Fix shadow mask fade and optimize it at same time

com.unity.render-pipelines.high-definition/CHANGELOG.md

@@ -23,6 +23,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed ambient occlusion strenght incorrectly using GTAOMultiBounce
 - Fixed range compression factor being clamped. (case 1365707)
 - Fixed tooltip not showing on labels in ShaderGraphs (1358483).
+- Fixed and optimize distance shadowmask fade.
 
 ## [13.0.0] - 2021-09-01
 

com.unity.render-pipelines.high-definition/Runtime/Lighting/LightDefinition.cs

@@ -95,6 +95,8 @@ struct DirectionalLightData
         [SurfaceDataAttributes(precision = FieldPrecision.Real)]
         public Vector4 shadowMaskSelector;      // Used with ShadowMask feature
 
+        public Vector2 cascadesBorderFadeScaleBias;
+
         public float diffuseDimmer;
         public float specularDimmer;
         public float penumbraTint;

com.unity.render-pipelines.high-definition/Runtime/Lighting/LightDefinition.cs.hlsl

@@ -72,6 +72,7 @@ struct DirectionalLightData
     real minRoughness;
     int screenSpaceShadowIndex;
     real4 shadowMaskSelector;
+    float2 cascadesBorderFadeScaleBias;
     float diffuseDimmer;
     float specularDimmer;
     float penumbraTint;

com.unity.render-pipelines.high-definition/Runtime/Lighting/LightEvaluation.hlsl

@@ -269,19 +269,9 @@ SHADOW_TYPE EvaluateShadow_Directional( LightLoopContext lightLoopContext, Posit
         shadow = lightLoopContext.shadowValue;
 
     #ifdef SHADOWS_SHADOWMASK
-        // TODO: Optimize this code! Currently it is a bit like brute force to get the last transistion and fade to shadow mask, but there is
-        // certainly more efficient to do
-        // We reuse the transition from the cascade system to fade between shadow mask at max distance
-        uint  payloadOffset;
-        real  fade;
-        int cascadeCount;
-        int shadowSplitIndex = 0;
-
-        shadowSplitIndex = EvalShadow_GetSplitIndex(lightLoopContext.shadowContext, light.shadowIndex, posInput.positionWS, fade, cascadeCount);
-
-        // we have a fade caclulation for each cascade but we must lerp with shadow mask only for the last one
-        // if shadowSplitIndex is -1 it mean we are outside cascade and should return 1.0 to use shadowmask: saturate(-shadowSplitIndex) return 0 for >= 0 and 1 for -1
-        fade = ((shadowSplitIndex + 1) == cascadeCount) ? fade : saturate(-shadowSplitIndex);
+        float3 camToPixel = posInput.positionWS - GetPrimaryCameraPosition();
+        float distanceCamToPixel2 = dot(camToPixel, camToPixel);
+        float fade = saturate(distanceCamToPixel2 * light.cascadesBorderFadeScaleBias.x + light.cascadesBorderFadeScaleBias.y);
 
         // In the transition code (both dithering and blend) we use shadow = lerp( shadow, 1.0, fade ) for last transition
         // mean if we expend the code we have (shadow * (1 - fade) + fade). Here to make transition with shadow mask

com.unity.render-pipelines.high-definition/Runtime/Lighting/LightLoop/LightLoop.cs

@@ -1252,6 +1252,22 @@ internal void GetDirectionalLightData(CommandBuffer cmd, HDCamera hdCamera, Visi
             {
                 lightData.shadowMaskSelector[lightComponent.bakingOutput.occlusionMaskChannel] = 1.0f;
                 lightData.nonLightMappedOnly = lightComponent.lightShadowCasterMode == LightShadowCasterMode.NonLightmappedOnly ? 1 : 0;
+                // Get shadow info from the volume stack.
+                var shadowSettings = hdCamera.volumeStack.GetComponent<HDShadowSettings>();
+                float maxDistanceSq = shadowSettings.maxShadowDistance.value * shadowSettings.maxShadowDistance.value;
+                float outBorderDistance = shadowSettings.cascadeShadowBorders[shadowSettings.cascadeShadowSplitCount.value - 1];
+                if (outBorderDistance < 1e-4f)
+                {
+                    lightData.cascadesBorderFadeScaleBias = new Vector2(1e6f, -maxDistanceSq * 1e6f);
+                }
+                else
+                {
+                    outBorderDistance = 1.0f - outBorderDistance;
+                    outBorderDistance *= outBorderDistance;
+                    float distanceFadeNear = outBorderDistance * maxDistanceSq;
+                    lightData.cascadesBorderFadeScaleBias.x = 1.0f / (maxDistanceSq - distanceFadeNear);
+                    lightData.cascadesBorderFadeScaleBias.y = -distanceFadeNear / (maxDistanceSq - distanceFadeNear);
+                }
             }
             else
             {