[MAX-MAT-002] Strip buggy emission=1.0 / emission_color=(0,0,0) from MaterialX export

[doug-smith-miris]

Summary

Stacked on top of #36 (MAX-MAT-001). Until that PR merges, this PR's diff includes its commits too — once MAX-MAT-001 merges into dev, this PR's diff will collapse to just the MAX-MAT-002 changes (1 commit, 4 files).

Adds a second post-parse normalization pass in MtlxShaderWriter::Write() that strips the spurious emission = 1.0 paired with emission_color = (0, 0, 0) that 3ds Max's MtlxIOUtil bridge writes on every ND_standard_surface_surfaceshader.

The MaterialX standard_surface nodedef defaults are emission = 0.0 and emission_color = (1, 1, 1). The buggy pair multiplies to (0, 0, 0) so it has no visual effect, but it is semantically wrong:

• A downstream override that bumps emission_color away from black inherits a full-strength emission = 1.0 the artist never asked for.
• A round-trip importer reads emission_color = (0, 0, 0) as "explicit black emission" rather than "no emission authored" — semantically different states that diverge under later edits.
• The values do not match the source DCC: PhysicalMaterial has no emission knob set to 1.0, and never asked for an emission color of pure black.

What changed

• src/translators/MtlxShaderWriter.cpp:
  • Adds _TryGetStaticColor3 — parses MaterialX color3 value strings ("r, g, b" or "r g b") when not connected; rejects strings with the wrong component count.
  • Adds _NormalizeStandardSurfaceEmissionDefault — walks standard_surface nodes and strips both emission and emission_color inputs when both are statically the buggy pair. Any deviation (connected input, non-1.0 emission, non-black color, missing half) leaves both untouched.
  • Invokes the new pass in Write() immediately after _NormalizeStandardSurfaceSpecularRotation.
• doc/translation-mapping.md: adds a new row + section + change log entry.
• doc/changelog.md: adds an Unreleased fix entry.
• src/Tests/Integration/mtlxShaderWriter_test.ms: adds test_export_physical_material_strips_buggy_emission_default.

Validation

3ds Max is Windows-only and this repo builds via Visual Studio; build/run was deferred (same constraint as #36). A Python validator that mirrors the C++ logic 1:1 at the USD layer was run on the diagnostic corpus and synthetic fixtures.

Diagnostic corpus (complex_export_mat001_postfix.usda, MAX-MAT-001-clean): 6/6 standard_surface shaders affected. The USDA diff is exactly 12 deletions (6 emission, 6 emission_color), zero additions, zero structural changes.

Karma renders of pre-fix and post-fix corpora are byte-identical (SHA-256 eaf38328...). This is the expected outcome and the success signal — the buggy pair multiplies to zero emission, the nodedef-default pair multiplies to zero emission, both produce the same BSDF lighting result.

Synthetic negative-test fixture (_synthetic_with_emission.usda):

• BrushedSteel emission_color set to (0.8, 0.4, 0.1) — intentional orange emission tint → kept.
• Copper emission set to 0.5 — intentional partial emission → kept.
• The 4 untouched buggy materials → stripped.

The MaxScript regression test extends the existing PhysicalMaterial test pattern from MAX-MAT-001.

Reference files (validator + corpus)

Under agent/arch-builds/f72c97e6-301f-400a-9e77-c768979e8fa5/ (outside the repo):

• normalize_emission_default.py — USD-layer mirror of the C++ logic.
• complex_export_mat001_postfix.usda — input (MAX-MAT-001-clean, MAX-MAT-002-dirty).
• complex_export_postfix.usda — output (MAX-MAT-001+002-clean).
• make_negative_fixture.py, _synthetic_with_emission.usda, _synthetic_postfix.usda.
• render_karma_prefix.png, render_karma_postfix.png (byte-identical), compare_side_by_side.png.
• intended_example.md — detailed observable / fixture description for the auditor.

Test plan

• Reviewer confirms the C++ pattern mirrors MAX-MAT-001 conservatively.
• On a Windows 3ds Max install, run the MaterialX export integration suite (mtlxShaderWriter_test.ms) and confirm the new test passes alongside the MAX-MAT-001 test.
• On a Windows 3ds Max install, export a fresh complex scene via the plugin and confirm no emission = 1.0 + emission_color = (0, 0, 0) pair appears on any standard_surface shader, while any intentionally authored emission is preserved.

🤖 Generated with Claude Code