Add dithering to Volumetric Fog to reduce aliasing

Content/Editor/MaterialTemplates/Features/ForwardShading.hlsl

@@ -20,6 +20,7 @@ LightData DirectionalLight;
 LightData SkyLight;
 EnvProbeData EnvironmentProbe;
 ExponentialHeightFogData ExponentialHeightFog;
+VolumetricFogData VolumetricFog;
 float3 Dummy2;
 uint LocalLightsCount;
 LightData LocalLights[MAX_LOCAL_LIGHTS];
@@ -35,6 +36,7 @@ LightData GetDirectionalLight() { return DirectionalLight; }
 LightData GetSkyLight() { return SkyLight; }
 EnvProbeData GetEnvironmentProbe() { return EnvironmentProbe; }
 ExponentialHeightFogData GetExponentialHeightFog() { return ExponentialHeightFog; }
+VolumetricFogData GetVolumetricFog() { return VolumetricFog; }
 uint GetLocalLightsCount() { return LocalLightsCount; }
 LightData GetLocalLight(uint i) { return LocalLights[i]; }
 @5// Forward Shading: Shaders
@@ -164,7 +166,7 @@ void PS_Forward(
 	{
 		// Sample volumetric fog and mix it in
 		float2 screenUV = materialInput.SvPosition.xy * ScreenSize.zw;
-		float4 volumetricFog = SampleVolumetricFog(VolumetricFogTexture, ExponentialHeightFog.VolumetricFogGrid, materialInput.WorldPosition - ViewPos, screenUV);
+		float4 volumetricFog = SampleVolumetricFog(VolumetricFogTexture, VolumetricFog, materialInput.WorldPosition - ViewPos, screenUV, TemporalAAJitter);
 		fog = CombineVolumetricFog(fog, volumetricFog);
 	}
 

Source/Engine/Graphics/Materials/MaterialShaderFeatures.cpp

@@ -29,7 +29,8 @@ void ForwardShadingFeature::Bind(MaterialShader::BindParameters& params, Span<by
     const bool canUseShadow = view.Pass != DrawPass::Depth;
 
     // Set fog input
-    data.ExponentialHeightFog = cache->Fog.ExponentialHeightFog;
+    data.ExponentialHeightFog = cache->Fog.ExponentialHeightFogData;
+    data.VolumetricFogData = cache->Fog.VolumetricFogData;
     params.GPUContext->BindSR(volumetricFogTextureRegisterIndex, cache->Fog.VolumetricFogTexture);
 
     // Set directional light input

Source/Engine/Graphics/Materials/MaterialShaderFeatures.h

@@ -33,6 +33,7 @@ struct ForwardShadingFeature : MaterialShaderFeature
         ShaderLightData SkyLight;
         ShaderEnvProbeData EnvironmentProbe;
         ShaderExponentialHeightFogData ExponentialHeightFog;
+        ShaderVolumetricFogData VolumetricFogData;
         Float3 Dummy2;
         uint32 LocalLightsCount;
         ShaderLightData LocalLights[MaxLocalLights];

Source/Engine/Level/Actors/ExponentialHeightFog.cpp

@@ -182,6 +182,8 @@ void ExponentialHeightFog::GetExponentialHeightFogData(const RenderView& view, S
 GPU_CB_STRUCT(Data {
     ShaderGBufferData GBuffer;
     ShaderExponentialHeightFogData ExponentialHeightFog;
+    ShaderVolumetricFogData VolumetricFog;
+    Float4 TemporalAAJitter;
     });
 
 void ExponentialHeightFog::DrawFog(GPUContext* context, RenderContext& renderContext, GPUTextureView* output)
@@ -193,7 +195,9 @@ void ExponentialHeightFog::DrawFog(GPUContext* context, RenderContext& renderCon
     // Setup shader inputs
     Data data;
     GBufferPass::SetInputs(renderContext.View, data.GBuffer);
-    data.ExponentialHeightFog = renderContext.List->Fog.ExponentialHeightFog;
+    data.ExponentialHeightFog = renderContext.List->Fog.ExponentialHeightFogData;
+    data.VolumetricFog = renderContext.List->Fog.VolumetricFogData;
+    data.TemporalAAJitter = renderContext.View.TemporalAAJitter;
     auto cb = _shader->GetShader()->GetCB(0);
     ASSERT_LOW_LAYER(cb->GetSize() == sizeof(Data));
     context->UpdateCB(cb, &data);

