Add comment about RoughnessToAlpha bug.

Issue #479.

Author: mmp

src/pbrt/util/scattering.h

@@ -188,6 +188,11 @@ class TrowbridgeReitzDistribution {
 
     std::string ToString() const;
 
+    // Note that this should probably instead be "return Sqr(roughness)" to
+    // be more perceptually uniform, though this wasn't noticed until some
+    // time after pbrt-v4 shipped: https://github.com/mmp/pbrt-v4/issues/479.
+    // therefore, we will leave it as is so that the rendered results with
+    // existing pbrt-v4 scenes doesn't change unexpectedly.
     PBRT_CPU_GPU
     static Float RoughnessToAlpha(Float roughness) { return std::sqrt(roughness); }