Standard Surface with colors outside of 0-1 renders incorrectly

[joaovbs96]

This Standard Surface material from physicallybased.info, the Gold material, has some color3 values beyond the 0-1 range, namely the base color and specular color.

<?xml version="1.0"?>
<materialx version="1.38" colorspace="lin_rec709" doc="Downloaded from https://physicallybased.info on 2026-07-10">
  <standard_surface name="SR_Gold" type="surfaceshader">
    <input name="base_color" type="color3" value="1.059, 0.773, 0.307" />
    <input name="specular_color" type="color3" value="1.001, 0.985, 0.523" />
    <input name="specular_roughness" type="float" value="0" />
    <input name="metalness" type="float" value="1" />
  </standard_surface>
  <surfacematerial name="Gold" type="material">
    <input name="surfaceshader" type="surfaceshader" nodename="SR_Gold" />
  </surfacematerial>
</materialx>

The material renders with a green circle in the middle. I've observed this on both GLSL backends (MaterialXView, MaterialXEditor and the webviewer):

and MDL:

Per discussion in the Slack channel, Jamie Portsmouth mentioned:

I think this is because Standard Surface uses the Gulbrandsen model for its metallic Fresnel (i.e. maps base and spec color to n, k), and this gives NaNs if the input colors are outside the [0,1] range. The fix would just be to clamp the channels before computing n, k.
It won't occur in OpenPBR (even if you supply color channels exceeding 1) since we use a different metallic Fresnel. But also we said the colors are assumed to be clamped anyway.

Note that, while I mentioned GLSL and MDL, it's possible this also happens in other backends as well, but I have no way of testing them.