Fix GLSL implementation of BSDF addition (#2334)

jstone-lucasfilm · web-flow · commit ef2f2211a84d · 2025-04-05T13:11:06.000-07:00
This changelist fixes the computation of throughput in the GLSL implementation of BSDF addition, with a comment describing the motivation for the provided math.
diff --git a/libraries/pbrlib/genglsl/mx_add_bsdf.glsl b/libraries/pbrlib/genglsl/mx_add_bsdf.glsl
@@ -2,6 +2,8 @@
 
 void mx_add_bsdf(ClosureData closureData, BSDF in1, BSDF in2, out BSDF result)
 {
+    // To provide a throughput computation for hardware shading languages,
+    // we reinterpret BSDF addition as in1 + in2 = mix(in1, in2, 0.5) * 2.
     result.response = in1.response + in2.response;
-    result.throughput = in1.throughput + in2.throughput;
+    result.throughput = mix(in1.throughput, in2.throughput, 0.5);
 }

Original file line number	Diff line number	Diff line change
`@@ -2,6 +2,8 @@`
`2`	`2`
`3`	`3`	`void mx_add_bsdf(ClosureData closureData, BSDF in1, BSDF in2, out BSDF result)`
`4`	`4`	`{`
	`5`	`+ // To provide a throughput computation for hardware shading languages,`
	`6`	`+ // we reinterpret BSDF addition as in1 + in2 = mix(in1, in2, 0.5) * 2.`
`5`	`7`	`result.response = in1.response + in2.response;`
`6`		`- result.throughput = in1.throughput + in2.throughput;`
	`8`	`+ result.throughput = mix(in1.throughput, in2.throughput, 0.5);`
`7`	`9`	`}`