Fix artifacts in volumetric lighting near opaque geometry (#493)

* Add view bias to volumetrics

* Implement geometry-distance-aware volume rendering

* Comment

* Fix the flickering issue

* Clean

* Fix

* Changelog

* Implement lookup bias for volumetrics

* Update CHANGELOG.md

* update test filter

* fix refernce screenshots

Co-authored-by: sebastienlagarde <[email protected]>

Author: EvgeniiG

TestProjects/HDRP_Tests/Assets/GraphicTests/Scenes/1x_Materials/1204_Lit_Fog.unity

@@ -661,6 +661,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed an exception happening when using RTSSS without using RTShadows.
 - Fix inconsistencies with transparent motion vectors and opaque by allowing camera only transparent motion vectors.
 - Fix reflection probe frame settings override
+- Fixed certain shadow bias artifacts present in volumetric lighting (case 1231885).
 
 ### Changed
 - Improve MIP selection for decals on Transparents

TestProjects/HDRP_Tests/Assets/GraphicTests/Scenes/5x_SkyAndFog/5001_Fog_FogFallback.unity

@@ -279,13 +279,13 @@ void EvaluateAtmosphericScattering(PositionInputs posInput, float3 V, out float3
         {
             float4 value = SampleVBuffer(TEXTURE3D_ARGS(_VBufferLighting, s_linear_clamp_sampler),
                                          posInput.positionNDC,
-                                         fogFragDist,   
+                                         fogFragDist,
                                          _VBufferViewportSize,
                                          _VBufferLightingViewportScale.xyz,
                                          _VBufferLightingViewportLimit.xyz,
                                          _VBufferDistanceEncodingParams,
                                          _VBufferDistanceDecodingParams,
-                                         true, false);
+                                         true, true, false);
 
             // TODO: add some slowly animated noise (dither?) to the reconstructed value.
             // TODO: re-enable tone mapping after implementing pre-exposure.

TestProjects/HDRP_Tests/Assets/ReferenceImages/Linear/OSXEditor/Metal/None/5008_FogFiltering.png

@@ -3,8 +3,12 @@
 
 #define SHADOW_OPTIMIZE_REGISTER_USAGE 1
 
-#ifndef SHADOW_USE_DEPTH_BIAS
-#define SHADOW_USE_DEPTH_BIAS                   1   // Enable clip space z biasing
+#ifndef SHADOW_AUTO_FLIP_NORMAL   // If (NdotL < 0), we flip the normal to correctly bias lit back-faces (used for transmission)
+#define SHADOW_AUTO_FLIP_NORMAL 1 // Externally define as 0 to disable
+#endif
+
+#ifndef SHADOW_USE_DEPTH_BIAS     // Enable clip space z biasing
+#define SHADOW_USE_DEPTH_BIAS   1 // Externally define as 0 to disable
 #endif
 
 #if SHADOW_OPTIMIZE_REGISTER_USAGE == 1
@@ -16,8 +20,9 @@
 // normalWS is the vertex normal if available or shading normal use to bias the shadow position
 float GetDirectionalShadowAttenuation(HDShadowContext shadowContext, float2 positionSS, float3 positionWS, float3 normalWS, int shadowDataIndex, float3 L)
 {
-    // If NdotL < 0, we flip the normal in case it is used for the transmission to correctly bias shadow position
+#if SHADOW_AUTO_FLIP_NORMAL
     normalWS *= FastSign(dot(normalWS, L));
+#endif
 #if defined(SHADOW_LOW) || defined(SHADOW_MEDIUM)
     return EvalShadow_CascadedDepth_Dither(shadowContext, _ShadowmapCascadeAtlas, s_linear_clamp_compare_sampler, positionSS, positionWS, normalWS, shadowDataIndex, L);
 #else
@@ -31,8 +36,9 @@ float GetPunctualShadowAttenuation(HDShadowContext shadowContext, float2 positio
     shadowDataIndex = WaveReadLaneFirst(shadowDataIndex);
 #endif
 
-    // If NdotL < 0, we flip the normal in case it is used for the transmission to correctly bias shadow position
+#if SHADOW_AUTO_FLIP_NORMAL
     normalWS *= FastSign(dot(normalWS, L));
+#endif
 
     // Note: Here we assume that all the shadow map cube faces have been added contiguously in the buffer to retreive the shadow information
     HDShadowData sd = shadowContext.shadowDatas[shadowDataIndex];

