UsdPreviewSurface metallic opacity handling different

[ld-kerley]

There is a discrepancy with respect to how ND_UsdPreviewSurface_surfaceshader is handling opacity for materials where metallic is used.

It appears that the USD version is using opacity to weight the metallic component - but the MaterialX version is not when opacityMode is set to transparent.

test files

USD version

MaterialX version

[meshula]

Hopefully the specification makes the interpretation clear, the key point is opacity must be considered in conjunction with opacityMode.

opacity - float - 1.0

When opacity is 1.0 then the prim is fully opaque, if it is smaller than 1.0 then the prim is translucent, when it is 0 the prim is fully transparent. Fully transparent objects will still receive a lighting response when the opacityMode is transparent. When the opacityMode is set to presence the lighting response will be scaled by the opacity.

opacityMode - token - transparent

This input dictates how materials with opacity less than one are interpreted and accepts one of two possible values: transparent or presence. When the value is transparent, opacity reduces the contribution of the diffuse lighting component in favor of transparency, while the specular response and emissive component remain at full weight. This models materials such as glass, where even perfectly clear glass still produces a specular reflection, and glowing glass glows. When set to presence, opacity scales the entire lighting response, including specular and emission, so the material response fades proportionally as opacity decreases. In this mode the surface transitions toward full transparency as opacity approaches zero, behaving as if the object itself is disappearing rather than remaining optically present through specular reflection and emission.