Implement Beer's law fog

Assets/ScriptableRenderPipeline/HDRenderPipeline/Lighting/Volumetrics/Resources/VolumetricLighting.compute

 #include "../HomogeneousFog.cs.hlsl"
 #include "../../../../Core/ShaderLibrary/Common.hlsl"
 #include "../../../../Core/ShaderLibrary/SpaceFillingCurves.hlsl"
+#include "../../../../Core/ShaderLibrary/VolumeRendering.hlsl"
 #include "../../../ShaderVariables.hlsl"
 
 //--------------------------------------------------------------------------------------------------
     uint2 tileAnchor = groupId * GROUP_SIZE_1D;
     uint2 pixelCoord = tileAnchor + localCoord;
 
+    PositionInputs posInput = GetPositionInput(pixelCoord, _ScreenSize.zw);
+
+    float z = LOAD_TEXTURE2D(_DepthTexture, pixelCoord).r;
+    float t = length(ComputeViewSpacePosition(posInput.positionSS, z, _InvProjMatrix));
+
+    float3 transmittance = Transmittance(OpticalDepthHomogeneous(_UnboundedVolume[0].extinction, t));
+
-    _LightingTexture[pixelCoord] += float4(0.25, 0.25, 0.25, 0);
+    _LightingTexture[pixelCoord] = float4(transmittance * _LightingTexture[pixelCoord].rgb, _LightingTexture[pixelCoord].a);
 }

Assets/ScriptableRenderPipeline/Core/ShaderLibrary/VolumeRendering.hlsl

+#ifndef UNITY_VOLUME_RENDERING_INCLUDED
+#define UNITY_VOLUME_RENDERING_INCLUDED
+
+float3 OpticalDepthHomogeneous(float3 extinction, float intervalLength)
+{
+    return extinction * intervalLength;
+}
+
+float3 Transmittance(float3 opticalDepth)
+{
+    return exp(-opticalDepth);
+}
+
+#endif // UNITY_VOLUME_RENDERING_INCLUDED

Assets/ScriptableRenderPipeline/Core/ShaderLibrary/VolumeRendering.hlsl.meta

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