0)) = 0.0
         _SmoothnessRemapMin1("SmoothnessRemapMin1", Range(0.0, 1.0)) = 0.0

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/LayeredLit/TerrainLitData.hlsl

 float4 _Splat3_ST;
 SAMPLER(sampler_Splat0);
 
+#define _BaseColor0 float4(1,1,1,1)
+#define _BaseColor1 float4(1,1,1,1)
+#define _BaseColor2 float4(1,1,1,1)
+#define _BaseColor3 float4(1,1,1,1)
+
 // Number of sampler are limited, we need to share sampler as much as possible with lit material
 // for this we put the constraint that the sampler are the same in a layered material for all textures of the same type
 // then we take the sampler matching the first textures use of this type
 #define ADD_ZERO_IDX(Name) Name##0
 #define _BaseColorMap _Splat
 #define sampler_BaseColorMap sampler_Splat
+#define ALPHA_USED_AS_SMOOTHNESS
 
 // include LitDataInternal multiple time to define the variation of GetSurfaceData for each layer
 #define LAYER_INDEX 0
 #undef _THICKNESSMAP_IDX
 #undef _MASKMAP_IDX
 #undef _BENTNORMALMAP_IDX
+
+#undef ALPHA_USED_AS_SMOOTHNESS
 
 float3 BlendLayeredVector3(float3 x0, float3 x1, float3 x2, float3 x3, float weight[4])
 {
 
     // We want to calculate the mean color of the texture. For this we will sample a low mipmap
     float textureBias = 15.0; // Use maximum bias
-    float3 baseMeanColor0 = SAMPLE_UVMAPPING_TEXTURE2D_BIAS(_Splat0, sampler_Splat0, layerTexCoord.base0, textureBias).rgb *_BaseColor0.rgb;
-    float3 baseMeanColor1 = SAMPLE_UVMAPPING_TEXTURE2D_BIAS(_Splat1, sampler_Splat0, layerTexCoord.base1, textureBias).rgb *_BaseColor1.rgb;
-    float3 baseMeanColor2 = SAMPLE_UVMAPPING_TEXTURE2D_BIAS(_Splat2, sampler_Splat0, layerTexCoord.base2, textureBias).rgb *_BaseColor2.rgb;
-    float3 baseMeanColor3 = SAMPLE_UVMAPPING_TEXTURE2D_BIAS(_Splat3, sampler_Splat0, layerTexCoord.base3, textureBias).rgb *_BaseColor3.rgb;
+    float3 baseMeanColor0 = SAMPLE_UVMAPPING_TEXTURE2D_BIAS(_Splat0, sampler_Splat0, layerTexCoord.base0, textureBias).rgb;
+    float3 baseMeanColor1 = SAMPLE_UVMAPPING_TEXTURE2D_BIAS(_Splat1, sampler_Splat0, layerTexCoord.base1, textureBias).rgb;
+    float3 baseMeanColor2 = SAMPLE_UVMAPPING_TEXTURE2D_BIAS(_Splat2, sampler_Splat0, layerTexCoord.base2, textureBias).rgb;
+    float3 baseMeanColor3 = SAMPLE_UVMAPPING_TEXTURE2D_BIAS(_Splat3, sampler_Splat0, layerTexCoord.base3, textureBias).rgb;
 
     float3 meanColor = BlendLayeredVector3(baseMeanColor0, baseMeanColor1, baseMeanColor2, baseMeanColor3, weights);
 

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

 {
     float alpha = SAMPLE_UVMAPPING_TEXTURE2D(ADD_IDX(_BaseColorMap), ADD_ZERO_IDX(sampler_BaseColorMap), ADD_IDX(layerTexCoord.base)).a * ADD_IDX(_BaseColor).a;
 
+#ifndef ALPHA_USED_AS_SMOOTHNESS
-#if defined(_ALPHATEST_ON) && !defined(LAYERED_LIT_SHADER)
-    float alphaCutoff = _AlphaCutoff;
-    #ifdef CUTOFF_TRANSPARENT_DEPTH_PREPASS
-    alphaCutoff = _AlphaCutoffPrepass;
-    #elif defined(CUTOFF_TRANSPARENT_DEPTH_POSTPASS)
-    alphaCutoff = _AlphaCutoffPostpass;
+    #if defined(_ALPHATEST_ON) && !defined(LAYERED_LIT_SHADER)
+        float alphaCutoff = _AlphaCutoff;
+        #ifdef CUTOFF_TRANSPARENT_DEPTH_PREPASS
+        alphaCutoff = _AlphaCutoffPrepass;
+        #elif defined(CUTOFF_TRANSPARENT_DEPTH_POSTPASS)
+        alphaCutoff = _AlphaCutoffPostpass;
+        #endif
+        DoAlphaTest(alpha, alphaCutoff);
-    DoAlphaTest(alpha, alphaCutoff);
+    surfaceData.perceptualSmoothness = 1;
+#else
+    surfaceData.perceptualSmoothness = alpha;
+    alpha = 1;
 #endif
 
     float3 detailNormalTS = float3(0.0, 0.0, 0.0);
     bentNormalTS = ADD_IDX(GetBentNormalTS)(input, layerTexCoord, normalTS, detailNormalTS, detailMask);
 
 #if defined(_MASKMAP_IDX)
-    surfaceData.perceptualSmoothness = SAMPLE_UVMAPPING_TEXTURE2D(ADD_IDX(_MaskMap), SAMPLER_MASKMAP_IDX, ADD_IDX(layerTexCoord.base)).a;
+    surfaceData.perceptualSmoothness *= SAMPLE_UVMAPPING_TEXTURE2D(ADD_IDX(_MaskMap), SAMPLER_MASKMAP_IDX, ADD_IDX(layerTexCoord.base)).a;
-    surfaceData.perceptualSmoothness = ADD_IDX(_Smoothness);
+    surfaceData.perceptualSmoothness *= ADD_IDX(_Smoothness);
 #endif
 
 #ifdef _DETAIL_MAP_IDX