Add forward shadow receiving support

engine/assets/builtin/shaders/forward-lit/forward-lit.ps.hlsl

@@ -1,6 +1,8 @@
 // XC_BUILTIN_FORWARD_LIT_D3D12_PS
 Texture2D gBaseColorTexture : register(t1);
 SamplerState gLinearSampler : register(s1);
+Texture2D gShadowMapTexture : register(t2);
+SamplerState gShadowMapSampler : register(s2);
 
 cbuffer PerObjectConstants : register(b1) {
     float4x4 gProjectionMatrix;
@@ -15,12 +17,45 @@ cbuffer MaterialConstants : register(b2) {
     float4 gBaseColorFactor;
 };
 
+cbuffer ShadowReceiverConstants : register(b3) {
+    float4x4 gWorldToShadowMatrix;
+    float4 gShadowBiasAndTexelSize;
+    float4 gShadowOptions;
+};
+
 struct PSInput {
     float4 position : SV_POSITION;
     float3 normalWS : TEXCOORD0;
     float2 texcoord : TEXCOORD1;
+    float3 positionWS : TEXCOORD2;
 };
 
+float ComputeShadowAttenuation(float3 positionWS) {
+    if (gShadowOptions.x < 0.5f) {
+        return 1.0f;
+    }
+
+    float4 shadowClip = mul(gWorldToShadowMatrix, float4(positionWS, 1.0f));
+    if (shadowClip.w <= 0.0f) {
+        return 1.0f;
+    }
+
+    float3 shadowNdc = shadowClip.xyz / shadowClip.w;
+    float2 shadowUv = float2(
+        shadowNdc.x * 0.5f + 0.5f,
+        shadowNdc.y * -0.5f + 0.5f);
+    if (shadowUv.x < 0.0f || shadowUv.x > 1.0f ||
+        shadowUv.y < 0.0f || shadowUv.y > 1.0f ||
+        shadowNdc.z < 0.0f || shadowNdc.z > 1.0f) {
+        return 1.0f;
+    }
+
+    const float shadowDepth = gShadowMapTexture.Sample(gShadowMapSampler, shadowUv).r;
+    const float receiverDepth = shadowNdc.z - gShadowBiasAndTexelSize.x;
+    const float shadowStrength = saturate(gShadowBiasAndTexelSize.w);
+    return receiverDepth <= shadowDepth ? 1.0f : (1.0f - shadowStrength);
+}
+
 float4 MainPS(PSInput input) : SV_TARGET {
     float4 baseColor = gBaseColorTexture.Sample(gLinearSampler, input.texcoord) * gBaseColorFactor;
     if (gMainLightColorAndFlags.a < 0.5f) {
@@ -30,7 +65,10 @@ float4 MainPS(PSInput input) : SV_TARGET {
     float3 normalWS = normalize(input.normalWS);
     float3 directionToLightWS = normalize(gMainLightDirectionAndIntensity.xyz);
     float diffuse = saturate(dot(normalWS, directionToLightWS));
+    float shadowAttenuation = diffuse > 0.0f
+        ? ComputeShadowAttenuation(input.positionWS)
+        : 1.0f;
     float3 lighting = float3(0.28f, 0.28f, 0.28f) +
-        gMainLightColorAndFlags.rgb * (diffuse * gMainLightDirectionAndIntensity.w);
+        gMainLightColorAndFlags.rgb * (diffuse * gMainLightDirectionAndIntensity.w * shadowAttenuation);
     return float4(baseColor.rgb * lighting, baseColor.a);
 }