decal normal buffer works, 1st draft

com.unity.render-pipelines.high-definition/HDRP/Editor/RenderPipeline/HDAssetFactory.cs

                 newAsset.shadowBlurMoments = Load<ComputeShader>(CorePath + "Shadow/ShadowBlurMoments.compute");
                 newAsset.debugShadowMapShader = Load<Shader>(CorePath + "Shadow/DebugDisplayShadowMap.shader");
 
+                // decal
+                newAsset.decalNormalBuffer = Load<Shader>(HDRenderPipelinePath + "Material/Decal/DecalNormalBuffer.shader");
+
                 AssetDatabase.CreateAsset(newAsset, pathName);
                 ProjectWindowUtil.ShowCreatedAsset(newAsset);
             }

com.unity.render-pipelines.high-definition/HDRP/Material/Decal/Decal.shader

 		Pass
 		{
 			Name "DBufferMesh_3RT"  // Name is not used
-			Tags{"LightMode" = "DBufferMesh_3RT"} // Smoothness 
+			Tags{"LightMode" = "DBufferMesh_3RT"} // Smoothness
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			ZWrite Off
 			ZTest LEqual
 			// using alpha compositing https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch23.html
 		Pass
 		{
 			Name "DBufferProjector_M"  // Name is not used
-			Tags{"LightMode" = "DBufferProjector_M"} // Metalness 
+			Tags{"LightMode" = "DBufferProjector_M"} // Metalness
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			// back faces with zfail, for cases when camera is inside the decal volume
 			Cull Front
 			ZWrite Off
 			Name "DBufferProjector_AO"  // Name is not used
 			Tags{"LightMode" = "DBufferProjector_AO"} // AO only
 													  // back faces with zfail, for cases when camera is inside the decal volume
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			Cull Front
 			ZWrite Off
 			ZTest Greater
 			Name "DBufferProjector_MAO"  // Name is not used
 			Tags{"LightMode" = "DBufferProjector_MAO"} // AO + Metalness
 													   // back faces with zfail, for cases when camera is inside the decal volume
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			Cull Front
 			ZWrite Off
 			ZTest Greater
 		Pass
 		{
 			Name "DBufferProjector_S"  // Name is not used
-			Tags{"LightMode" = "DBufferProjector_S"} // Smoothness 
+			Tags{"LightMode" = "DBufferProjector_S"} // Smoothness
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			// back faces with zfail, for cases when camera is inside the decal volume
 			Cull Front
 			ZWrite Off
 		Pass
 		{
 			Name "DBufferProjector_MS"  // Name is not used
-			Tags{"LightMode" = "DBufferProjector_MS"} // Smoothness and Metalness 
+			Tags{"LightMode" = "DBufferProjector_MS"} // Smoothness and Metalness
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			// back faces with zfail, for cases when camera is inside the decal volume
 			Cull Front
 			ZWrite Off
 		{
 			Name "DBufferProjector_AOS"  // Name is not used
 			Tags{"LightMode" = "DBufferProjector_AOS"} // AO + Smoothness
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			// back faces with zfail, for cases when camera is inside the decal volume
 			Cull Front
 			ZWrite Off
         {
             Name "DBufferProjector_MAOS"  // Name is not used
             Tags { "LightMode" = "DBufferProjector_MAOS" } // Metalness AO and Smoothness
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
             // back faces with zfail, for cases when camera is inside the decal volume
             Cull Front
             ZWrite Off
 		Pass
 		{
 			Name "DBufferMesh_M"  // Name is not used
-			Tags{"LightMode" = "DBufferMesh_M"} // Metalness 
+			Tags{"LightMode" = "DBufferMesh_M"} // Metalness
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			ZWrite Off
 			ZTest LEqual
 			// using alpha compositing https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch23.html
 		{
 			Name "DBufferMesh_AO"  // Name is not used
 			Tags{"LightMode" = "DBufferMesh_AO"} // AO only
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			ZWrite Off
 			ZTest LEqual
 			// using alpha compositing https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch23.html
 		{
 			Name "DBufferMesh_MAO"  // Name is not used
 			Tags{"LightMode" = "DBufferMesh_MAO"} // AO + Metalness
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			ZWrite Off
 			ZTest LEqual
 			// using alpha compositing https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch23.html
 		Pass
 		{
 			Name "DBufferMesh_S"  // Name is not used
-			Tags{"LightMode" = "DBufferMesh_S"} // Smoothness 
+			Tags{"LightMode" = "DBufferMesh_S"} // Smoothness
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			ZWrite Off
 			ZTest LEqual
 			// using alpha compositing https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch23.html
 		Pass
 		{
 			Name "DBufferMesh_MS"  // Name is not used
-			Tags{"LightMode" = "DBufferMesh_MS"} // Smoothness and Metalness 
+			Tags{"LightMode" = "DBufferMesh_MS"} // Smoothness and Metalness
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			ZWrite Off
 			ZTest LEqual
 			// using alpha compositing https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch23.html
 		{
 			Name "DBufferMesh_AOS"  // Name is not used
 			Tags{"LightMode" = "DBufferMesh_AOS"} // AO + Smoothness
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			ZWrite Off
 			ZTest LEqual
 			// using alpha compositing https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch23.html
 		{
 			Name "DBufferMesh_MAOS"  // Name is not used
 			Tags{"LightMode" = "DBufferMesh_MAOS"} // Metalness AO and Smoothness
+
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
 			ZWrite Off
 			ZTest LEqual
 			// using alpha compositing https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch23.html

