Jungfraujoch/image_analysis/spot_finding/ImageSpotFinderCPU.cpp

// SPDX-FileCopyrightText: 2024 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
// SPDX-License-Identifier: GPL-3.0-only

#include <algorithm>
#include <bit>
#include <bitset>
#include <cmath>

#include "ImageSpotFinderCPU.h"
#include "StrongPixelSet.h"

ImageSpotFinderCPU::ImageSpotFinderCPU(int32_t in_width, int32_t in_height)
    : ImageSpotFinder(in_width, in_height), first_pass_buffer(OutputSize(), 0) {}

void ImageSpotFinderCPU::Detect(const ImagePreprocessorBuffer &image,
                                const SpotFindingSettings &settings) {
    // Two passes, as ImageSpotFinderGPU::Detect does. The second recomputes every local background
    // with the pixels the first found strong taken out of it, and keeps those pixels strong. It
    // matters because a spot wide enough to reach into its own background window inflates the mean
    // and variance it is then tested against, so its outer pixels fail the SNR test on a single
    // pass. The GPU has always done this; running one pass here made the two finders return
    // different spot lists for the same frame.
    DetectPass(image, settings, nullptr, first_pass_buffer);
    DetectPass(image, settings, first_pass_buffer.data(), output_buffer);
}

namespace {

// The local-box SNR test of one pixel. sum/sum2/valid are its window's, centre pixel included.
bool StrongInWindow(int64_t pxl_val, int64_t sum, int64_t sum2, int64_t valid,
                    const SpotFindingSettings &settings, float strong2) {
    const int64_t sum_local = sum - pxl_val;
    const int64_t sum2_local = sum2 - pxl_val * pxl_val;
    const int64_t valid_local = valid - 1;

    const int64_t var = valid_local * sum2_local - (sum_local * sum_local);
    const int64_t in_minus_mean = pxl_val * valid_local - sum_local;

    return (pxl_val == INT32_MAX) // saturated pixel, or strong in the previous pass, is accepted always
           || ((pxl_val != INT32_MIN && // pixel is not bad pixel
                valid_local > ImageSpotFinder::MIN_VALID_PIXELS && // too many bad pixels around will give poor statistics
                (pxl_val > settings.photon_count_threshold) && // pixel is above count threshold
                (in_minus_mean > 0) && // pixel value is larger than mean
                (in_minus_mean * in_minus_mean > static_cast<int64_t>(std::ceil(var * strong2)))));
                // pixel is above SNR threshold
}

// Adds row `in` to the vertical sums and takes row `out` out of them; a row of INT32_MIN is no row. A
// bad or saturated pixel adds 0. Written without branches - both rows are always read, so no load is
// conditional - and as a function of its own, so that the loop vectorises whatever it is inlined into;
// integer sums, so the same totals as pixel by pixel.
void SlideVertical(const int32_t *in, const int32_t *out, int32_t width,
                   int64_t *sum, int64_t *sum2, uint16_t *valid) {
    for (int32_t col = 0; col < width; col++) {
        const int64_t a = in[col];
        const int64_t r = out[col];
        const bool a_ok = a != INT32_MAX && a != INT32_MIN;
        const bool r_ok = r != INT32_MAX && r != INT32_MIN;
        const int64_t a_v = a_ok ? a : 0;
        const int64_t r_v = r_ok ? r : 0;
        sum[col] += a_v - r_v;
        sum2[col] += a_v * a_v - r_v * r_v;
        valid[col] += static_cast<uint16_t>(a_ok - r_ok);
    }
}

} // namespace

void ImageSpotFinderCPU::DetectAt(const ImagePreprocessorBuffer &image, const SpotFindingSettings &settings,
                                  const std::vector<uint32_t> &candidates,
                                  const std::function<void(int32_t)> &fill_row) {
    candidate_windows.clear();
    // Filled in by DetectPass as the candidates of each row become known.
    first_pass_needed.assign(static_cast<size_t>(height) * ((width + 31) / 32), 0);
    DetectPass(image, settings, nullptr, first_pass_buffer, candidates.data(), fill_row);

    std::fill(output_buffer.begin(), output_buffer.end(), 0);
    const float strong2 = settings.signal_to_noise_threshold * settings.signal_to_noise_threshold;
    const auto first_pass = [&](int32_t pxl) { return (first_pass_buffer[pxl / 32] >> (pxl % 32)) & 1U; };

