Fix issues in specular pre-filter convolution (missing PI in GGX dist…

…ribution; wrong pdf calculation)
TaaTT4 · May 4, 2022 · 58dfd9b · 58dfd9b
1 parent d426474
commit 58dfd9b
Showing 1 changed file with 9 additions and 6 deletions.
diff --git a/Samples/Media/Compute/Algorithms/IBL/SpecularIblIntegrator_piece_cs.any b/Samples/Media/Compute/Algorithms/IBL/SpecularIblIntegrator_piece_cs.any
@@ -116,7 +116,8 @@
 		float a = roughness * roughness;
 
 		float Phi = 2 * PI * Xi.x;
-		float CosTheta = sqrt( ( 1 - Xi.y ) / ( 1 + ( a * a - 1 ) * Xi.y ) );
+		// NOTE: (aa-1) == (a-1)(a+1) produces better fp accuracy
+		float CosTheta = sqrt( ( 1 - Xi.y ) / ( 1 + ( a + 1 ) * ( ( a - 1 ) * Xi.y ) ) );
 		float SinTheta = sqrt( 1 - CosTheta * CosTheta );
 
 		float3 H;
@@ -141,9 +142,11 @@
 	// Smoth GGX
 	INLINE float specularD( float roughness, float NoH )
 	{
-		float NoH2 = NoH * NoH;
-		float r2 = roughness * roughness;
-		return r2 / pow( NoH2 * ( r2 - 1.0 ) + 1.0, 2.0 );
+		float a = roughness * roughness;
+
+		// NOTE: (aa-1) == (a-1)(a+1) produces better fp accuracy
+		float f = ( a - 1 ) * ( ( a + 1 ) * ( NoH * NoH ) ) + 1;
+		return ( a * a ) / ( PI * f * f );
 	}
 
 	INLINE float4 importanceSample( float3 R PARAMS_ARG_DECL )
@@ -165,7 +168,7 @@
 			float NoL = max( dot( N, L ), 0.0 );
 			// float VoL = max( dot( V, L ), 0.0 );
 			float NoH = max( dot( N, H ), 0.0 );
-			float VoH = max( dot( V, H ), 0.0 );
+			// float VoH = max( dot( V, H ), 0.0 );
 			if( NoL > 0 )
 			{
 				//
@@ -174,7 +177,7 @@
 				// http://blog.selfshadow.com/publications/s2013-shading-course/karis/s2013_pbs_epic_notes_v2.pdf
 				//
 				float Dh = specularD( p_convolutionRoughness, NoH );
-				float pdf = Dh * NoH / ( 4 * VoH );
+				float pdf = Dh / 4;
 				float solidAngleTexel = 4 * PI / ( 6 * p_inputResolution.x * p_inputResolution.x );
 				float solidAngleSample = 1.0 / ( p_convolutionSampleCount * pdf );
 				float lod =