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

#include "Decal.hlsl"

DECLARE_DBUFFER_TEXTURE(_DBufferTexture);

DecalData FetchDecal(uint start, uint i)
{
#ifdef LIGHTLOOP_TILE_PASS
    int j = FetchIndex(start, i);
#else
    int j = start + i;
#endif
    return _DecalDatas[j];
}

// Caution: We can't compute LOD inside a dynamic loop. The gradient are not accessible.
// we need to find a way to calculate mips. For now just fetch first mip of the decals
void ApplyBlendNormal(inout float4 dst, inout int matMask, float2 texCoords, int sliceIndex, int mapMask, float3x3 decalToWorld, float blend)
{
	float4 src;
	src.xyz =  mul(decalToWorld, UnpackNormalmapRGorAG(SAMPLE_TEXTURE2D_ARRAY_LOD(_DecalAtlas, sampler_DecalAtlas, texCoords, sliceIndex, 0 /* ComputeTextureLOD(texCoords) */))) * 0.5f + 0.5f;
	src.w = blend;
	dst.xyz = src.xyz * src.w + dst.xyz * (1.0f - src.w);
	dst.w = dst.w * (1.0f - src.w);
	matMask |= mapMask;
}

void ApplyBlendDiffuse(inout float4 dst, inout int matMask, float2 texCoords, int sliceIndex, int mapMask, float blend)
{
	float4 src = SAMPLE_TEXTURE2D_ARRAY_LOD(_DecalAtlas, sampler_DecalAtlas, texCoords, sliceIndex, 0 /* ComputeTextureLOD(texCoords) */);
	src.w *= blend;
	dst.xyz = src.xyz * src.w + dst.xyz * (1.0f - src.w);
	dst.w = dst.w * (1.0f - src.w);
	matMask |= mapMask;
}

void ApplyBlendMask(inout float4 dst, inout int matMask, float2 texCoords, int sliceIndex, int mapMask, float blend)
{
	float4 src = SAMPLE_TEXTURE2D_ARRAY_LOD(_DecalAtlas, sampler_DecalAtlas, texCoords, sliceIndex, 0 /* ComputeTextureLOD(texCoords) */);
	src.z = src.w;
	src.w = blend;
	dst.xyz = src.xyz * src.w + dst.xyz * (1.0f - src.w);
	dst.w = dst.w * (1.0f - src.w);
	matMask |= mapMask;
}

void AddDecalContribution(PositionInputs posInput, inout SurfaceData surfaceData)
{
	if(_EnableDBuffer)
	{
		DecalSurfaceData decalSurfaceData;
		int mask = 0;
		// the code in the macros, gets moved inside the conditionals by the compiler
		FETCH_DBUFFER(DBuffer, _DBufferTexture, posInput.positionSS);

#ifdef _SURFACE_TYPE_TRANSPARENT	// forward transparent using clustered decals
        uint decalCount, decalStart;
		DBuffer0 = float4(0.0f, 0.0f, 0.0f, 1.0f);
		DBuffer1 = float4(0.5f, 0.5f, 0.5f, 1.0f);
		DBuffer2 = float4(0.0f, 0.0f, 0.0f, 1.0f);

    #ifdef LIGHTLOOP_TILE_PASS
        GetCountAndStart(posInput, LIGHTCATEGORY_DECAL, decalStart, decalCount);
    #else
        decalCount = _DecalCount;
        decalStart = 0;
    #endif
		float3 positionWS = GetAbsolutePositionWS(posInput.positionWS);
		uint i = 0;
        for (i = 0; i < decalCount; i++)
        {
            DecalData decalData = FetchDecal(decalStart, i);
			float3 positionDS = mul(decalData.worldToDecal, float4(positionWS, 1.0)).xyz;
			positionDS = positionDS * float3(1.0, -1.0, 1.0) + float3(0.5, 0.0f, 0.5);
			float decalBlend = decalData.normalToWorld[0][3];
			int diffuseIndex = decalData.normalToWorld[1][3];
			int normalIndex = decalData.normalToWorld[2][3];
			int maskIndex = decalData.normalToWorld[3][3];
			if((all(positionDS.xyz > 0.0f) && all(1.0f - positionDS.xyz > 0.0f))) // clip to decal space
			{
				if(diffuseIndex != -1)
				{
					ApplyBlendDiffuse(DBuffer0, mask, positionDS.xz, diffuseIndex, DBUFFERHTILEBIT_DIFFUSE, decalBlend);
				}

				if(normalIndex != -1)
				{
					ApplyBlendNormal(DBuffer1, mask, positionDS.xz, normalIndex, DBUFFERHTILEBIT_NORMAL, (float3x3)decalData.normalToWorld, decalBlend);
				}

				if(maskIndex != -1)
				{
					ApplyBlendMask(DBuffer2, mask, positionDS.xz, maskIndex, DBUFFERHTILEBIT_MASK, decalBlend);
				}
			}
		}
#else
		mask = UnpackByte(LOAD_TEXTURE2D(_DecalHTileTexture, posInput.positionSS / 8).r);
#endif
		DECODE_FROM_DBUFFER(DBuffer, decalSurfaceData);
		// using alpha compositing https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch23.html
		if(mask & DBUFFERHTILEBIT_DIFFUSE)
		{
			surfaceData.baseColor.xyz = surfaceData.baseColor.xyz * decalSurfaceData.baseColor.w + decalSurfaceData.baseColor.xyz;
		}

		if(mask & DBUFFERHTILEBIT_NORMAL)
		{
			surfaceData.normalWS.xyz = normalize(surfaceData.normalWS.xyz * decalSurfaceData.normalWS.w + decalSurfaceData.normalWS.xyz);
		}
		if(mask & DBUFFERHTILEBIT_MASK)
		{
			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;
		}
	}
}