PMREM: Simplify GGX VNDF importance sampling. (mrdoob#32737)

Author: Mugen87

src/extras/PMREMGenerator.js

@@ -845,24 +845,20 @@ function _getGGXShader( lodMax, width, height ) {
 			vec3 importanceSampleGGX_VNDF(vec2 Xi, vec3 V, float roughness) {
 				float alpha = roughness * roughness;
 
-				// Section 3.2: Transform view direction to hemisphere configuration
-				vec3 Vh = normalize(vec3(alpha * V.x, alpha * V.y, V.z));
-
 				// Section 4.1: Orthonormal basis
-				float lensq = Vh.x * Vh.x + Vh.y * Vh.y;
-				vec3 T1 = lensq > 0.0 ? vec3(-Vh.y, Vh.x, 0.0) / sqrt(lensq) : vec3(1.0, 0.0, 0.0);
-				vec3 T2 = cross(Vh, T1);
+				vec3 T1 = vec3(1.0, 0.0, 0.0);
+				vec3 T2 = cross(V, T1);
 
 				// Section 4.2: Parameterization of projected area
 				float r = sqrt(Xi.x);
 				float phi = 2.0 * PI * Xi.y;
 				float t1 = r * cos(phi);
 				float t2 = r * sin(phi);
-				float s = 0.5 * (1.0 + Vh.z);
+				float s = 0.5 * (1.0 + V.z);
 				t2 = (1.0 - s) * sqrt(1.0 - t1 * t1) + s * t2;
 
 				// Section 4.3: Reprojection onto hemisphere
-				vec3 Nh = t1 * T1 + t2 * T2 + sqrt(max(0.0, 1.0 - t1 * t1 - t2 * t2)) * Vh;
+				vec3 Nh = t1 * T1 + t2 * T2 + sqrt(max(0.0, 1.0 - t1 * t1 - t2 * t2)) * V;
 
 				// Section 3.4: Transform back to ellipsoid configuration
 				return normalize(vec3(alpha * Nh.x, alpha * Nh.y, max(0.0, Nh.z)));

src/nodes/pmrem/PMREMUtils.js

@@ -312,30 +312,24 @@ const hammersley = /*@__PURE__*/ Fn( ( [ i, N ] ) => {
 // GGX VNDF importance sampling (Eric Heitz 2018)
 // "Sampling the GGX Distribution of Visible Normals"
 // https://jcgt.org/published/0007/04/01/
-const importanceSampleGGX_VNDF = /*@__PURE__*/ Fn( ( [ Xi, V_immutable, roughness_immutable ] ) => {
+const importanceSampleGGX_VNDF = /*@__PURE__*/ Fn( ( [ Xi, V, roughness ] ) => {
 
-	const V = vec3( V_immutable ).toVar();
-	const roughness = float( roughness_immutable );
-	const alpha = roughness.mul( roughness ).toVar();
-
-	// Section 3.2: Transform view direction to hemisphere configuration
-	const Vh = normalize( vec3( alpha.mul( V.x ), alpha.mul( V.y ), V.z ) ).toVar();
+	const alpha = roughness.mul( roughness ).toConst();
 
 	// Section 4.1: Orthonormal basis
-	const lensq = Vh.x.mul( Vh.x ).add( Vh.y.mul( Vh.y ) );
-	const T1 = select( lensq.greaterThan( 0.0 ), vec3( Vh.y.negate(), Vh.x, 0.0 ).div( sqrt( lensq ) ), vec3( 1.0, 0.0, 0.0 ) ).toVar();
-	const T2 = cross( Vh, T1 ).toVar();
+	const T1 = vec3( 1.0, 0.0, 0.0 ).toConst();
+	const T2 = cross( V, T1 ).toConst();
 
 	// Section 4.2: Parameterization of projected area
-	const r = sqrt( Xi.x );
-	const phi = mul( 2.0, 3.14159265359 ).mul( Xi.y );
-	const t1 = r.mul( cos( phi ) ).toVar();
+	const r = sqrt( Xi.x ).toConst();
+	const phi = mul( 2.0, 3.14159265359 ).mul( Xi.y ).toConst();
+	const t1 = r.mul( cos( phi ) ).toConst();
 	const t2 = r.mul( sin( phi ) ).toVar();
-	const s = mul( 0.5, Vh.z.add( 1.0 ) );
+	const s = mul( 0.5, V.z.add( 1.0 ) ).toConst();
 	t2.assign( s.oneMinus().mul( sqrt( t1.mul( t1 ).oneMinus() ) ).add( s.mul( t2 ) ) );
 
 	// Section 4.3: Reprojection onto hemisphere
-	const Nh = T1.mul( t1 ).add( T2.mul( t2 ) ).add( Vh.mul( sqrt( max( 0.0, t1.mul( t1 ).add( t2.mul( t2 ) ).oneMinus() ) ) ) );
+	const Nh = T1.mul( t1 ).add( T2.mul( t2 ) ).add( V.mul( sqrt( max( 0.0, t1.mul( t1 ).add( t2.mul( t2 ) ).oneMinus() ) ) ) );
 
 	// Section 3.4: Transform back to ellipsoid configuration
 	return normalize( vec3( alpha.mul( Nh.x ), alpha.mul( Nh.y ), max( 0.0, Nh.z ) ) );