com.unity.render-pipelines.high-definition/HDRP/Material/GBufferManager.cs

 
             for (int gbufferIndex = 0; gbufferIndex < m_BufferCount; ++gbufferIndex)
             {
-                m_RTs[gbufferIndex] = RTHandles.Alloc(Vector2.one, colorFormat: rtFormat[gbufferIndex], sRGB: sRGBFlags[gbufferIndex], filterMode: FilterMode.Point, name: string.Format("GBuffer{0}", gbufferIndex));
+                m_RTs[gbufferIndex] = RTHandles.Alloc(Vector2.one, colorFormat: rtFormat[gbufferIndex], sRGB: sRGBFlags[gbufferIndex], filterMode: FilterMode.Point, name: string.Format("GBuffer{0}", gbufferIndex), enableRandomWrite: gbufferIndex == 1);
                 m_RTIDs[gbufferIndex] = m_RTs[gbufferIndex].nameID;
                 m_TextureShaderIDs[gbufferIndex] = HDShaderIDs._GBufferTexture[gbufferIndex];
 

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

 
             ZWrite On
 
+            Stencil
+            {
+                WriteMask 255
+                Ref[_StencilRef]
+                Comp Always
+                Pass IncrSat
+            }
+
             HLSLPROGRAM
 
             // In deferred, depth only pass don't output anything.

com.unity.render-pipelines.high-definition/HDRP/RenderPipeline/HDCustomSamplerId.cs

         GBuffer,
         DBufferRender,
         DBufferPrepareDrawData,
+        DBufferNormal,
         DisplayDebugDecalsAtlas,
         DisplayDebugViewMaterial,
         DebugViewMaterialGBuffer,

com.unity.render-pipelines.high-definition/HDRP/RenderPipeline/HDRenderPipeline.cs

         int m_applyDistortionKernel;
 
         Material m_CameraMotionVectorsMaterial;
+        Material m_DecalNormalBufferMaterial;
 
         // Debug material
         Material m_DebugViewMaterialGBuffer;
             m_CopyStencilForNoLighting.SetInt(HDShaderIDs._StencilRef, (int)StencilLightingUsage.NoLighting);
             m_CopyStencilForNoLighting.SetInt(HDShaderIDs._StencilMask, (int)StencilBitMask.LightingMask);
             m_CameraMotionVectorsMaterial = CoreUtils.CreateEngineMaterial(asset.renderPipelineResources.cameraMotionVectors);
+            m_DecalNormalBufferMaterial = CoreUtils.CreateEngineMaterial(asset.renderPipelineResources.decalNormalBuffer);
 
             m_CopyDepth = CoreUtils.CreateEngineMaterial(asset.renderPipelineResources.copyDepthBuffer);
 
 
             CoreUtils.Destroy(m_CopyStencilForNoLighting);
             CoreUtils.Destroy(m_CameraMotionVectorsMaterial);
+            CoreUtils.Destroy(m_DecalNormalBufferMaterial);
 
             CoreUtils.Destroy(m_DebugViewMaterialGBuffer);
             CoreUtils.Destroy(m_DebugViewMaterialGBufferShadowMask);
 