Source/Engine/Renderer/Config.h

@@ -42,8 +42,15 @@ GPU_CB_STRUCT(ShaderExponentialHeightFogData {
     float VolumetricFogMaxDistance;
     float DirectionalInscatteringStartDistance;
     float StartDistance;
+    });
 
-    Float4 VolumetricFogGrid;
+/// <summary>
+/// Structure that contains information about volumetric fog
+/// </summary>
+GPU_CB_STRUCT(ShaderVolumetricFogData {
+    Float4 GridSliceParameters;
+    Float2 ScreenSize;
+    Float2 VolumeTexelSize; // Scaled for dithering
     });
 
 /// <summary>

Source/Engine/Renderer/RenderList.cpp

@@ -183,19 +183,20 @@ RenderFogData::RenderFogData()
 {
     Renderer = nullptr;
     VolumetricFogTexture = nullptr;
-    ExponentialHeightFog.FogMinOpacity = 1.0f;
-    ExponentialHeightFog.FogDensity = 0.0f;
-    ExponentialHeightFog.FogCutoffDistance = 0.1f;
-    ExponentialHeightFog.StartDistance = 0.0f;
-    ExponentialHeightFog.ApplyDirectionalInscattering = 0.0f;
-    ExponentialHeightFog.VolumetricFogMaxDistance = -1.0f;
-    ExponentialHeightFog.VolumetricFogGrid = Float4::One;
+    ExponentialHeightFogData.FogMinOpacity = 1.0f;
+    ExponentialHeightFogData.FogDensity = 0.0f;
+    ExponentialHeightFogData.FogCutoffDistance = 0.1f;
+    ExponentialHeightFogData.StartDistance = 0.0f;
+    ExponentialHeightFogData.ApplyDirectionalInscattering = 0.0f;
+    ExponentialHeightFogData.VolumetricFogMaxDistance = -1.0f;
+    VolumetricFogData.GridSliceParameters = Float4::One;
+    VolumetricFogData.ScreenSize = VolumetricFogData.VolumeTexelSize = Float2::Zero;
 }
 
 void RenderFogData::Init(const RenderView& view, IFogRenderer* renderer)
 {
     Renderer = renderer;
-    renderer->GetExponentialHeightFogData(view, ExponentialHeightFog);
+    renderer->GetExponentialHeightFogData(view, ExponentialHeightFogData);
     renderer->GetVolumetricFogOptions(VolumetricFog);
 }
 

Source/Engine/Renderer/RenderList.h

@@ -179,7 +179,8 @@ struct RenderFogData
 {
     IFogRenderer* Renderer;
     GPUTextureView* VolumetricFogTexture;
-    ShaderExponentialHeightFogData ExponentialHeightFog;
+    ShaderExponentialHeightFogData ExponentialHeightFogData;
+    ShaderVolumetricFogData VolumetricFogData;
     VolumetricFogOptions VolumetricFog;
 
     RenderFogData();

Source/Engine/Renderer/VolumetricFogPass.cpp

@@ -679,8 +679,11 @@ void VolumetricFogPass::Render(RenderContext& renderContext)
     renderContext.Buffers->LastFrameVolumetricFog = Engine::FrameCount;
 
     // Update fog to be used by other passes
+    const float ditherNoiseScale = 0.2f; // Scales noise in SampleVolumetricFog
     renderContext.List->Fog.VolumetricFogTexture = integratedLightScattering->ViewVolume();
-    renderContext.List->Fog.ExponentialHeightFog.VolumetricFogGrid = cache.Data.GridSliceParameters;
+    renderContext.List->Fog.VolumetricFogData.GridSliceParameters = cache.Data.GridSliceParameters;
+    renderContext.List->Fog.VolumetricFogData.ScreenSize = renderContext.Buffers->GetSize();
+    renderContext.List->Fog.VolumetricFogData.VolumeTexelSize = Float2(1.0f / cache.GridSize.X, 1.0f / cache.GridSize.Y) * ditherNoiseScale;
 
     groupCountX = Math::DivideAndRoundUp((int32)cache.GridSize.X, VolumetricFogIntegrationGroupSize);
     groupCountY = Math::DivideAndRoundUp((int32)cache.GridSize.Y, VolumetricFogIntegrationGroupSize);

