test(engine): lock down sRGB→BT.2020 LUT with byte-exact reference values (#377)

mirror of https://github.com/heygen-com/hyperframes.git synced 2026-09-05 00:56:23 +00:00

## Summary

Add a 12-row reference table covering the full sRGB range with byte-exact 16-bit HLG and PQ signal values, plus three guard tests, locking down the `buildSrgbToHdrLut()` math.

## Why

`Chunk 9F` of `plans/hdr-followups.md`. The matrix-free fast path through `blitRgba8OverRgb48le` runs every DOM pixel through `buildSrgbToHdrLut()` (sRGB EOTF → linear → HDR OETF → 16-bit). Any drift in the EOTF/OETF math — constant changes, branch swaps, rounding-mode regressions — would silently corrupt every text / UI / overlay pixel composited onto an HDR frame.

Existing tests covered structural invariants (transparent passthrough, opaque overwrite, alpha blending, channel symmetry, HLG ≠ PQ) but no byte-exact reference values, so a uniform scale or constant tweak could pass everything.

## What changed

- 12-row reference table in `alphaBlit.test.ts` covering black, shadow, mid-grays, highlight, near-white, and white with exact 16-bit HLG and PQ signal values.
- Three guard tests:
- **Asymmetric R/G/B (HLG):** each channel hits the LUT independently.
- **Asymmetric R/G/B (PQ):** same, on the PQ path.
- **BT.2408 SDR-white invariant:** PQ caps sRGB 255 at 38055 (~203 nits), well below HLG's 65535. This is the load-bearing detail that makes PQ headroom work — locking the exact value prevents a future "fix" that would re-scale PQ to peak-at-SDR-white and clip every real HDR pixel.

Reference values mirror `buildSrgbToHdrLut()` exactly and were verified against the existing HLG mid-gray comment in the file.

## Test plan

- [x] All new tests pass against the current LUT.
- [x] Existing `alphaBlit.test.ts` invariants unchanged.

## Stack

Chunk 9F of `plans/hdr-followups.md`. Test-only change, independent of all other chunks.

This commit is contained in:

Vance Ingalls

2026-04-23 10:37:45 -07:00

committed by

GitHub

parent 3089c8ee3a

commit bb9e6bdf05

4 changed files with 274 additions and 2 deletions

									
										packages/engine/src/services/chunkEncoder.test.ts
									
		-1
	
												View File
												
				@@ -439,7 +439,6 @@ describe("buildEncoderArgs HDR color space", () => {

				    expect(args[paramIdx + 1]).not.toContain("max-cll");

				  });

				  it("strips HDR and tags as SDR/BT.709 when codec=h264 (libx264 has no HDR support)", () => {

				    // libx264 cannot encode HDR. Rather than emit a "half-HDR" file (BT.2020

				    // container tags + BT.709 VUI inside the bitstream — confusing to HDR-aware