Use alpha compositing to blend decals

ScriptableRenderPipeline/HDRenderPipeline/HDRP/HDRenderPipeline.cs

             return m_ColorMRTs;
         }
 
+		public void ClearNormalTarget(Color clearColor, CommandBuffer cmd)
+		{
+			// index 1 is normals
+			CoreUtils.SetRenderTarget(cmd, m_ColorMRTs[1], ClearFlag.Color, clearColor);
+		}
+
         public void PushGlobalParams(CommandBuffer cmd)
         {
             cmd.SetGlobalInt(HDShaderIDs._EnableDBuffer, vsibleDecalCount > 0 ? 1 : 0);
                 // Depth texture is now ready, bind it.
                 cmd.SetGlobalTexture(HDShaderIDs._MainDepthTexture, GetDepthTexture());
 
-                CoreUtils.SetRenderTarget(cmd, m_DbufferManager.GetDBuffers(), m_CameraDepthStencilBufferRT, ClearFlag.Color, CoreUtils.clearColorAllBlack);
+				// for alpha compositing, color is cleared to 0, alpha to 1 
+				// https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch23.html
+
+				Color clearColor = new Color(0.0f, 0.0f, 0.0f, 1.0f); 
+				CoreUtils.SetRenderTarget(cmd, m_DbufferManager.GetDBuffers(), m_CameraDepthStencilBufferRT, ClearFlag.Color, clearColor);
+
+				// we need to do a separate clear for normals, because they are cleared to a different color
+				Color clearColorNormal = new Color(0.5f, 0.5f, 0.5f, 1.0f); // for normals 0.5 is neutral
+				m_DbufferManager.ClearNormalTarget(clearColorNormal, cmd);
+
+				CoreUtils.SetRenderTarget(cmd, m_DbufferManager.GetDBuffers(), m_CameraDepthStencilBufferRT); // do not clear anymore
 				DecalSystem.instance.Render(renderContext, camera, cmd);
             }
         }

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Decal/Decal.shader

             Cull Off
 			ZWrite Off
 			ZTest Always
+			// using alpha compositing https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch23.html
+			Blend SrcAlpha OneMinusSrcAlpha, Zero OneMinusSrcAlpha
 
 			HLSLPROGRAM
 

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Decal/DecalData.hlsl

 #endif
 #if _MASKMAP
 	surfaceData.mask = SAMPLE_TEXTURE2D(_MaskMap, sampler_MaskMap, texCoordDS.xy); 
-	surfaceData.mask.z = totalBlend;
+	surfaceData.mask.z = surfaceData.mask.w;
+	surfaceData.mask.w = totalBlend;
 #endif
 
 }

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Decal/DecalProperties.hlsl

 
 float _DecalBlend;
 
-CBUFFER_START(Decal)
-float4x4 _WorldToDecal;
-float4x4 _DecalToWorldR;
-CBUFFER_END
-
 #endif 

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Decal/DecalUtilities.hlsl

 		DecalSurfaceData decalSurfaceData;
 		DECODE_FROM_DBUFFER(DBuffer, decalSurfaceData);
 
-		surfaceData.baseColor.xyz = lerp(surfaceData.baseColor.xyz, decalSurfaceData.baseColor.xyz, decalSurfaceData.baseColor.w);
-		surfaceData.normalWS.xyz = normalize(lerp(surfaceData.normalWS.xyz, decalSurfaceData.normalWS.xyz, decalSurfaceData.normalWS.w));
-		surfaceData.metallic = lerp(surfaceData.metallic, decalSurfaceData.mask.x, decalSurfaceData.mask.z);
-		surfaceData.ambientOcclusion = lerp(surfaceData.ambientOcclusion, decalSurfaceData.mask.y, decalSurfaceData.mask.z);
-		surfaceData.perceptualSmoothness = lerp(surfaceData.perceptualSmoothness, decalSurfaceData.mask.w, decalSurfaceData.mask.z);
+		// using alpha compositing https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch23.html
+		surfaceData.baseColor.xyz = surfaceData.baseColor.xyz * decalSurfaceData.baseColor.w + decalSurfaceData.baseColor.xyz;
+		surfaceData.normalWS.xyz = normalize(surfaceData.normalWS.xyz * decalSurfaceData.normalWS.w + decalSurfaceData.normalWS.xyz);
+		surfaceData.metallic = surfaceData.metallic * decalSurfaceData.mask.w + decalSurfaceData.mask.x;
+		surfaceData.ambientOcclusion = surfaceData.ambientOcclusion * decalSurfaceData.mask.w + decalSurfaceData.mask.y;
+		surfaceData.perceptualSmoothness = surfaceData.perceptualSmoothness * decalSurfaceData.mask.w + decalSurfaceData.mask.z;
 	}
 }