    for (const auto &c : candidate_windows) {
        const int32_t line = c.pxl / width;
        const int32_t col = c.pxl % width;
        int64_t sum = c.sum, sum2 = c.sum2, valid = c.valid;
        // Take out of the window what the second pass does not count: the first pass's strong pixels.
        for (int32_t y = std::max(line - NBX, 0); y <= std::min(line + NBX, height - 1); y++) {
            const int32_t first = y * width + std::max(col - NBX, 0);
            const int32_t last = y * width + std::min(col + NBX, width - 1);
            for (int32_t w = first / 32; w <= last / 32; w++) {
                uint32_t bits = first_pass_buffer[w];
                if (w == first / 32)
                    bits &= UINT32_MAX << (first % 32);
                if (w == last / 32 && last % 32 != 31)
                    bits &= (1U << (last % 32 + 1)) - 1;
                while (bits) {
                    const int32_t q = w * 32 + std::countr_zero(bits);
                    bits &= bits - 1;
                    const int64_t v = image[q];
                    if (v != INT32_MAX && v != INT32_MIN) {
                        sum -= v;
                        sum2 -= v * v;
                        valid -= 1;
                    }
                }
            }
        }
        const int64_t pxl_val = first_pass(c.pxl) ? INT32_MAX : image[c.pxl];
        if (StrongInWindow(pxl_val, sum, sum2, valid, settings, strong2))
            output_buffer[c.pxl / 32] |= 1U << (c.pxl % 32);
    }
}

void ImageSpotFinderCPU::DetectPass(const ImagePreprocessorBuffer &image,
                                    const SpotFindingSettings &settings,
                                    const uint32_t *prev_strong,
                                    std::vector<uint32_t> &out_buffer,
                                    const uint32_t *candidates,
                                    const std::function<void(int32_t)> &fill_row) {
    for (int i = 0; i < OutputSize(); i++)
        out_buffer[i] = 0;

    // The first pass's bits are read only inside the window of a candidate (and at the candidate), so
    // with candidates it tests only the pixels of the row/column blocks such a window reaches; the bits
    // it leaves unset there are never read. Those blocks are marked from each row's candidates as the
    // row enters the vertical sums below, which is before any row within NBX of it is tested - so the
    // candidates themselves can be filled in a row at a time (fill_row), while the row is in cache.
    const int32_t nblocks = (width + 31) / 32;
    const auto new_row = [&](int32_t r) {
        if (!candidates)
            return;
        if (fill_row)
            fill_row(r);
        const int32_t first = r * width;
        const int32_t last = first + width - 1;
        for (int32_t w = first / 32; w <= last / 32; w++) {
            uint32_t bits = candidates[w];
            if (w == first / 32)
                bits &= UINT32_MAX << (first % 32);
            if (w == last / 32 && last % 32 != 31)
                bits &= (1U << (last % 32 + 1)) - 1;
            for (; bits; bits &= bits - 1) {
                const int32_t col = w * 32 + std::countr_zero(bits) - first;
                for (int32_t y = std::max(r - NBX, 0); y <= std::min(r + NBX, height - 1); y++)
                    for (int32_t b = std::max(col - NBX, 0) / 32; b <= std::min(col + NBX, width - 1) / 32; b++)
                        first_pass_needed[static_cast<size_t>(y) * nblocks + b] = 1;
            }
        }
    };

    // A pixel found strong by the previous pass reads as INT32_MAX, which the accumulation below
    // already skips and the acceptance test below already takes as strong - the same substitution
    // the GPU kernel makes when it reads prev_out.
    auto value_at = [&](int32_t pxl) -> int32_t {
        if (prev_strong && (prev_strong[pxl / 32] & (1U << (pxl % 32))))
            return INT32_MAX;
        return image[pxl];
    };

    std::bitset<32> out = 0;

    if (settings.signal_to_noise_threshold <= 0.0) {
        if (settings.photon_count_threshold > 0) {
            for (int pxl = 0; pxl < height * width; pxl++) {
                int32_t bit = pxl % 32;
                int32_t pxl_val = value_at(pxl);
                if (pxl_val == INT32_MAX || (pxl_val > settings.photon_count_threshold && pxl_val != INT32_MIN))
                    out.set(bit);

                if (bit == 31) {
                    out_buffer[pxl / 32] = out.to_ulong();
                    out.reset();
                }
            }
        }
    } else {
        float strong2 = settings.signal_to_noise_threshold * settings.signal_to_noise_threshold;

        // Sum and sum of squares of (2*NBY+1) vertical elements
        // These are updated after each line is finished
        // 64-bit integer guarantees calculations are made without rounding errors
        std::vector<int64_t> sum_vert(width, 0);
        std::vector<int64_t> sum2_vert(width, 0);
        std::vector<uint16_t> valid_vert(width, 0);