Source/Shaders/ExponentialHeightFog.hlsl

@@ -27,8 +27,6 @@ struct ExponentialHeightFogData
     float VolumetricFogMaxDistance;
     float DirectionalInscatteringStartDistance;
     float StartDistance;
-
-    float4 VolumetricFogGrid;
 };
 
 float4 GetExponentialHeightFog(ExponentialHeightFogData exponentialHeightFog, float3 posWS, float3 camWS, float skipDistance, float sceneDistance)

Source/Shaders/Fog.shader

@@ -16,6 +16,8 @@
 META_CB_BEGIN(0, Data)
 GBufferData GBuffer;
 ExponentialHeightFogData ExponentialHeightFog;
+VolumetricFogData VolumetricFog;
+float4 TemporalAAJitter;
 META_CB_END
 
 DECLARE_GBUFFERDATA_ACCESS(GBuffer)
@@ -46,7 +48,7 @@ float4 PS_Fog(Quad_VS2PS input) : SV_Target0
 
 #if VOLUMETRIC_FOG
     // Sample volumetric fog and mix it in
-	float4 volumetricFog = SampleVolumetricFog(VolumetricFogTexture, ExponentialHeightFog.VolumetricFogGrid, worldPos - GBuffer.ViewPos, input.TexCoord);
+	float4 volumetricFog = SampleVolumetricFog(VolumetricFogTexture, VolumetricFog, worldPos - GBuffer.ViewPos, input.TexCoord, TemporalAAJitter);
 	fog = CombineVolumetricFog(fog, volumetricFog);
 #endif
 

Source/Shaders/VolumetricFog.hlsl

@@ -3,8 +3,18 @@
 #ifndef __VOLUMETRIC_FOG__
 #define __VOLUMETRIC_FOG__
 
+#include "./Flax/Noise.hlsl"
+
 #define VOLUMETRIC_FOG_GRID_Z_LINEAR 1
 
+// Structure that contains information about volumetric fog
+struct VolumetricFogData
+{
+    float4 GridSliceParameters;
+    float2 ScreenSize;
+    float2 VolumeTexelSize; // Scaled for dithering
+};
+
 float GetDepthFromSlice(float4 gridSliceParameters, float zSlice)
 {
 #if VOLUMETRIC_FOG_GRID_Z_LINEAR
@@ -23,11 +33,19 @@ float GetSliceFromDepth(float4 gridSliceParameters, float sceneDepth)
 #endif
 }
 
-float4 SampleVolumetricFog(Texture3D volumetricFogTexture, float4 gridSliceParameters, float3 viewVector, float2 uv)
+float4 SampleVolumetricFog(Texture3D volumetricFogTexture, VolumetricFogData volumetricFogData, float3 viewVector, float2 uv, float4 temporalAAJitter = 0)
 {
+    // Project view vector to get 3D frustum UVW coordinates
     float sceneDepth = length(viewVector);
-    float zSlice = GetSliceFromDepth(gridSliceParameters, sceneDepth) * gridSliceParameters.w;
+    float zSlice = GetSliceFromDepth(volumetricFogData.GridSliceParameters, sceneDepth) * volumetricFogData.GridSliceParameters.w;
     float3 volumeUV = float3(uv, zSlice);
+
+    // Dither to reduce banding artifacts
+    float2 noiseUV = volumeUV.xy + temporalAAJitter.xy;
+    float2 noise = rand2dTo2d(noiseUV * volumetricFogData.ScreenSize) * 2.0f - 1.0f;
+    volumeUV.xy += noise * volumetricFogData.VolumeTexelSize;
+
+    // Sample 3D texture
     return volumetricFogTexture.SampleLevel(SamplerLinearClamp, volumeUV, 0);
 }