Jungfraujoch/image_analysis/indexing/SpindleBlindFraction.cpp

// SPDX-FileCopyrightText: 2026 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
// SPDX-License-Identifier: GPL-3.0-only

#include <algorithm>
#include <cmath>

#include "SpindleBlindFraction.h"

namespace {
    // Only rows short enough to be a plausible symmetry axis count. The cut is relative to the
    // crystal's own shortest row, not an absolute length, so it works the same for a 40 A cell and
    // a 200 A one: measured over 107 solved cells, 2.5 x the shortest row covers 88% of all
    // symmetry axes and 98% of crystals' shortest one.
    constexpr float MAX_ROW_LENGTH_RATIO = 2.5f;
    // The reference length is taken over the strong rows only. A long-cell still makes the pass
    // invent short spurious rows - measured at 0.4-0.6 of the true row's peak - and taking the
    // reference over every row would let one of those shrink the window until the real aligned row
    // fell outside it, turning a severe orientation into a reported zero.
    constexpr float MIN_MAGNITUDE_RATIO = 0.5f;
    // Where the search grid can no longer resolve the crystal's rows, the pass stops returning them
    // and starts returning short spurious ones instead, and the shortlist becomes internally
    // inconsistent: its strong rows are many times longer than its shortest entry. Measured on the
    // stills of 22 solved crystals - 22 independent mounts, so 22 is the sample size, not the
    // several hundred frames they contributed - the ratio never exceeded 2.34 and was 1.00 at the
    // median; on a
    // synthetic still whose cell is past the grid's reach it runs 4-13. Past this the score would be
    // reporting a cone it cannot see into, so it reports nothing instead.
    constexpr float MAX_REFERENCE_LENGTH_RATIO = 3.0f;
}

float SpindleThetaMax_deg(float wavelength_A, float d_min_A) {
    if (wavelength_A <= 0 || d_min_A <= 0)
        return 0;
    const float sin_theta = std::min(1.0f, wavelength_A / (2.0f * d_min_A));
    return static_cast<float>(std::asin(sin_theta) * 180.0 / M_PI);
}

float BlindConeSelfOverlap(float x) {
    if (x >= 1.0f)
        return 0.0f;
    if (x <= 0.0f)
        return 1.0f;
    return static_cast<float>(2.0 / M_PI) *
           (std::acos(x) - x * std::sqrt(1.0f - x * x));
}

std::optional<SpindleSeverity> SpindleBlindFraction(const std::vector<Coord> &rows,
                                                    const std::vector<float> &magnitudes,
                                                    const Coord &spindle,
                                                    float theta_max_deg) {
    if (rows.empty() || rows.size() != magnitudes.size() || theta_max_deg <= 0)
        return {};

    const float axis_length = spindle.Length();
    if (axis_length < 1e-6f)
        return {};
    const Coord axis = spindle.Normalize();

    float max_magnitude = 0;
    for (const auto &m : magnitudes)
        max_magnitude = std::max(max_magnitude, m);

    float reference_length = 0;
    for (size_t i = 0; i < rows.size(); i++)
        if (magnitudes[i] >= MIN_MAGNITUDE_RATIO * max_magnitude) {
            const float l = rows[i].Length();
            if (reference_length == 0 || l < reference_length)
                reference_length = l;
        }
    if (reference_length == 0)
        return {};

    float shortest_length = reference_length;
    for (const auto &r : rows)
        shortest_length = std::min(shortest_length, r.Length());
    if (reference_length > MAX_REFERENCE_LENGTH_RATIO * shortest_length)
        return {};

    std::vector<size_t> eligible;
    for (size_t i = 0; i < rows.size(); i++) {
        const float length = rows[i].Length();
        if (length <= 0 || length > MAX_ROW_LENGTH_RATIO * reference_length)
            continue;
        if (magnitudes[i] < MIN_MAGNITUDE_RATIO * max_magnitude)
            continue;
        eligible.push_back(i);
    }

    SpindleSeverity ret;
    bool scored = false;
    const auto consider = [&](const Coord &direction, float row_length_A) {
        const float len = direction.Length();
        const float cos_beta = std::min(1.0f, std::fabs(direction * axis) / len);
        const float beta_deg = static_cast<float>(std::acos(cos_beta) * 180.0 / M_PI);
        // A direction PERPENDICULAR to the spindle is as damaging as one along it, and far more
        // common: a lone 2-fold about it carries the blind cone onto the cone's opposite lobe, which
        // the same sweep leaves equally unmeasured. That is Friedel's rescue, which is no rescue -
        // the cone is double-sided. (An axis of order >= 3 there DOES repair the cone - measured
        // unrepaired fraction 0.000 for orders 3, 4 and 6 against 1.000 for order 2 - but a still
        // cannot know the order, and the lone diad is the worst case this bound assumes.) Both ends
        // of the range are the bad case and the safe zone lies between them, so the miss-angle is
        // folded about 45 deg. Checked against a Monte-Carlo of the true spherical overlap the
        // folded form is within 0.006 to theta_max = 20 deg and 0.024 to 45 deg; unfolded it is
        // wrong by a full 1.0 at beta = 90 deg, reporting the worst case as the best.
        const float fold_deg = std::min(beta_deg, 90.0f - beta_deg);
        const float score = BlindConeSelfOverlap(fold_deg / theta_max_deg);
        if (!scored || score > ret.score) {
            ret.score = score;
            ret.row_length_A = row_length_A;
            ret.miss_angle_deg = beta_deg;
            scored = true;
        }
    };

    for (const auto i : eligible)
        consider(rows[i], rows[i].Length());

    // A lone 2-fold on an axis LONGER than the length window is invisible above - not among the
    // shortlist's rows, and excluded by the window even when it is - but its direction is still
    // recoverable: the normal to two direct-lattice rows is itself a reciprocal-lattice row, and a
    // symmetry axis is parallel in the direct and reciprocal bases, so for a monoclinic cell
    // cross(a, c) IS the unique-axis direction whatever the length of b. Score the normals of the
    // strong in-window row pairs alongside the rows themselves; measured on a synthetic lone-diad
    // crystal with a 300 A unique axis, the fraction of severe mounts reported severe at the 0.5
    // trigger rises from 0.60 to 1.00 and the engagement rate on harmless mounts of that class
    // does not move. The guard only rejects a numerically degenerate normal; the shortlist already
    // keeps its rows 5 deg apart.
    for (size_t a = 0; a < eligible.size(); a++)
        for (size_t b = a + 1; b < eligible.size(); b++) {
            const Coord n = rows[eligible[a]] % rows[eligible[b]];
            if (n.Length() > 1e-4f * rows[eligible[a]].Length() * rows[eligible[b]].Length())
                consider(n, 0.0f);   // 0 = a direction inferred from a pair, not a measured row
        }

    return ret;
}