explain sign flip in Rodrigues formula.

Author: bhouston

src/nodes/materialx/MaterialXCore.js

@@ -27,7 +27,9 @@ export const mx_rotate3d = ( input, amount = 0, axis = vec3( 0, 1, 0 ) ) => {
 	const c = cos( rotationRadians );
 	const oc = sub( 1, c );
 
-	// https://en.wikipedia.org/wiki/Rodrigues%27_rotation_formula
+	// based on https://en.wikipedia.org/wiki/Rodrigues%27_rotation_formula
+    // but the code in the Wikipedia article is for v' = M * v, where as 
+    // MaterialX is v' = v * M, thus the order of parameters into the first cross are reversed
 	return input.mul( c )
 		.add( input.cross( normalizedAxis ).mul( s ) )
 		.add( normalizedAxis.mul( normalizedAxis.dot( input ).mul( oc ) ) );