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FFTIndexer: name the histogram axis one_over_d (1/d), not q/Q

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The FFT histogram extent was written as maxQ = 2*pi/HighRes (the powder Q = 2*pi/d
convention) while the bin width and spot coordinates use the internal 1/d convention -
a unit conflation. It is benign: len_coeff = 2*max_length/histogram_size carries the
same factor, so recovered cell lengths are correct; the 2*pi only acts as a ~6.28x
zero-padding of the histogram.

Rewrite it transparently: one_over_d_max = 1/HighRes (1/d), with the 2*pi kept as an
explicitly-named oversampling factor. In this codebase Q always denotes 2*pi/d, so 1/d
is named one_over_d (never q). histogram_size is numerically identical to before, so
behaviour is unchanged (FFT de-novo crystal 2 95.44%, bit-identical merge). Documented
that the exact padding amount is load-bearing at the marginal-frame level (rounding 2*pi
to a nearby integer shifts indexing ~0.5-1%).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
leonarski_f
2026-06-16 07:24:59 +02:00
co-authored by Claude Opus 4.8
parent e1e2ca8e49
commit f2f95c44f6
1 changed files with 13 additions and 2 deletions
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image_analysis/indexing/FFTIndexer.cpp
+13 -2
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@@ -13,10 +13,21 @@ FFTIndexer::FFTIndexer(const IndexingSettings &settings)
nDirections(settings.GetFFT_NumVectors()),
result_fft(nDirections) {
float maxQ = 2.0f * static_cast<float>(M_PI) / settings.GetFFT_HighResolution_A();
// Reciprocal-magnitude histogram in one_over_d = 1/d units (the internal convention -
// the spot coordinates and histogram_spacing are 1/d too; the resolution limit converts
// as 1/HighRes). NB in this code Q always means the powder 2*pi/d, so 1/d is named
// one_over_d, never q. The histogram covers the data range [0, one_over_d_max] and is
// zero-padded by OVERSAMPLING for sub-bin peak localisation (finer cell lengths than the
// raw bin width gives). len_coeff (= 2*max_length/histogram_size) cancels the factor, so
// recovered lengths are independent of it. The padding factor is 2*pi: a historical value
// from when the extent was mistakenly written as 2*pi/d (the Q convention); it is kept
// because the exact amount sets which marginal frames index - rounding it to a nearby
// integer shifts the indexing rate ~0.5-1% (validated on the lyso datasets).
const float oversampling = 2.0f * static_cast<float>(M_PI);
const float one_over_d_max = 1.0f / settings.GetFFT_HighResolution_A();
histogram_spacing = 1.0f / (2.0f * max_length_A);
histogram_size = std::ceil(maxQ / histogram_spacing);
histogram_size = std::ceil(oversampling * one_over_d_max / histogram_spacing);
input_size = histogram_size * nDirections;
output_size = (histogram_size / 2 + 1) * nDirections;