rendering: improve main light shadow receiver filtering

engine/assets/builtin/shaders/forward-lit.shader

@@ -93,6 +93,49 @@ Shader "Builtin Forward Lit"
         return output;
     }
 
+    float SampleShadowVisibility(float2 sampleUv, float receiverDepth)
+    {
+        if (sampleUv.x < 0.0f || sampleUv.x > 1.0f ||
+            sampleUv.y < 0.0f || sampleUv.y > 1.0f) {
+            return 1.0f;
+        }
+
+        const float shadowDepth = ShadowMapTexture.Sample(ShadowMapSampler, sampleUv).r;
+        return receiverDepth <= shadowDepth ? 1.0f : 0.0f;
+    }
+
+    float ResolveShadowPcfAxisWeight(int offset)
+    {
+        const int absoluteOffset = abs(offset);
+        if (absoluteOffset == 0) {
+            return 6.0f;
+        }
+        if (absoluteOffset == 1) {
+            return 4.0f;
+        }
+        return 1.0f;
+    }
+
+    float ComputeShadowPcfVisibility(float2 shadowUv, float receiverDepth, float2 shadowTexelSize)
+    {
+        float weightedVisibility = 0.0f;
+        float weightSum = 0.0f;
+    [unroll]
+        for (int offsetY = -2; offsetY <= 2; ++offsetY) {
+            const float weightY = ResolveShadowPcfAxisWeight(offsetY);
+        [unroll]
+            for (int offsetX = -2; offsetX <= 2; ++offsetX) {
+                const float weight = weightY * ResolveShadowPcfAxisWeight(offsetX);
+                const float2 sampleUv =
+                    shadowUv + float2((float)offsetX, (float)offsetY) * shadowTexelSize;
+                weightedVisibility += SampleShadowVisibility(sampleUv, receiverDepth) * weight;
+                weightSum += weight;
+            }
+        }
+
+        return weightSum > 0.0f ? weightedVisibility / weightSum : 1.0f;
+    }
+
     float ComputeShadowAttenuation(float3 positionWS, float3 normalWS, float3 lightDirectionWS)
     {
     #ifndef XC_MAIN_LIGHT_SHADOWS
@@ -136,25 +179,10 @@ Shader "Builtin Forward Lit"
         const float receiverDepth = shadowNdc.z - gShadowSampling.receiverDepthBias;
     #endif
         const float2 shadowTexelSize = gShadowMapMetrics.inverseMapSize;
-        float visibility = 0.0f;
-    [unroll]
-        for (int offsetY = -1; offsetY <= 1; ++offsetY) {
-        [unroll]
-            for (int offsetX = -1; offsetX <= 1; ++offsetX) {
-                const float2 sampleUv =
-                    shadowUv + float2((float)offsetX, (float)offsetY) * shadowTexelSize;
-                if (sampleUv.x < 0.0f || sampleUv.x > 1.0f ||
-                    sampleUv.y < 0.0f || sampleUv.y > 1.0f) {
-                    visibility += 1.0f;
-                    continue;
-                }
-
-                const float shadowDepth = ShadowMapTexture.Sample(ShadowMapSampler, sampleUv).r;
-                visibility += receiverDepth <= shadowDepth ? 1.0f : 0.0f;
-            }
-        }
-
-        visibility *= (1.0f / 9.0f);
+        const float visibility = ComputeShadowPcfVisibility(
+            shadowUv,
+            receiverDepth,
+            shadowTexelSize);
         const float shadowStrength = saturate(gShadowSampling.shadowStrength);
         return lerp(1.0f - shadowStrength, 1.0f, visibility);
     #endif