TestProjects/HDRP_Tests/Assets/ReferenceImages/Linear/WindowsEditor/Direct3D11/None/5003_Fog_DensityVolumesShadows.png

@@ -12,6 +12,8 @@
 // if (clampToBorder), samples outside of the buffer return 0 (border color).
 // Otherwise, the sampler simply clamps the texture coordinate to the edge of the texture.
 // Warning: clamping to border may not work as expected with the quadratic filter due to its extent.
+//
+// if (biasLookup), we apply a constant bias to the look-up to avoid light leaks through geometry.
 float4 SampleVBuffer(TEXTURE3D_PARAM(VBuffer, clampSampler),
                      float2 positionNDC,
                      float  linearDistance,
@@ -20,13 +22,22 @@ float4 SampleVBuffer(TEXTURE3D_PARAM(VBuffer, clampSampler),
                      float3 VBufferViewportLimit,
                      float4 VBufferDistanceEncodingParams,
                      float4 VBufferDistanceDecodingParams,
+                     bool   biasLookup,
                      bool   quadraticFilterXY,
                      bool   clampToBorder)
 {
     // These are the viewport coordinates.
     float2 uv = positionNDC;
     float  w  = EncodeLogarithmicDepthGeneralized(linearDistance, VBufferDistanceEncodingParams);
 
+    if (biasLookup)
+    {
+        // The value is higher than 0.5 (we use half of the length the diagonal of a unit cube).
+        // to account for varying angles of incidence.
+        // TODO: XR?
+        w -= (sqrt(3)/2) * _VBufferRcpSliceCount;
+    }
+
     bool coordIsInsideFrustum;
 
     if (clampToBorder)
@@ -79,6 +90,7 @@ float4 SampleVBuffer(TEXTURE3D_PARAM(VBuffer, clampSampler),
 
             // The sampler clamps to the edge (so UVWs < 0 are OK).
             // TODO: perform per-sample (4, in this case) bilateral filtering, rather than per-pixel. This should reduce leaking.
+            // Currently we don't do it, since it is expensive and doesn't appear to be helpful/necessary in practice.
             result = (weights[0].x * weights[0].y) * SAMPLE_TEXTURE3D_LOD(VBuffer, clampSampler, min(float3(texUv0, texW), VBufferViewportLimit), 0)
                    + (weights[1].x * weights[0].y) * SAMPLE_TEXTURE3D_LOD(VBuffer, clampSampler, min(float3(texUv1, texW), VBufferViewportLimit), 0)
                    + (weights[0].x * weights[1].y) * SAMPLE_TEXTURE3D_LOD(VBuffer, clampSampler, min(float3(texUv2, texW), VBufferViewportLimit), 0)
@@ -105,6 +117,7 @@ float4 SampleVBuffer(TEXTURE3D_PARAM(VBuffer, clampSampler),
                      float3   VBufferViewportLimit,
                      float4   VBufferDistanceEncodingParams,
                      float4   VBufferDistanceDecodingParams,
+                     bool     biasLookup,
                      bool     quadraticFilterXY,
                      bool     clampToBorder)
 {
@@ -119,6 +132,7 @@ float4 SampleVBuffer(TEXTURE3D_PARAM(VBuffer, clampSampler),
                          VBufferViewportLimit,
                          VBufferDistanceEncodingParams,
                          VBufferDistanceDecodingParams,
+                         biasLookup,
                          quadraticFilterXY,
                          clampToBorder);
 }

TestProjects/HDRP_Tests/Assets/ReferenceImages/Linear/WindowsEditor/Direct3D12/None/5008_FogFiltering.png

@@ -28,10 +28,14 @@
     #define VBUFFER_VOXEL_SIZE 8
 #endif
 
-#define GROUP_SIZE_1D         8
-#define SUPPORT_LOCAL_LIGHTS  1 // Local lights contribute to fog lighting
-#define SHADOW_USE_DEPTH_BIAS 0
-#define PREFER_HALF           0
+#define PREFER_HALF             0
+#define GROUP_SIZE_1D           8
+#define SUPPORT_LOCAL_LIGHTS    1 // Local lights contribute to fog lighting
+#define SHADOW_USE_DEPTH_BIAS   0 // Too expensive, not particularly effective
+#define SHADOW_LOW                // Too expensive
+#define SHADOW_AUTO_FLIP_NORMAL 0 // No normal information, so no need to flip
+#define SHADOW_VIEW_BIAS        1 // Prevents light leaking through thin geometry. Not as good as normal bias at grazing angles, but cheaper and independent from the geometry.
+#define USE_DEPTH_BUFFER        1 // Accounts for opaque geometry along the camera ray
 