        // Add the row entering the window (line_in) and take out the one leaving it (line_out); -1 is
        // no row. A pixel strong in the previous pass is not counted either; SlideVertical adds it with
        // the rest of its row and it is taken out again below - there are few, and the sums are
        // integers, so the totals are the same as never adding it.
        const int32_t *img = image.data();
        const int32_t w = width;
        int64_t *sv = sum_vert.data();
        int64_t *sv2 = sum2_vert.data();
        uint16_t *vv = valid_vert.data();
        const std::vector<int32_t> no_row(width, INT32_MIN);
        auto slide_vert = [&](int line_in, int line_out) {
            SlideVertical(line_in >= 0 ? img + static_cast<size_t>(line_in) * w : no_row.data(),
                          line_out >= 0 ? img + static_cast<size_t>(line_out) * w : no_row.data(), w, sv, sv2, vv);
            if (!prev_strong)
                return;
            // sign = +1 takes out what line_in added, -1 puts back what line_out took out.
            const auto uncount_strong = [&](int line, int64_t sign) {
                const int32_t first = line * w, last = first + w - 1;
                for (int32_t word = first / 32; word <= last / 32; word++) {
                    uint32_t bits = prev_strong[word];
                    if (word == first / 32)
                        bits &= UINT32_MAX << (first % 32);
                    if (word == last / 32 && last % 32 != 31)
                        bits &= (1U << (last % 32 + 1)) - 1;
                    for (; bits; bits &= bits - 1) {
                        const int32_t pxl = word * 32 + std::countr_zero(bits);
                        const int64_t v = img[pxl];
                        if (v == INT32_MAX || v == INT32_MIN)
                            continue;
                        sv[pxl - first] -= sign * v;
                        sv2[pxl - first] -= sign * v * v;
                        vv[pxl - first] -= static_cast<uint16_t>(sign);
                    }
                }
            };
            if (line_in >= 0)
                uncount_strong(line_in, 1);
            if (line_out >= 0)
                uncount_strong(line_out, -1);
        };

        for (int line = 0; line < NBX; line++) {
            new_row(line);
            slide_vert(line, -1);
        }

        for (int line = 0; line < height; line++) {
            if (line < height - NBX)
                new_row(line + NBX);
            slide_vert(line < height - NBX ? line + NBX : -1, line >= NBX + 1 ? line - (NBX + 1) : -1);

            if (candidates) {
                // Only the runs of 32-column blocks whose result is read (first_pass_needed; every
                // candidate lies in one). Each run starts its window from the vertical sums over the
                // columns it covers there - integers, so the same sums the running window holds at
                // that column - and slides it as below; the bits outside the runs stay 0, as below.
                const uint8_t *needed = first_pass_needed.data() + static_cast<size_t>(line) * nblocks;
                for (int32_t b = 0; b < nblocks;) {
                    if (!needed[b]) { b++; continue; }
                    const int32_t c0 = b * 32;
                    while (b < nblocks && needed[b]) b++;
                    const int32_t c1 = std::min(b * 32, width);
                    int64_t sum = 0, sum2 = 0, valid = 0;
                    for (int32_t c = std::max(c0 - NBX, 0); c <= std::min(c0 + NBX, width - 1); c++) {
                        sum += sum_vert[c];
                        sum2 += sum2_vert[c];
                        valid += valid_vert[c];
                    }
                    for (int32_t col = c0; col < c1; col++) {
                        if (col > c0) {
                            if (col < width - NBX) {
                                sum += sum_vert[col + NBX];
                                sum2 += sum2_vert[col + NBX];
                                valid += valid_vert[col + NBX];
                            }
                            if (col >= NBX + 1) {
                                sum -= sum_vert[col - NBX - 1];
                                sum2 -= sum2_vert[col - NBX - 1];
                                valid -= valid_vert[col - NBX - 1];
                            }
                        }
                        const int32_t pxl = line * width + col;
                        if (candidates[pxl / 32] & (1U << (pxl % 32)))
                            candidate_windows.push_back({pxl, sum, sum2, valid});
                        if (StrongInWindow(value_at(pxl), sum, sum2, valid, settings, strong2))
                            out_buffer[pxl / 32] |= 1U << (pxl % 32);
                    }
                }
                continue;
            }

            int64_t sum = 0;
            int64_t sum2 = 0;
            int64_t valid = 0;

            for (int col = 0; col < NBX; col++) {
                sum += sum_vert[col];
                sum2 += sum2_vert[col];
                valid += valid_vert[col];
            }

            for (int col = 0; col < width; col++) {
                if (col < width - NBX) {
                    sum += sum_vert[col + NBX];
                    sum2 += sum2_vert[col + NBX];
                    valid += valid_vert[col + NBX];
                }

                if (col >= NBX + 1) {
                    sum -= sum_vert[col - NBX - 1];
                    sum2 -= sum2_vert[col - NBX - 1];
                    valid -= valid_vert[col - NBX - 1];
                }
                const int32_t pxl = line * width + col;
                const int32_t bit = pxl % 32;

                if (StrongInWindow(value_at(pxl), sum, sum2, valid, settings, strong2))
                    out.set(bit);

                if (bit == 31) {
                    out_buffer[pxl / 32] = out.to_ulong();
                    out.reset() ;
                }
            }
        }
    }

    if (height * width % 32 != 0)
        out_buffer[OutputSize() - 1] |= out.to_ulong();   // zeroed above; the candidates path sets its bits directly
}