Add volumetrics to the HDRI sky

ScriptableRenderPipeline/Core/ShaderLibrary/VolumeRendering.hlsl

            HenyeyGreensteinPhasePartVarying(asymmetry, LdotD);
 }
 
+// Absorption coefficient from Disney: http://blog.selfshadow.com/publications/s2015-shading-course/burley/s2015_pbs_disney_bsdf_notes.pdf
+float3 TransmittanceColorAtDistanceToAbsorption(float3 transmittanceColor, float atDistance)
+{
+    return -log(transmittanceColor + 0.00001) / max(atDistance, 0.000001);
+}
+
+#ifndef USE_LEGACY_UNITY_SHADER_VARIABLES
+    #define VOLUMETRIC_LIGHTING_ENABLED
+#endif
+
 // TODO: share this...
 #define PRESET_ULTRA 0
 
 
 float4 GetInScatteredRadianceAndTransmittance(float2 positionSS, float depthVS,
                                               TEXTURE3D(VBufferLighting), SAMPLER3D(linearClampSampler),
-                                              float4 VBufferDepthEncodingParams, float2 VBufferScale)
+                                              float2 VBufferScale, float4 VBufferDepthEncodingParams)
-    float slice0 = clamp(floor(d * n - 0.5), 0, n - 1); // TODO: somehow avoid the clamp...
-    float slice1 = clamp( ceil(d * n - 0.5), 0, n - 1); // TODO: somehow avoid the clamp...
-
+    // Therefore, we perform 2 bilinear taps.
-    float d0 = slice0 * rcp(n) + (0.5 * rcp(n));
-    float d1 = slice1 * rcp(n) + (0.5 * rcp(n));
+    float s0 = clamp(floor(d * n - 0.5), 0, n - 1); // TODO: somehow avoid the clamp...
+    float s1 = clamp( ceil(d * n - 0.5), 0, n - 1); // TODO: somehow avoid the clamp...
+    float d0 = s0 * rcp(n) + (0.5 * rcp(n));
+    float d1 = s1 * rcp(n) + (0.5 * rcp(n));
     float z0 = DecodeLogarithmicDepth(d0, VBufferDepthEncodingParams);
     float z1 = DecodeLogarithmicDepth(d1, VBufferDepthEncodingParams);
 
-    // Perform 2 bilinear taps. The sampler should clamp the values at the boundaries of the 3D texture.
-    float4 v0 = SAMPLE_TEXTURE3D_LOD(VBufferLighting, linearClampSampler, float3(uv, d0), 0);
-    float4 v1 = SAMPLE_TEXTURE3D_LOD(VBufferLighting, linearClampSampler, float3(uv, d1), 0);
-    float4 vt = lerp(v0, v1, saturate((z - z0) / (z1 - z0)));
+    // The sampler should clamp to edge.
+    float4 L0 = SAMPLE_TEXTURE3D_LOD(VBufferLighting, linearClampSampler, float3(uv, d0), 0);
+    float4 L1 = SAMPLE_TEXTURE3D_LOD(VBufferLighting, linearClampSampler, float3(uv, d1), 0);
+    float4 L  = lerp(L0, L1, saturate((z - z0) / (z1 - z0)));
-    return float4(vt.rgb, Transmittance(vt.a));
+    return float4(L.rgb, Transmittance(L.a));
-// Absorption coefficient from Disney: http://blog.selfshadow.com/publications/s2015-shading-course/burley/s2015_pbs_disney_bsdf_notes.pdf
-float3 TransmittanceColorAtDistanceToAbsorption(float3 transmittanceColor, float atDistance)
+// A version without depth - returns the value for the far plane.
+float4 GetInScatteredRadianceAndTransmittance(float2 positionSS,
+                                              TEXTURE3D(VBufferLighting), SAMPLER3D(linearClampSampler),
+                                              float2 VBufferScale)
-    return -log(transmittanceColor + 0.00001) / max(atDistance, 0.000001);
+    // Account for the visible area of the VBuffer.
+    float2 uv = positionSS * VBufferScale;
+
+    // The sampler should clamp to edge.
+    float4 L = SAMPLE_TEXTURE3D_LOD(VBufferLighting, linearClampSampler, float3(uv, 1), 0);
+
+    return float4(L.rgb, Transmittance(L.a));
 }
 