 //--------------------------------------------------------------------------------------------------
 // Included headers
@@ -56,8 +60,6 @@
 
 #include "Packages/com.unity.render-pipelines.high-definition/Runtime/Sky/PhysicallyBasedSky/PhysicallyBasedSkyCommon.hlsl"
 
-// Use low quality shadow when doing volumetric
-#define SHADOW_LOW
 #include "Packages/com.unity.render-pipelines.high-definition/Runtime/Lighting/Lighting.hlsl"
 #include "Packages/com.unity.render-pipelines.high-definition/Runtime/Lighting/LightLoop/LightLoopDef.hlsl"
 #include "Packages/com.unity.render-pipelines.high-definition/Runtime/Lighting/LightEvaluation.hlsl"
@@ -83,6 +85,7 @@ struct JitteredRay
     float3 jitterDirWS;
     float3 xDirDerivWS;
     float3 yDirDerivWS;
+    float  geomDist;
 };
 
 struct VoxelLighting
@@ -91,6 +94,11 @@ struct VoxelLighting
     float3 radianceNoPhase;
 };
 
+bool IsInRange(float x, float2 range)
+{
+    return clamp(x, range.x, range.y) == x;
+}
+
 float ComputeHistoryWeight()
 {
     // Compute the exponential moving average over 'n' frames:
@@ -149,9 +157,13 @@ VoxelLighting EvaluateVoxelLightingDirectional(LightLoopContext context, uint fe
         // Is it worth sampling the shadow map?
         if ((light.volumetricLightDimmer > 0) && (light.volumetricShadowDimmer > 0))
         {
-            // Pass the light direction as the normal to avoid issues with shadow biasing code.
-            // TODO: does it make sense?
-            float3 shadowN = L;
+            #if SHADOW_VIEW_BIAS
+                // Our shadows only support normal bias. Volumetrics has no access to the surface normal.
+                // We fake view bias by invoking the normal bias code with the view direction.
+                float3 shadowN = -ray.jitterDirWS;
+            #else
+                float3 shadowN = 0; // No bias
+            #endif // SHADOW_VIEW_BIAS
 
             context.shadowValue = GetDirectionalShadowAttenuation(context.shadowContext,
                                                                   posInput.positionSS, posInput.positionWS, shadowN,
@@ -182,9 +194,13 @@ VoxelLighting EvaluateVoxelLightingDirectional(LightLoopContext context, uint fe
             lightColor.a *= light.volumetricLightDimmer;
             lightColor.rgb *= lightColor.a; // Composite
 
-            // Pass the light direction as the normal to avoid issues with shadow biasing code.
-            // TODO: does it make sense?
-            float3 shadowN = L;
+            #if SHADOW_VIEW_BIAS
+                // Our shadows only support normal bias. Volumetrics has no access to the surface normal.
+                // We fake view bias by invoking the normal bias code with the view direction.
+                float3 shadowN = -ray.jitterDirWS;
+            #else
+                float3 shadowN = 0; // No bias
+            #endif // SHADOW_VIEW_BIAS
 
             // This code works for both surface reflection and thin object transmission.
             DirectionalShadowType shadow = EvaluateShadow_Directional(context, posInput, light, unused, shadowN);
@@ -298,9 +314,13 @@ VoxelLighting EvaluateVoxelLightingLocal(LightLoopContext context, uint featureF
                 lightColor.a *= light.volumetricLightDimmer;
                 lightColor.rgb *= lightColor.a; // Composite
 
-                // Pass the light direction as the normal to avoid issues with shadow biasing code.
-                // TODO: does it make sense?
-                float3 shadowN = L;
+            #if SHADOW_VIEW_BIAS
+                // Our shadows only support normal bias. Volumetrics has no access to the surface normal.
+                // We fake view bias by invoking the normal bias code with the view direction.
+                float3 shadowN = -ray.jitterDirWS;
+            #else
+                float3 shadowN = 0; // No bias
+            #endif // SHADOW_VIEW_BIAS
 
                 float shadow = EvaluateShadow_Punctual(context, posInput, light, unused, shadowN, L, distances);
                 lightColor.rgb *= ComputeShadowColor(shadow, light.shadowTint, light.penumbraTint);
@@ -379,9 +399,13 @@ VoxelLighting EvaluateVoxelLightingLocal(LightLoopContext context, uint featureF
                 lightColor.a *= light.volumetricLightDimmer;
                 lightColor.rgb *= lightColor.a; // Composite
 
