Add CascadeBlendSize to Directional Light and fix sun shadow fade at distance

Source/Engine/Level/Actors/DirectionalLight.cpp

@@ -48,6 +48,7 @@ void DirectionalLight::Draw(RenderContext& renderContext)
         data.Cascade2Spacing = Cascade2Spacing;
         data.Cascade3Spacing = Cascade3Spacing;
         data.Cascade4Spacing = Cascade4Spacing;
+        data.CascadeBlendSize = CascadeBlendSize;
         data.PartitionMode = PartitionMode;
         data.ContactShadowsLength = ContactShadowsLength;
         data.StaticFlags = GetStaticFlags();
@@ -69,6 +70,7 @@ void DirectionalLight::Serialize(SerializeStream& stream, const void* otherObj)
     SERIALIZE(Cascade2Spacing);
     SERIALIZE(Cascade3Spacing);
     SERIALIZE(Cascade4Spacing);
+    SERIALIZE(CascadeBlendSize);
     SERIALIZE(PartitionMode);
 }
 
@@ -82,6 +84,7 @@ void DirectionalLight::Deserialize(DeserializeStream& stream, ISerializeModifier
     DESERIALIZE(Cascade2Spacing);
     DESERIALIZE(Cascade3Spacing);
     DESERIALIZE(Cascade4Spacing);
+    DESERIALIZE(CascadeBlendSize);
     DESERIALIZE(PartitionMode);
 }
 

Source/Engine/Level/Actors/DirectionalLight.h

@@ -48,6 +48,12 @@ public:
     API_FIELD(Attributes="EditorOrder(69), DefaultValue(1.0f), VisibleIf(nameof(ShowCascade4)), Limit(0, 1, 0.001f), EditorDisplay(\"Shadow\")")
     float Cascade4Spacing = 1.0f;
 
+    /// <summary>
+    /// Percentage of the cascade distance over which cascades will blend together. This helps to hide the transition between cascades.
+    /// </summary>
+    API_FIELD(Attributes = "EditorOrder(70), EditorDisplay(\"Shadow\", \"Cascade Blend Distance\"), Limit(0.0f, 1.0f)")
+    float CascadeBlendSize = 0.1f;
+
 public:
     // [LightWithShadow]
     void Draw(RenderContext& renderContext) override;

Source/Engine/Renderer/RenderList.h

@@ -79,6 +79,7 @@ struct RenderDirectionalLightData : RenderLightData
 
     PartitionMode PartitionMode;
     int32 CascadeCount;
+    float CascadeBlendSize;
 
     RenderDirectionalLightData()
     {

Source/Engine/Renderer/ShadowsPass.cpp

@@ -195,7 +195,7 @@ struct ShadowAtlasLight
     bool BlendCSM;
     mutable StaticStates StaticState;
     BoundingSphere Bounds;
-    float Sharpness, Fade, NormalOffsetScale, Bias, FadeDistance, Distance, TileBorder;
+    float Sharpness, Fade, NormalOffsetScale, Bias, FadeDistance, Distance, TileBorder, CascadeBlendSize;
     Float4 CascadeSplits;
     ShadowAtlasLightTile Tiles[SHADOWS_MAX_TILES];
     ShadowAtlasLightCache Cache;
@@ -846,6 +846,7 @@ void ShadowsPass::SetupLight(ShadowsCustomBuffer& shadows, RenderContext& render
             cascadeSplits[i] = (cascadeSplits[i] - renderContext.View.Near) / viewRange;
     }
     atlasLight.CascadeSplits = renderContext.View.Near + Float4(cascadeSplits) * viewRange;
+    atlasLight.CascadeBlendSize = Math::Saturate(light.CascadeBlendSize);
 
     // Update cached state (invalidate it if the light changed)
     atlasLight.ValidateCache(renderContext.View, light);
@@ -917,10 +918,10 @@ void ShadowsPass::SetupLight(ShadowsCustomBuffer& shadows, RenderContext& render
 
