Coherent SSB reduction on Metal

Coherent SSB reduction on Metal#

Question and change#

Can repeated full-field aberration previews avoid writing a dense corrected spectrum for every bright-field pixel? The MPS coherent estimator now sums groups of 32 corrected spectra in registers and writes one spectrum per group. The final Fourier transform and phase extraction are unchanged. Batches retain all BF pixels, including incomplete groups and single-pixel tails.

This changes the floating-point reduction order. It does not change the estimator, float32/complex64 precision, calibration, scan resolution, aperture support or sampling rule. The native-grid mean-of-phase loss still evaluates the individual BF waves. Higher-order aberrations retain the full complex spectrum; the existing symmetry guard is unchanged.

Matched measurements#

Two complete 512 × 512 × 192 × 192 acquisitions were tested on Apple Silicon through the Python MPS backend. Case A used 8,807 BF pixels and 1536 × 1536 output; case B used 8,930 BF pixels and 2048 × 2048 output. The frozen baseline was 3362fd55. Ten new phase evaluations per row alternated baseline and candidate on the same prepared data, including zero and both focus signs and both rotation candidates. Times include GPU completion and phase return, but exclude loading, initial preparation, fitting and the separate native loss.

Case / model

Before p50 / p95 / max (s)

After p50 / p95 / max (s)

A ordinary

0.753 / 0.879 / 0.928

0.434 / 0.614 / 0.650

A tilt/depth

0.746 / 0.778 / 0.793

0.432 / 0.457 / 0.458

B ordinary

1.359 / 1.494 / 1.548

0.770 / 0.865 / 0.883

B tilt/depth

1.411 / 1.466 / 1.471

0.809 / 0.891 / 0.918

Median speedup was 1.7–1.8×. The complete interleaved validation took 72.70 s, including preparation and comparisons. Peak MLX allocation was 9.70 → 9.67 GB for A and 9.92 → 10.17 GB for B; these figures include resident prepared data. These are observed interactive-system measurements, not a 60 Hz claim or a complete acquisition-processing benchmark.

Numerical and workflow checks#

  • Forty complete-array comparisons in the interleaved test passed the existing absolute tolerance of 2e-5 rad, with maximum error 3.52e-6 rad.

  • Twelve additional frozen full-array comparisons covered ordinary, tilt/depth and higher-order reconstruction at native and automatic sampling. Maximum error was 3.49e-6 rad; all twelve native loss values were unchanged.

  • All 31 MPS upsampling tests passed, including factors 1, 2, 3, 4 and 8, the symmetry guard and a new incomplete-BF-group regression. Repeated candidate results were identical.

  • The legacy frozen-fit script built the native Metal target, then could not run because its external fixture strict-parity/arina-128-full-disk/case.json was unavailable. This is an unexecuted gate, not a pass. Native Swift and CUDA execution are unchanged by this MPS-only patch.

Private evidence: experiment 20261017-ssb-coherent-reduction. The application acceptance and HTTP timing are recorded there separately from these prepared GPU measurements.