-                // Pass the light direction as the normal to avoid issues with shadow biasing code.
-                // TODO: does it make sense?
-                float3 shadowN = L;
+            #if SHADOW_VIEW_BIAS
+                // Our shadows only support normal bias. Volumetrics has no access to the surface normal.
+                // We fake view bias by invoking the normal bias code with the view direction.
+                float3 shadowN = -ray.jitterDirWS;
+            #else
+                float3 shadowN = 0; // No bias
+            #endif // SHADOW_VIEW_BIAS
 
                 float shadow = EvaluateShadow_Punctual(context, posInput, light, unused, shadowN, L, distances);
                 lightColor.rgb *= ComputeShadowColor(shadow, light.shadowTint, light.penumbraTint);
@@ -474,7 +498,22 @@ void FillVolumetricLightingBuffer(LightLoopContext context, uint featureFlags,
 
         float e1 = slice * de + de; // (slice + 1) / sliceCount
         float t1 = DecodeLogarithmicDepthGeneralized(e1, _VBufferDistanceDecodingParams);
-        float dt = t1 - t0;
+        float tNext = t1;
+
+    #if USE_DEPTH_BUFFER
+        bool containsOpaqueGeometry = IsInRange(ray.geomDist, float2(t0, t1));
+
+        if (containsOpaqueGeometry)
+        {
+            // Only integrate up to the opaque surface (make the voxel shorter, but not completely flat).
+            // Note that we can NOT completely stop integrating when the ray reaches geometry, since
+            // otherwise we get flickering at geometric discontinuities if reprojection is enabled.
+            // In this case, a temporally stable light leak is better than flickering.
+            t1 = max(t0 * 1.0001, ray.geomDist);
+        }
+    #endif
+
+        float dt = t1 - t0; // Is geometry-aware
 
         // Accurately compute the center of the voxel in the log space. It's important to perform
         // the inversion exactly, since the accumulated value of the integral is stored at the center.
@@ -553,7 +592,7 @@ void FillVolumetricLightingBuffer(LightLoopContext context, uint featureFlags,
                                            _VBufferHistoryViewportLimit.xyz,
                                            _VBufferPrevDistanceEncodingParams,
                                            _VBufferPrevDistanceDecodingParams,
-                                           false, true) * float4(GetInversePreviousExposureMultiplier().xxx, 1);
+                                           false, false, true) * float4(GetInversePreviousExposureMultiplier().xxx, 1);
 
         bool reprojSuccess = (_VBufferHistoryIsValid != 0) && (reprojValue.a != 0);
 
@@ -592,7 +631,7 @@ void FillVolumetricLightingBuffer(LightLoopContext context, uint featureFlags,
         blendValue.rgb *= phaseCurrFrame;
     #endif // ENABLE_ANISOTROPY
 
-    #else // ENABLE_REPROJECTION
+    #else // NO REPROJECTION
 
     #ifdef ENABLE_ANISOTROPY
         float4 blendValue = float4(aggregateLighting.radianceComplete, extinction * dt);
@@ -615,6 +654,7 @@ void FillVolumetricLightingBuffer(LightLoopContext context, uint featureFlags,
         // Integral{a, b}{Transmittance(0, t) * L_s(t) dt} = Transmittance(0, a) * Integral{a, b}{Transmittance(0, t - a) * L_s(t) dt}.
         float3 probeRadiance = probeInScatteredRadiance * TransmittanceIntegralHomogeneousMedium(extinction, dt);
 
+        // Accumulate radiance along the ray.
         totalRadiance += transmittance * scattering * (phase * blendValue.rgb + probeRadiance);
 
         // Compute the optical depth up to the center of the interval.
@@ -630,7 +670,7 @@ void FillVolumetricLightingBuffer(LightLoopContext context, uint featureFlags,
         // Compute the optical depth up to the end of the interval.
         opticalDepth += 0.5 * blendValue.a;
 
