[miogds]

Summary

The model pass draws without culling, so an open mesh shows its back faces: a dart's flight, a leaf, a sheet seen from behind. Those fragments arrived with the front side's normal, pointing away from the viewer, and shaded as if unlit — on a headset one wing of a dart's flight was dark while the next was lit. A face whose normal points away from the viewer is now lit with its normal turned around (modelShader.metal, before the TBN is built so a normal map follows).

The test is geometric (dot(N, camera − position) < 0) rather than [[front_facing]]: the pass sets no winding, and assets arrive wound either way — a [[front_facing]] flip turned every face of the shading test scenes dark.

Checklist

This project follows the Engineering Principles.

• Relevant tests have been added or updated — the shading suites (base colour, normal map, emissive, reflection, glass, transparency, post-FX, MSAA) pass unchanged, which pins that front faces shade exactly as before; a back-face shading test would be a welcome addition.
• Existing functionality and API compatibility have been considered — front faces are untouched; only fragments whose interpolated normal faces away change, which for a closed mesh is a silhouette fringe.
• Relevant builds and tests pass locally — kernels rebuilt for every platform; 75 render tests across the shading suites.
• Recoverable failures are handled appropriately — not applicable.
• Significant architectural changes and trade-offs are explained below — none.
• Documentation has been updated where necessary — the shader comment.
• Performance-sensitive changes have been measured — one dot product per fragment.

[miogds]

Superseded by #1359, which carries this work linearised together with the rest of the headset rendering set.