MaterialX UsdTransform2D rotation direction does not align with USD specification

[ld-kerley]

The USD specification states explicitly that the rotation component in UsdTransform2D is counter-clockwise and specified in degrees around the origin. The USD specification also has an explicit callout that its transforming the texture coordinates, not the texture.

NOTE : The UsdTransform2d node transforms texture coordinates, not the textures themselves, so to get the effect of moving, resizing, or rotating an image being applied to a surface, one must apply the inverse transformation of how you expect the image to move.

It appears that the MaterialX implementation doesn't follow this convention.

rotate_test_file.zip

The test files zip contain both a USD version where you can see the rotation occurs in opposing directions between USD and MaterialX.

USD version

MaterialX version

Caveat - the MaterialX version does have some modifications to work around other shortcomings in the MaterialX implementation of the UsdPreviewSurface node.

[ld-kerley]

Actually turns out that on closer inspection all of the transforms are being applied inverted - likely due to a mis-understanding that is clarified in the NOTE from the UsdPreviewSurface spec referenced above.

[jstone-lucasfilm]

This sounds like a good observation to me, @ld-kerley, and I'm CC'ing @kwokcb for his thoughts, as he was the original author of the USD texturing and placement nodes in MaterialX.

[kwokcb]

Review looks good and should be faster to compute as well.