-        t0 = t1;
+        t0 = tNext;
     }
 }
 
@@ -643,17 +683,6 @@ void VolumetricLighting(uint3 dispatchThreadId : SV_DispatchThreadID,
 
     uint2 groupOffset = groupId * GROUP_SIZE_1D;
     uint2 voxelCoord  = groupOffset + groupThreadId;
-#ifdef VL_PRESET_OPTIMAL
-    // The entire thread group is within the same light tile.
-    uint2 tileCoord = groupOffset * VBUFFER_VOXEL_SIZE / TILE_SIZE_BIG_TILE;
-#else
-    // No compile-time optimizations, no scalarization.
-    // If _VBufferVoxelSize is not a power of 2 or > TILE_SIZE_BIG_TILE, a voxel may straddle
-    // a tile boundary. This means different voxel subsamples may belong to different tiles.
-    // We accept this error, and simply use the coordinates of the center of the voxel.
-    uint2 tileCoord = (uint2)((voxelCoord + 0.5) * _VBufferVoxelSize / TILE_SIZE_BIG_TILE);
-#endif
-    uint  tileIndex = tileCoord.x + _NumTileBigTileX * tileCoord.y;
 
     // Reminder: our voxels are sphere-capped right frustums (truncated right pyramids).
     // The curvature of the front and back faces is quite gentle, so we can use
@@ -665,6 +694,7 @@ void VolumetricLighting(uint3 dispatchThreadId : SV_DispatchThreadID,
 
     float3 F = GetViewForwardDir();
     float3 U = GetViewUpDir();
+    float3 R = cross(F, U);
 
     float2 centerCoord = voxelCoord + float2(0.5, 0.5);
 
@@ -685,14 +715,45 @@ void VolumetricLighting(uint3 dispatchThreadId : SV_DispatchThreadID,
     ray.yDirDerivWS = cross(ray.xDirDerivWS, ray.centerDirWS); // Will have the length of 'unitDistFaceSize' by construction
 
 #ifdef ENABLE_REPROJECTION
-    ray.jitterDirWS = normalize(ray.centerDirWS + _VBufferSampleOffset.x * ray.xDirDerivWS
-                                                + _VBufferSampleOffset.y * ray.yDirDerivWS);
+    float2 sampleOffset = _VBufferSampleOffset.xy;
 #else
-    ray.jitterDirWS = ray.centerDirWS;
+    float2 sampleOffset = 0;
 #endif
 
+    ray.jitterDirWS = normalize(ray.centerDirWS + sampleOffset.x * ray.xDirDerivWS
+                                                + sampleOffset.y * ray.yDirDerivWS);
+
+    // We would like to determine the screen pixel (at the full resolution) which
+    // the jittered ray corresponds to. The exact solution can be obtained by intersecting
+    // the ray with the screen plane, e.i. (ViewSpace(jitterDirWS).z = 1). That's a little expensive.
+    // So, as an approximation, we ignore the curvature of the frustum.
+    uint2 pixelCoord = (uint2)((voxelCoord + 0.5 + sampleOffset) * _VBufferVoxelSize);
+
+#ifdef VL_PRESET_OPTIMAL
+    // The entire thread group is within the same light tile.
+    uint2 tileCoord = groupOffset * VBUFFER_VOXEL_SIZE / TILE_SIZE_BIG_TILE;
+#else
+    // No compile-time optimizations, no scalarization.
+    uint2 tileCoord = pixelCoord / TILE_SIZE_BIG_TILE;
+#endif
+    uint  tileIndex = tileCoord.x + _NumTileBigTileX * tileCoord.y;
+
+    // Do not jitter 'voxelCoord' else. It's expected to correspond to the center of the voxel.
     PositionInputs posInput = GetPositionInput(voxelCoord, _VBufferViewportSize.zw, tileCoord);
 
+    ray.geomDist = FLT_INF;
+
+#if USE_DEPTH_BUFFER
+    float deviceDepth = LoadCameraDepth(pixelCoord);
+
+    if (deviceDepth > 0) // Skip the skybox
+    {
+        // Convert it to distance along the ray. Doesn't work with tilt shift, etc.
+        float linearDepth = LinearEyeDepth(deviceDepth, _ZBufferParams);
+        ray.geomDist = linearDepth * rcp(dot(ray.jitterDirWS, F));
+    }
+#endif
+
     // TODO
     LightLoopContext context;
     context.shadowContext = InitShadowContext();