MPS coherent SSB redraw — 2026-10-17

MPS coherent SSB redraw — 2026-10-17#

Experiment: 20261017-mps-coherent-redraw (private evidence).

Question and contract#

Can full-data, tilt-aware SSB redraw approach a 16.7 ms frame budget on Apple M5 Max? The measured acquisition has a 512×512 scan, 192×192 detector, 4,697 selected bright-field pixels, and a 1024×1024 reconstructed phase. Inputs, calibration, BF selection, complex64 arithmetic, precise trigonometry, output sampling, and the phase_of_mean estimator are unchanged.

Change#

The previous path materialized a corrected spectrum for every BF pixel, then reduced hundreds of chunks. The coherent path now corrects and sums each BF group in one Metal kernel, using bounded partial arrays and ordered group accumulation. It skips only frequencies outside the maximum support of both antialiased probe apertures. Native phase-variance diagnostics retain their separate per-detector inverse FFTs. Source Hermitian storage is expanded when reading the source; the corrected spectrum still uses a complex inverse FFT.

Measurements and parity#

  • Initial warm reconstruction: about 236 ms before and 63 ms after.

  • A later interleaved before/after run under variable desktop load: medians 328 ms and 114 ms (2.9×). These are measurements on a shared desktop, not exclusive-device throughput guarantees.

  • Live notebook drag: median reconstruction 68 ms; median image paint interval 106 ms, with additional pauses at release for the unchanged loss diagnostic. Controls retained a roughly 16.7 ms animation interval. Images do not reach 60 fps.

  • Three frozen full-data tilt settings: maximum phase difference below 1e-7 radians. The inner float32 reduction order changes; this is not bit-exact phase reconstruction. Native losses and the two fixed-optics regional fit results were bit-exact in the before/after comparison.

  • Native sizes 128, 256, 512, and 1024, each at output factors 1 and 2, passed frozen same-backend phase comparisons. Native mean-phase and loss outputs were bit-exact. Small BF counts remain dominated by dispatch overhead.

  • Higher-order, alias/DC, anisotropic aperture, and bounded-launch checks pass. The broader MPS SSB run had 170 passes, two missing-simulation skips, and three fixture/calibration failures also reproduced with the frozen backend. The native Metal workflow gate passes. The frozen fit-trajectory gate could not run because its external fixture was absent; it is not counted as a pass.

Larger BF groups gave only small or unstable gains and changed accumulation rounding more. Per-detector bounding tests and additional aperture branches were not retained. The remaining compute cost is the full transfer-function correction; the final FFT and ordered partial reduction are much smaller.

Reconciliation with the guarded half-spectrum path#

The combined implementation retains the ordered detector-group accumulation and adds the canonical anti-Hermitian symmetry optimization for even C10/C12 probes. It evaluates the unique frequency half only when both Nyquist edges lie beyond the entire soft aperture support. Higher-order probes and cases that do not satisfy this guard evaluate the full spectrum. Both paths retain the complex inverse FFT, all BF contributions, and the same output sampling.

An interleaved comparison used the same three frozen full-data tilt settings, six evaluations per implementation and setting, excluding each first use:

Phase-only reconstruction

Mean

Median

p95

Frozen per-detector path

340 ms

292 ms

506 ms

Fused full-spectrum path

121 ms

102 ms

202 ms

Fused guarded half-spectrum path

62 ms

54 ms

98 ms

The combined path took about 34 ms in the quieter first setting. Later measurements slowed across all three implementations under sustained desktop load. These script measurements do not establish 60 fps or browser paint throughput. The combined median is 5.4× faster than the frozen path.

All three phase outputs remain within the unchanged 1e-7 radian bound, with a maximum error of 8.94e-8 radians. Native losses and both regional 200-trial Newton fits remain bit-exact against the earlier records. Fresh full-source loading took 0.39 s, preparation plus first phase/loss 5.17 s, and the two regional fits 2.38 s and 1.26 s. Filesystem cache state was not controlled. The focused integration suite passed 73 tests with one missing-data skip. Raw integration evidence is retained privately as 20261017-regional-reconciliation.