Add forward shadow receiving support

engine/assets/builtin/shaders/forward-lit/forward-lit.frag.glsl

@@ -1,6 +1,7 @@
 // XC_BUILTIN_FORWARD_LIT_OPENGL_PS
 #version 430
 layout(binding = 1) uniform sampler2D uBaseColorTexture;
+layout(binding = 2) uniform sampler2D uShadowMapTexture;
 
 layout(std140, binding = 1) uniform PerObjectConstants {
     mat4 gProjectionMatrix;
@@ -15,11 +16,44 @@ layout(std140, binding = 2) uniform MaterialConstants {
     vec4 gBaseColorFactor;
 };
 
+layout(std140, binding = 3) uniform ShadowReceiverConstants {
+    mat4 gWorldToShadowMatrix;
+    vec4 gShadowBiasAndTexelSize;
+    vec4 gShadowOptions;
+};
+
 in vec3 vNormalWS;
 in vec2 vTexCoord;
+in vec3 vPositionWS;
 
 layout(location = 0) out vec4 fragColor;
 
+float ComputeShadowAttenuation(vec3 positionWS) {
+    if (gShadowOptions.x < 0.5) {
+        return 1.0;
+    }
+
+    vec4 shadowClip = gWorldToShadowMatrix * vec4(positionWS, 1.0);
+    if (shadowClip.w <= 0.0) {
+        return 1.0;
+    }
+
+    vec3 shadowNdc = shadowClip.xyz / shadowClip.w;
+    vec2 shadowUv = vec2(
+        shadowNdc.x * 0.5 + 0.5,
+        shadowNdc.y * -0.5 + 0.5);
+    if (shadowUv.x < 0.0 || shadowUv.x > 1.0 ||
+        shadowUv.y < 0.0 || shadowUv.y > 1.0 ||
+        shadowNdc.z < 0.0 || shadowNdc.z > 1.0) {
+        return 1.0;
+    }
+
+    float shadowDepth = texture(uShadowMapTexture, shadowUv).r;
+    float receiverDepth = shadowNdc.z - gShadowBiasAndTexelSize.x;
+    float shadowStrength = clamp(gShadowBiasAndTexelSize.w, 0.0, 1.0);
+    return receiverDepth <= shadowDepth ? 1.0 : (1.0 - shadowStrength);
+}
+
 void main() {
     vec4 baseColor = texture(uBaseColorTexture, vTexCoord) * gBaseColorFactor;
     if (gMainLightColorAndFlags.w < 0.5) {
@@ -30,7 +64,10 @@ void main() {
     vec3 normalWS = normalize(vNormalWS);
     vec3 directionToLightWS = normalize(gMainLightDirectionAndIntensity.xyz);
     float diffuse = max(dot(normalWS, directionToLightWS), 0.0);
+    float shadowAttenuation = diffuse > 0.0
+        ? ComputeShadowAttenuation(vPositionWS)
+        : 1.0;
     vec3 lighting = vec3(0.28) +
-        gMainLightColorAndFlags.rgb * (diffuse * gMainLightDirectionAndIntensity.w);
+        gMainLightColorAndFlags.rgb * (diffuse * gMainLightDirectionAndIntensity.w * shadowAttenuation);
     fragColor = vec4(baseColor.rgb * lighting, baseColor.a);
 }