         // Calculate cascade split frustum corners in view space
         Float3 cascadeCornersVs[8];
-        float csmOverlap = atlasLight.BlendCSM ? 0.2f : 0.1f;
+        float blendDistance = atlasLight.CascadeBlendSize;
         for (int32 j = 0; j < 4; j++)
         {
-            float overlapWithPrevSplit = csmOverlap * (splitMinRatio - oldSplitMinRatio);
+            float overlapWithPrevSplit = blendDistance * (splitMinRatio - oldSplitMinRatio);
             const Float3 frustumRangeVS = frustumCornersVs[j + 4] - frustumCornersVs[j];
             cascadeCornersVs[j] = frustumCornersVs[j] + frustumRangeVS * (splitMinRatio - overlapWithPrevSplit);
             cascadeCornersVs[j + 4] = frustumCornersVs[j] + frustumRangeVS * splitMaxRatio;
@@ -1430,7 +1431,7 @@ RETRY_ATLAS_SETUP:
         {
             // Shadow info
             auto* packed = shadows.ShadowsBuffer.WriteReserve<Float4>(2);
-            Color32 packed0x((byte)(atlasLight.Sharpness * (255.0f / 10.0f)), (byte)(atlasLight.Fade * 255.0f), (byte)atlasLight.TilesCount, 0);
+            Color32 packed0x((byte)(atlasLight.Sharpness * (255.0f / 10.0f)), (byte)(atlasLight.Fade * 255.0f), (byte)atlasLight.TilesCount, (byte)(atlasLight.CascadeBlendSize * 255.0f));
             packed[0] = Float4(*(const float*)&packed0x, atlasLight.FadeDistance, atlasLight.NormalOffsetScale, atlasLight.Bias);
             packed[1] = atlasLight.CascadeSplits;
         }

Source/Shaders/ShadowsCommon.hlsl

@@ -21,6 +21,7 @@ struct ShadowData
     float FadeDistance;
     float NormalOffsetScale;
     float Bias;
+    float CascadeBlendSize;
     uint TilesCount;
     float4 CascadeSplits;
 };
@@ -43,6 +44,7 @@ ShadowData LoadShadowsBuffer(Buffer<float4> shadowsBuffer, uint shadowsBufferAdd
     shadow.Sharpness = (packed0x & 0x000000ff) * (10.0f / 255.0f);
     shadow.Fade = ((packed0x & 0x0000ff00) >> 8) * (1.0f / 255.0f);
     shadow.TilesCount = ((packed0x & 0x00ff0000) >> 16);
+    shadow.CascadeBlendSize = ((packed0x & 0xff000000) >> 24) * (1.0f / 255.0f);
     shadow.FadeDistance = vector0.y;
     shadow.NormalOffsetScale = vector0.z;
     shadow.Bias = vector0.w;

Source/Shaders/ShadowsSampling.hlsl

@@ -293,11 +293,10 @@ ShadowSample SampleDirectionalLightShadow(LightData light, Buffer<float4> shadow
 	float splitDist = (nextSplit - viewDepth) / splitSize;
 #endif
 #if SHADOWS_CSM_DITHERING && !SHADOWS_CSM_BLENDING
-	const float BlendThreshold = 0.05f;
+    if (splitDist <= shadow.CascadeBlendSize && cascadeIndex != shadow.TilesCount - 1)
-    if (splitDist <= BlendThreshold && cascadeIndex != shadow.TilesCount - 1)
     {
         // Dither with the next cascade but with screen-space dithering (gets cleaned out by TAA)
-        float lerpAmount = 1 - splitDist / BlendThreshold;
+        float lerpAmount = 1 - splitDist / shadow.CascadeBlendSize;
         if (step(RandN2(gBuffer.ViewPos.xy + dither).x, lerpAmount))
             cascadeIndex++;
     }
@@ -312,17 +311,19 @@ ShadowSample SampleDirectionalLightShadow(LightData light, Buffer<float4> shadow
     result = SampleDirectionalLightShadowCascade(light, shadowsBuffer, shadowMap, gBuffer, shadow, samplePosition, cascadeIndex);
 
 #if SHADOWS_CSM_BLENDING
-	const float BlendThreshold = 0.1f;
+    if (splitDist <= shadow.CascadeBlendSize && cascadeIndex != shadow.TilesCount - 1)
-    if (splitDist <= BlendThreshold && cascadeIndex != shadow.TilesCount - 1)
     {
 	    // Sample the next cascade, and blend between the two results to smooth the transition
         ShadowSample nextResult = SampleDirectionalLightShadowCascade(light, shadowsBuffer, shadowMap, gBuffer, shadow, samplePosition, cascadeIndex + 1);
-		float blendAmount = splitDist / BlendThreshold;
+		float blendAmount = splitDist / shadow.CascadeBlendSize;
 		result.SurfaceShadow = lerp(nextResult.SurfaceShadow, result.SurfaceShadow, blendAmount);
 		result.TransmissionShadow = lerp(nextResult.TransmissionShadow, result.TransmissionShadow, blendAmount);
     }
 #endif
 
+	result.SurfaceShadow = lerp(result.SurfaceShadow, 1, fade);
+	result.TransmissionShadow = lerp(result.TransmissionShadow, 1, fade);
+
     return result;
 }