Update detail map formula for albedo and smoothness

com.unity.render-pipelines.high-definition/HDRP/Material/Lit/LitDataIndividualLayer.hlsl

         detailMask = SAMPLE_UVMAPPING_TEXTURE2D(ADD_IDX(_MaskMap), SAMPLER_MASKMAP_IDX, ADD_IDX(layerTexCoord.base)).b;
     #endif
     float2 detailAlbedoAndSmoothness = SAMPLE_UVMAPPING_TEXTURE2D(ADD_IDX(_DetailMap), SAMPLER_DETAILMAP_IDX, ADD_IDX(layerTexCoord.details)).rb;
-    float detailAlbedo = detailAlbedoAndSmoothness.r;
-    float detailSmoothness = detailAlbedoAndSmoothness.g;
+    float detailAlbedo = detailAlbedoAndSmoothness.r * 2.0 - 1.0;
+    float detailSmoothness = detailAlbedoAndSmoothness.g * 2.0 - 1.0;
     // Resample the detail map but this time for the normal map. This call should be optimize by the compiler
     // We split both call due to trilinear mapping
     detailNormalTS = SAMPLE_UVMAPPING_NORMALMAP_AG(ADD_IDX(_DetailMap), SAMPLER_DETAILMAP_IDX, ADD_IDX(layerTexCoord.details), ADD_IDX(_DetailNormalScale));
 #ifdef _DETAIL_MAP_IDX
-    // Use overlay blend mode for detail abledo: (base < 0.5 ? (2.0 * base * blend) : (1.0 - 2.0 * (1.0 - base) * (1.0 - blend)))
-    float3 baseColorOverlay = (detailAlbedo < 0.5) ?
-                                surfaceData.baseColor * PositivePow(2.0 * detailAlbedo, ADD_IDX(_DetailAlbedoScale)) :
-                                1.0 - (1.0 - surfaceData.baseColor) * PositivePow(2.0 * (1.0 - detailAlbedo), ADD_IDX(_DetailAlbedoScale));
+	
+	// Goal: we want the detail albedo map to be able to darken down to black and brighten up to white the surface albedo.
+	// The scale control the speed of the gradient. We simply remap detailAlbedo from [0..1] to [-1..1] then perform a lerp to black or white
+	// with a factor based on speed.
+    // For base color we interpolate in sRGB space (approximate here as square) as it get a nicer perceptual gradient
+	float albedoDetailSpeed = saturate(abs(detailAlbedo) * ADD_IDX(_DetailAlbedoScale));
+	float3 baseColorOverlay = lerp(sqrt(surfaceData.baseColor), (detailAlbedo < 0.0) ? float3(0.0, 0.0, 0.0) : float3(1.0, 1.0, 1.0), albedoDetailSpeed * albedoDetailSpeed);
+	baseColorOverlay *= baseColorOverlay;							   
     // Lerp with details mask
     surfaceData.baseColor = lerp(surfaceData.baseColor, saturate(baseColorOverlay), detailMask);
 #endif
 #endif
 
 #ifdef _DETAIL_MAP_IDX
-    // Use overlay blend mode for detail abledo: (base < 0.5 ? (2.0 * base * blend) : (1.0 - 2.0 * (1.0 - base) * (1.0 - blend)))
-    float smoothnessOverlay = (detailSmoothness < 0.5) ?
-                                surfaceData.perceptualSmoothness * PositivePow(2.0 * detailSmoothness, ADD_IDX(_DetailSmoothnessScale)) :
-                                1.0 - (1.0 - surfaceData.perceptualSmoothness) * PositivePow(2.0 * (1.0 - detailSmoothness), ADD_IDX(_DetailSmoothnessScale));
+    // See comment for baseColorOverlay
+    float smoothnessDetailSpeed = saturate(abs(detailSmoothness) * ADD_IDX(_DetailSmoothnessScale));
+	float3 smoothnessOverlay = lerp(surfaceData.perceptualSmoothness, (detailSmoothness < 0.0) ? 0.0 : 1.0, smoothnessDetailSpeed);
     // Lerp with details mask
     surfaceData.perceptualSmoothness = lerp(surfaceData.perceptualSmoothness, saturate(smoothnessOverlay), detailMask);
 #endif