 #endif // UNITY_VOLUME_RENDERING_INCLUDED

ScriptableRenderPipeline/HDRenderPipeline/Lighting/Deferred.shader

                                                                                    posInput.depthVS,
                                                                                    _VBufferLighting,
                                                                                    s_linear_clamp_sampler,
-                                                                                   _VBufferDepthEncodingParams,
-                                                                                   _VBufferResolutionAndScale.zw);
-                diffuseLighting  *= volumetricLighting.a;
+                                                                                   _VBufferResolutionAndScale.zw,
+                                                                                   _VBufferDepthEncodingParams);
+                // TODO: apply volumetrics after SSS.
+                // diffuseLighting  *= volumetricLighting.a;
                 specularLighting *= volumetricLighting.a;
                 specularLighting += volumetricLighting.rgb;
             #endif

ScriptableRenderPipeline/HDRenderPipeline/Lighting/TilePass/Deferred.compute

                                                                        posInput.depthVS,
                                                                        _VBufferLighting,
                                                                        s_linear_clamp_sampler,
-                                                                       _VBufferDepthEncodingParams,
-                                                                       _VBufferResolutionAndScale.zw);
-    diffuseLighting  *= volumetricLighting.a;
+                                                                       _VBufferResolutionAndScale.zw,
+                                                                       _VBufferDepthEncodingParams);
+    // TODO: apply volumetrics after SSS.
+    // diffuseLighting  *= volumetricLighting.a;
     specularLighting *= volumetricLighting.a;
     specularLighting += volumetricLighting.rgb;
 #endif

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.hlsl

-#ifndef USE_LEGACY_UNITY_SHADER_VARIABLES
-#define VOLUMETRIC_LIGHTING_ENABLED
-#endif
-
 //-----------------------------------------------------------------------------
 // SurfaceData and BSDFData
 //-----------------------------------------------------------------------------
 #include "SubsurfaceScatteringSettings.cs.hlsl"
 
+#include "../../../Core/ShaderLibrary/VolumeRendering.hlsl"
+
-    #include "../../../Core/ShaderLibrary/VolumeRendering.hlsl"
 #endif
 
 // Define refraction keyword helpers

ScriptableRenderPipeline/HDRenderPipeline/Sky/HDRISky/Resources/SkyHDRI.shader

     #include "../../../../Core/ShaderLibrary/Color.hlsl"
     #include "../../../../Core/ShaderLibrary/Common.hlsl"
     #include "../../../../Core/ShaderLibrary/CommonLighting.hlsl"
+    #include "../../../../Core/ShaderLibrary/VolumeRendering.hlsl"
+    #include "../../../ShaderVariables.hlsl"
+
+#ifdef VOLUMETRIC_LIGHTING_ENABLED
+    TEXTURE3D(_VBufferLighting);
+    SamplerState s_linear_clamp_sampler;
+#endif
+
 
     float4   _SkyParam; // x exposure, y multiplier, z rotation
     float4x4 _PixelCoordToViewDirWS; // Actually just 3x3, but Unity can only set 4x4
         dir = float3(dot(rotDirX, dir), dir.y, dot(rotDirY, dir));
 
         float3 skyColor = ClampToFloat16Max(SAMPLE_TEXTURECUBE_LOD(_Cubemap, sampler_Cubemap, dir, 0).rgb * exp2(_SkyParam.x) * _SkyParam.y);
+
+    #ifdef VOLUMETRIC_LIGHTING_ENABLED
+        PositionInputs posInput = GetPositionInput(input.positionCS.xy, _ScreenSize.zw);
+        float4 volumetricLighting = GetInScatteredRadianceAndTransmittance(posInput.positionSS,
+                                                                           _VBufferLighting,
+                                                                           s_linear_clamp_sampler,
+                                                                           _VBufferResolutionAndScale.zw);
+        // TODO: apply volumetrics after SSS.
+        // diffuseLighting  *= volumetricLighting.a;
+        skyColor *= volumetricLighting.a;
+        skyColor += volumetricLighting.rgb;
+    #endif
+
         return float4(skyColor, 1.0);
     }
 

Tests/GraphicsTests/RenderPipeline/HDRenderPipeline/Scenes/0xxx.meta

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