Adding environment prefiltering shader

res/shaders/EnvironmentMapping.slang

@@ -123,4 +123,70 @@ float4 convolveCubemap(float3 localPos : LOCALPOS) : SV_Target
     irradiance = PI * irradiance * (1.0 / nrSamples);
 
     return float4(irradiance, 1);
+}
+
+float RadicalInverse_VdC(uint bits) 
+{
+    bits = (bits << 16u) | (bits >> 16u);
+    bits = ((bits & 0x55555555u) << 1u) | ((bits & 0xAAAAAAAAu) >> 1u);
+    bits = ((bits & 0x33333333u) << 2u) | ((bits & 0xCCCCCCCCu) >> 2u);
+    bits = ((bits & 0x0F0F0F0Fu) << 4u) | ((bits & 0xF0F0F0F0u) >> 4u);
+    bits = ((bits & 0x00FF00FFu) << 8u) | ((bits & 0xFF00FF00u) >> 8u);
+    return float(bits) * 2.3283064365386963e-10; // / 0x100000000
+}
+
+float2 Hammersley(uint i, uint N)
+{
+    return float2(float(i)/float(N), RadicalInverse_VdC(i));
+}
+
+float3 ImportanceSampleGGX(float2 Xi, float3 N, float roughness)
+{
+    float a = roughness*roughness;
+	
+    float phi = 2.0 * PI * Xi.x;
+    float cosTheta = sqrt((1.0 - Xi.y) / (1.0 + (a*a - 1.0) * Xi.y));
+    float sinTheta = sqrt(1.0 - cosTheta*cosTheta);
+	
+    // from spherical coordinates to cartesian coordinates
+    float3 H;
+    H.x = cos(phi) * sinTheta;
+    H.y = sin(phi) * sinTheta;
+    H.z = cosTheta;
+	
+    // from tangent-space vector to world-space sample vector
+    float3 up        = abs(N.z) < 0.999 ? float3(0.0, 0.0, 1.0) : float3(1.0, 0.0, 0.0);
+    float3 tangent   = normalize(cross(up, N));
+    float3 bitangent = cross(N, tangent);
+	
+    float3 sampleVec = tangent * H.x + bitangent * H.y + N * H.z;
+    return normalize(sampleVec);
+}
+
+[shader("pixel")]
+float4 computePrefilteredCubemap(float3 localPos : LOCALPOS) : SV_Target
+{
+    float3 N = normalize(localPos);    
+    float3 R = N;
+    float3 V = R;
+
+    const uint SAMPLE_COUNT = 1024u;
+    float totalWeight = 0.0;   
+    float3 prefilteredColor = float3(0.0);     
+    for(uint i = 0u; i < SAMPLE_COUNT; ++i)
+    {
+        float2 Xi = Hammersley(i, SAMPLE_COUNT);
+        float3 H  = ImportanceSampleGGX(Xi, N, roughness);
+        float3 L  = normalize(2.0 * dot(V, H) * H - V);
+
+        float NdotL = max(dot(N, L), 0.0);
+        if(NdotL > 0.0)
+        {
+            prefilteredColor += pViewParams.cubeMap.Sample(pViewParams.sampler, L).rgb * NdotL;
+            totalWeight      += NdotL;
+        }
+    }
+    prefilteredColor = prefilteredColor / totalWeight;
+
+    return float4(prefilteredColor, 1.0);
 }