                     // We can now bind the normal buffer to be use by any effect
                     m_NormalBufferManager.BindNormalBuffers(cmd);
+
+                    if (hdCamera.frameSettings.enableForwardRenderingOnly)
+                    {
+                        using (new ProfilingSample(cmd, "DBuffer Normal (forward)", CustomSamplerId.DBufferNormal.GetSampler()))
+                        {
+                            HDUtils.SetRenderTarget(cmd, hdCamera, m_CameraDepthStencilBuffer);
+                            cmd.SetRandomWriteTarget(1, m_NormalBufferManager.GetNormalBuffer(0));                                                   
+                            cmd.DrawProcedural(Matrix4x4.identity, m_DecalNormalBufferMaterial, 0, MeshTopology.Triangles, 3, 1);
+                            cmd.ClearRandomWriteTargets();
+                        }
+                    }
 
                     // In both forward and deferred, everything opaque should have been rendered at this point so we can safely copy the depth buffer for later processing.
                     CopyDepthBufferIfNeeded(cmd);

com.unity.render-pipelines.high-definition/HDRP/RenderPipelineResources/RenderPipelineResources.cs

         public Shader shadowClearShader;
         public ComputeShader shadowBlurMoments;
         public Shader debugShadowMapShader;
+
+        // Decal
+        public Shader decalNormalBuffer;
     }
 }

com.unity.render-pipelines.high-definition/HDRP/Material/Decal/DecalNormalBuffer.shader

+Shader "Hidden/HDRenderPipeline/Material/Decal/DecalNormalBuffer"
+{
+    HLSLINCLUDE
+
+        #pragma target 4.5
+        #pragma only_renderers d3d11 ps4 xboxone vulkan metal switch
+        #include "CoreRP/ShaderLibrary/Common.hlsl"
+        #include "../../ShaderVariables.hlsl"
+        #include "Decal.hlsl"
+        #include "../NormalBuffer.hlsl"
+
+        TEXTURE2D(_DBufferTexture1);
+        RW_TEXTURE2D(float4, _NormalBuffer);
+        
+
+        SamplerState sampler_PointClamp;
+
+        struct Attributes
+        {
+            uint vertexID : SV_VertexID;
+        };
+
+        struct Varyings
+        {
+            float4 positionCS : SV_POSITION;
+            float2 texcoord   : TEXCOORD0;
+        };
+
+        Varyings Vert(Attributes input)
+        {
+            Varyings output;
+            output.positionCS = GetFullScreenTriangleVertexPosition(input.vertexID);
+            output.texcoord = GetFullScreenTriangleTexCoord(input.vertexID);// *_BlitScaleBias.xy + _BlitScaleBias.zw;
+            return output;
+        }
+
+        float4 FragNearest(Varyings input) : SV_Target
+        {
+            float4 DBufferNormal = SAMPLE_TEXTURE2D(_DBufferTexture1, sampler_PointClamp, input.texcoord) * float4(2.0f, 2.0f, 2.0f, 1.0f) - float4(1.0f, 1.0f, 1.0f, 0.0f);
+            float4 GBufferNormal = _NormalBuffer[input.texcoord * _ScreenSize.xy];
+            NormalData normalData;
+            DecodeFromNormalBuffer(GBufferNormal, uint2(0, 0), normalData);
+            normalData.normalWS.xyz = normalize(normalData.normalWS.xyz * DBufferNormal.w + DBufferNormal.xyz);
+            EncodeIntoNormalBuffer(normalData, uint2(0, 0), GBufferNormal);
+            _NormalBuffer[input.texcoord * _ScreenSize.xy] = GBufferNormal;
+            return GBufferNormal;
+        }
+
+    ENDHLSL
+
+    SubShader
+    {
+        Tags{ "RenderPipeline" = "HDRenderPipeline" }
+
+        Pass
+        {
+            ZWrite Off
+            ZTest Always
+            Blend Off
+            Cull Off
+            Stencil
+            {
+                WriteMask 255
+                Ref 2
+                Comp Equal
+                Pass Zero
+            }
+
+            HLSLPROGRAM
+                #pragma vertex Vert
+                #pragma fragment FragNearest
+            ENDHLSL
+        }
+    }
+
+    Fallback Off
+}

com.unity.render-pipelines.high-definition/HDRP/Material/Decal/DecalNormalBuffer.shader.meta

+fileFormatVersion: 2
+guid: fd532bf1795188c4daaa66ea798b8b0a
+ShaderImporter:
+  externalObjects: {}
+  defaultTextures: []
+  nonModifiableTextures: []
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: