Fix base color and reflectivity calculations for PBR materials

[jjhembd]

Description

Our calculation of the base diffuse color for a material was scaled by the transmission coefficient (1.0 - f0) (where f0 is the specular reflection coefficient at normal incidence):

material.diffuse = material.diffuse * (1.0 - f0) * (1.0 - metalness);

This intuitively seems to make sense at normal incidence. The incident light that actually interacts with the microfacets of the surface (in the diffuse model) will be reduced by the amount of light already reflected specularly.

However, this simple 1D scaling is an oversimplification of something that is already accounted for in the BRDF, so including the scaling results in PBR materials being rendered slightly too dim.

This PR instead computes the base diffuse color using a formula more similar to the glTF Sample Viewer:

material.diffuse = mix(material.baseColor.rgb, vec3(0), metalness);

Note the new baseColor property on the czm_modelMaterial struct. This helps avoid confusion from overwriting the diffuse property too many times, and fixes #12041, where the KHR_materials_specular extension was using the diffuse property as if it were the raw base color.

Issue number and link

Resolves #12041

Testing plan

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• [ ] I have added or updated unit tests to ensure consistent code coverage
• [ ] I have update the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjhembd!

✅ We can confirm we have a CLA on file for you.

[ggetz]

Even though they were fixed in one PR, I would suggest listing this as two different items. The former (Fixed diffuse color calculation for PBR materials) can link to the PR. The later (fixes an issue affecting the KHR_materials_specular extension) can link to the issue.

[ggetz]

Shouldn't this be?

Suggested change

	material.diffuse = mix(material.baseColor.rgb, vec3(0), metalness);
	material.diffuse = mix(material.baseColor.rgb, vec3(0.0), metalness);

[jjhembd]

Thanks @ggetz! I addressed both of those comments.

[ggetz]

All looks good! Thanks @jjhembd!

[jjhembd]

For the record: the extra (1 - f0) factor has appeared in other implementations as well. See KhronosGroup/glTF#1945 and KhronosGroup/glTF#2022 for discussion about how it was caught and corrected in the glTF spec implementation notes.