Improve UsdPreviewSurface shading logic #1082
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pablode
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jstone-lucasfilm
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Sep 29, 2022
Michaelredaa
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Oct 21, 2023
This changelist fixes discrepancies between the MaterialX graph for UsdPreviewSurface and Pixar's current shading logic.
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I've compared parts of MaterialX's UsdPreviewSurface BSDF implementation against Pixar's current shading logic and believe to have found some discrepancies:
Commit be438a4 addresses the calculation of F0 and F90 values for the specular BSDF when the metallic workflow is selected. It now implements the reference logic:
https://github.com/PixarAnimationStudios/USD/blob/3abc46452b1271df7650e9948fef9f0ce602e3b2/pxr/usdImaging/plugin/usdShaders/shaders/previewSurface.glslfx#L219-L227
Commit 25fb016 properly attenuates the weight of the diffuse BSDF when the metallic workflow is selected: https://github.com/PixarAnimationStudios/USD/blob/3abc46452b1271df7650e9948fef9f0ce602e3b2/pxr/usdImaging/plugin/usdShaders/shaders/previewSurface.glslfx#L229-L230
Commit dd4eba2 changes the clearcoat layer implementation to Pixar's reference logic:
https://github.com/PixarAnimationStudios/USD/blob/3abc46452b1271df7650e9948fef9f0ce602e3b2/pxr/usdImaging/plugin/usdShaders/shaders/previewSurface.glslfx#L247-L252