ADR-0526: Add enable_lcs and enable_chroma to float_ms_ssim SYCL twin¶

Status: Accepted
Date: 2026-05-17
Deciders: lusoris
Tags: sycl, parity, options

Context¶

The option-parity audit (.workingdir/feature-option-parity-audit-2026-05-16.md) identified that the SYCL twin of float_ms_ssim was missing enable_lcs and enable_chroma, both of which the CUDA twin already exposed (added in PR #933 and via the enable_chroma cross-backend pass). The SYCL collect_fex_sycl() already computed per-scale L, C, S means internally for the Wang product; the only gap was that the values were never forwarded to the feature collector when enable_lcs=true. The enable_chroma option is structural (MS-SSIM is luma-only by construction) and is accepted for option-table symmetry without semantic effect.

Decision¶

Add enable_lcs and enable_chroma fields to MsSsimStateSycl, expose them in options_ms_ssim_sycl[], and in collect_fex_sycl() emit the 15 per-scale intermediates (float_ms_ssim_{l,c,s}_scale{0..4}) when enable_lcs=true. The default-path output (both flags false) is bit-for-bit unchanged.

Alternatives considered¶

Option	Pros	Cons	Why not chosen
Defer until SYCL MS-SSIM is fully refactored	No churn now	Leaves a silent option-drop bug longer	The fix is three lines; deferral adds no value
Add only `enable_lcs`, skip `enable_chroma`	Slightly smaller diff	Options table stays asymmetric with CUDA	Symmetry is the whole point of the parity pass

Consequences¶

Positive: SYCL float_ms_ssim now matches CUDA's option table; callers using enable_lcs=true get the same 15 per-scale outputs on both GPU backends.
Negative: None — default-path is unchanged; enable_chroma is a no-op accepted for future compatibility.
Neutral / follow-ups: The audit row for SYCL enable_lcs can be marked DONE.

References¶

Option-parity audit: .workingdir/feature-option-parity-audit-2026-05-16.md
ADR-0485: enable_db/clip_db parity (this PR extends the same SYCL file)
CUDA reference implementation: core/src/feature/cuda/integer_ms_ssim_cuda.c