The centrosymmetry score of the pre-scan projection is a self-convolution, so one FFT set
scores every candidate centre on the detector at half-pixel spacing - the cost is O(N log N)
and independent of how far the header centre is from the truth, where every existing search
pays per pixel of error. BeamCenterFFT computes the 2D point-inversion score (a background
measurement, the physics FindBeamCenterFromBackground fits locally) and the two 1D line-mirror
scores (for the along-spindle coordinate this is exact Friedel physics, sharp exactly where
the indexing count is blind), and returns a non-maximum-suppressed shortlist plus a
peak-to-runner-up margin per surface - a shortlist and a margin, never a centre: measured on
17 datasets the surface can be locally flat (~25 px), and the margin is what says so.
Measured on the gross-header cases that motivate it (offline, float64 reference): a header
351 px wrong is captured at rank 1 within 1.0 px from 30 frames, and still from a 30 deg
wedge; two 73 px placeholder headers at rank 1 within 0.5 px; a dataset whose header is
~170 px wrong but which no centre can index is flagged by the lowest margin of the set
(0.8 %) instead of being answered confidently. Capture survives 30-120 deg wedges on all
four sets tried (union of 8 candidates within 6 px everywhere).
Numerics: fftwf (the tree's FFTW is single precision) with the valid-pixel mean subtracted
before the transform - the masked Pearson is exactly invariant under a global shift, and the
subtraction removes the large-term cancellation - and the per-element combination done in
double. Against the float64 numpy reference the shortlist positions are identical and the
margins agree to 3e-6 absolute on the two controls whose margins are 0.3 %. An overlap with
no image variance (a mirrored empty region) carries no evidence and minted r values of 4-274
in float64 as much as float32; such centres are now not scored (VARIANCE_FLOOR).
Wired report-only into the pre-scan next to the background estimate: one log line with the
strongest candidates, the margins and the wall time. Nothing consumes it.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011GxZqDiFP3KqriBhNdcR56
leonarski_fandClaude Opus 5
1d1d7fd686