beam centre: the FFT captures the whole detector, the walk refines what it captured

image_analysis/geom_refinement/BeamCenterFromBackground.cpp

@@ -82,7 +82,8 @@ float median_of(std::vector<float> &v) {
 
 std::optional<BeamCenterEstimate>
 FindBeamCenterFromBackground(const DiffractionExperiment &experiment, const PixelMask &mask,
-                             const std::vector<float> &mean, size_t nthreads) {
+                             const std::vector<float> &mean, size_t nthreads,
+                             std::optional<std::pair<float, float>> start) {
     if (nthreads == 0)
         nthreads = std::max(1u, std::thread::hardware_concurrency());
     const auto W = static_cast<int>(experiment.GetXPixelsNumConv());
@@ -106,8 +107,8 @@ FindBeamCenterFromBackground(const DiffractionExperiment &experiment, const Pixe
     const float pixel_size = geom.GetPixelSize_mm();
     const float distance = geom.GetDetectorDistance_mm();
 
-    float beam_x = geom.GetBeamX_pxl();
-    float beam_y = geom.GetBeamY_pxl();
+    float beam_x = start ? start->first : geom.GetBeamX_pxl();
+    float beam_y = start ? start->second : geom.GetBeamY_pxl();
 
     constexpr int n_cells = RADIAL_BINS * SECTORS;
     std::vector<int32_t> cell_of(n_pixels);
@@ -359,3 +360,38 @@ FindBeamCenterFromBackground(const DiffractionExperiment &experiment, const Pixe
     return BeamCenterEstimate{beam_x, beam_y,
                               std::max({sigma_x, sigma_y, std::hypot(step_x, step_y)})};
 }
+
+std::optional<BeamCenterEstimate>
+FindBeamCenter(const DiffractionExperiment &experiment, const PixelMask &mask,
+               const std::vector<float> &mean, size_t nthreads, BeamCenterFFTResult *capture) {
+    // The image the capture scores: the projection with everything the mask excludes taken out.
+    // The beam stop is in that mask by the time this runs, and it is the part that matters - an
+    // opaque region on one side of the beam is centrosymmetric about ITS own centre, and where it
+    // is large enough that preference beats the background's.
+    std::vector<float> scored = mean;
+    const auto &pixel_mask = mask.GetMask(experiment);
+    if (pixel_mask.size() == scored.size())
+        for (size_t i = 0; i < scored.size(); i++)
+            if (pixel_mask[i] != 0)
+                scored[i] = NAN;
+
+    const BeamCenterFFTResult fft = FindBeamCenterFFT(experiment, scored);
+    if (capture)
+        *capture = fft;
+    if (fft.point.empty())
+        return FindBeamCenterFromBackground(experiment, mask, mean, nthreads);
+
+    const std::pair<float, float> seed{fft.point[0].beam_x_pxl, fft.point[0].beam_y_pxl};
+    if (auto refined = FindBeamCenterFromBackground(experiment, mask, mean, nthreads, seed))
+        return refined;
+
+    // The walk has nothing to fit AT THE CAPTURE. That is a statement about the capture as much as
+    // about the background: a centre far enough out leaves too few fully covered rings inside the
+    // fitted band for the profile to exist at all. Measured on one small-molecule set whose capture
+    // lands 536 px out - the walk from the file's centre answers there to 2.9 px, and taking the
+    // capture instead would have been the composition's only regression on the corpus. So ask the
+    // walk again where it used to be asked, and keep the capture only where neither start works.
+    if (auto from_file = FindBeamCenterFromBackground(experiment, mask, mean, nthreads))
+        return from_file;
+    return BeamCenterEstimate{seed.first, seed.second, BEAM_CENTER_CAPTURE_SIGMA_PXL};
+}