use copysign when making onb

[buggy213]

• upstream luisacompute commit (12647ff0c96775770c23c705e76cb24e6ba82970) changes defn of sign, not merged yet but it causes div by zero

other uses of sign look fine to me? they mainly surface when wo / wh have z = 0, in which case its a grazing ray and has no contribution anyways

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the robustness of the Frame::make function by addressing a potential division-by-zero scenario. It proactively adapts to an anticipated change in an upstream sign function definition, ensuring that the calculation of orthonormal bases remains stable and error-free, particularly for grazing rays where the z-component might be zero.

Highlights

• Division by Zero Prevention: Replaced the sign function with copysign(1.f, n.z) in Frame::make to prevent a potential division-by-zero error. This change anticipates an upcoming upstream modification to the sign function's definition.
• Robustness in ONB Creation: Ensured the robust creation of orthonormal bases (ONB) by correctly handling the sign of the n.z component, especially when n.z approaches zero.

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[gemini-code-assist]

Code Review

This pull request addresses a critical issue in Frame::make that could lead to a division-by-zero error. The replacement of sign(n.z) with copysign(1.f, n.z) is the correct approach to handle the case where n.z is zero, which, as noted in the description, is a behavior change in an upstream dependency. This change makes the orthonormal basis construction more robust.

[buggy213]

additional change: make eta_scale part of path state so that it is carried through multiple bounces

[Mike-Leo-Smith]

cool! thank you!