[apotema]

Adds Blend.modulate2x to game-owned shader materials: each RGB channel of what is already drawn is multiplied by 2 * src, and the destination alpha is kept. A source of 0.5 leaves the pixel unchanged, 0 blacks it out, 1 doubles it.

Why: Flying Platform is moving its night lighting from a shader on every lit sprite to one baked light overlay per room (one quad drawn over the room's contents). An overlay has to both darken (the 60% ambient) and brighten (lamp beams, up to ~1.5x) whatever is under it. Alpha and additive blending can't do that; a 2x multiply can. On an SM-T505 this takes a 17-room night scene from 31.6 ms to 16.7 ms a frame, and a 1000-citizen colony from a frozen ~400 ms to normal frame times.

Compatibility: additive enum value; the default stays .alpha. Backends that switch exhaustively on Blend need the new case: labelle-bgfx gets it in a companion PR (the (DstColor, SrcColor) / (Zero, One) blend state). The assembler's material.json schema gets "blend": "modulate2x" in its own PR.

Tests: zig build test 230/230. The bgfx companion builds and passes 253/253 against this branch (-Dcore-source).

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-24T17:47:19.963372Z	c481b7b	PR opened

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Reviewing files that changed from the base of the PR and between 1cd1942 and c481b7b.

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• src/shader_material.zig

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📝 Walkthrough

Walkthrough

The Blend enum adds a modulate2x option. Its documentation describes RGB output as destination color multiplied by twice the source color, with destination alpha preserved and shader alpha ignored.

Changes

Blend option

Layer / File(s)	Summary
Add the modulate2x blend option src/shader_material.zig	Blend adds the modulate2x variant. Its documentation describes RGB and alpha handling and the output for overlay values 0.5, 0, and 1.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to c481b

This change adds the documented blend option, and the reviewed repository shows no actionable build or rendering issue. Mergeability risk is minimal.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the addition of modulate2x blending and states its main behavior and intended lighting-overlay use.
Description check	✅ Passed	The description directly explains the Blend.modulate2x change, its rendering behavior, motivation, compatibility impact, and reported test results.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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