sort.cpp

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// © OpenShot Studios, LLC
//
// SPDX-License-Identifier: LGPL-3.0-or-later
 
#include "sort.hpp"
 
using namespace std;
 
// Constructor
SortTracker::SortTracker(int max_age, int min_hits, int max_missed, double min_iou, double nms_iou_thresh, double min_conf)
{
    _min_hits = min_hits;
    _max_age = max_age;
    _max_missed = max_missed;
    _min_iou = min_iou;
    _nms_iou_thresh = nms_iou_thresh;
    _min_conf = min_conf;
    _next_id = 0;
    alive_tracker = true;
}
 
// Computes IOU between two bounding boxes
double SortTracker::GetIOU(cv::Rect_<float> bb_test, cv::Rect_<float> bb_gt)
{
    float in = (bb_test & bb_gt).area();
    float un = bb_test.area() + bb_gt.area() - in;
 
    if (un < DBL_EPSILON)
        return 0;
 
    return (double)(in / un);
}
 
// Computes centroid distance between two bounding boxes
double SortTracker::GetCentroidsDistance(
    cv::Rect_<float> bb_test,
    cv::Rect_<float> bb_gt)
{
    float bb_test_centroid_x = (bb_test.x + bb_test.width / 2);
    float bb_test_centroid_y = (bb_test.y + bb_test.height / 2);
 
    float bb_gt_centroid_x = (bb_gt.x + bb_gt.width / 2);
    float bb_gt_centroid_y = (bb_gt.y + bb_gt.height / 2);
 
    double distance = (double)sqrt(pow(bb_gt_centroid_x - bb_test_centroid_x, 2) + pow(bb_gt_centroid_y - bb_test_centroid_y, 2));
 
    return distance;
}
 
// Function to apply NMS on detections
void apply_nms(vector<TrackingBox>& detections, double nms_iou_thresh) {
    if (detections.empty()) return;
 
    // Sort detections by confidence descending
    std::sort(detections.begin(), detections.end(), [](const TrackingBox& a, const TrackingBox& b) {
        return a.confidence > b.confidence;
    });
 
    vector<bool> suppressed(detections.size(), false);
 
    for (size_t i = 0; i < detections.size(); ++i) {
        if (suppressed[i]) continue;
 
        for (size_t j = i + 1; j < detections.size(); ++j) {
            if (suppressed[j]) continue;
 
            if (detections[i].classId == detections[j].classId &&
                SortTracker::GetIOU(detections[i].box, detections[j].box) > nms_iou_thresh) {
                suppressed[j] = true;
            }
        }
    }
 
    // Remove suppressed detections
    vector<TrackingBox> filtered;
    for (size_t i = 0; i < detections.size(); ++i) {
        if (!suppressed[i]) {
            filtered.push_back(detections[i]);
        }
    }
    detections = filtered;
}
 
void SortTracker::update(vector<cv::Rect> detections_cv, int frame_count, double image_diagonal, std::vector<float> confidences, std::vector<int> classIds)
{
    vector<TrackingBox> detections;
    if (trackers.size() == 0) // the first frame met
    {
        alive_tracker = false;
        // initialize kalman trackers using first detections.
        for (unsigned int i = 0; i < detections_cv.size(); i++)
        {
            if (confidences[i] < _min_conf) continue; // filter low conf
 
            TrackingBox tb;
 
            tb.box = cv::Rect_<float>(detections_cv[i]);
            tb.classId = classIds[i];
            tb.confidence = confidences[i];
            detections.push_back(tb);
 
            KalmanTracker trk = KalmanTracker(detections.back().box, detections.back().confidence, detections.back().classId, _next_id++);
            trackers.push_back(trk);
        }
        return;
    }
    else
    {
        for (unsigned int i = 0; i < detections_cv.size(); i++)
        {
            if (confidences[i] < _min_conf) continue; // filter low conf
 
            TrackingBox tb;
            tb.box = cv::Rect_<float>(detections_cv[i]);
            tb.classId = classIds[i];
            tb.confidence = confidences[i];
            detections.push_back(tb);
        }
 
        // Apply NMS to remove duplicates
        apply_nms(detections, _nms_iou_thresh);
 
        for (auto it = frameTrackingResult.begin(); it != frameTrackingResult.end(); it++)
        {
            int frame_age = frame_count - it->frame;
            if (frame_age >= _max_age || frame_age < 0)
            {
                dead_trackers_id.push_back(it->id);
            }
        }
    }
 
    // 3.1. get predicted locations from existing trackers.
    predictedBoxes.clear();
    for (unsigned int i = 0; i < trackers.size();)
    {
        cv::Rect_<float> pBox = trackers[i].predict();
        if (pBox.x >= 0 && pBox.y >= 0)
        {
            predictedBoxes.push_back(pBox);
            i++;
            continue;
        }
        trackers.erase(trackers.begin() + i);
    }
 
    trkNum = predictedBoxes.size();
    detNum = detections.size();
 
    cost_matrix.clear();
    cost_matrix.resize(trkNum, vector<double>(detNum, 0));
 
    for (unsigned int i = 0; i < trkNum; i++) // compute cost matrix using 1 - IOU with gating
    {
        for (unsigned int j = 0; j < detNum; j++)
        {
            double iou = GetIOU(predictedBoxes[i], detections[j].box);
            double dist = GetCentroidsDistance(predictedBoxes[i], detections[j].box) / image_diagonal;
            if (trackers[i].classId != detections[j].classId || dist > max_centroid_dist_norm)
            {
                cost_matrix[i][j] = 1e9; // large cost for gating
            }
            else
            {
                cost_matrix[i][j] = 1 - iou + (1 - detections[j].confidence) * 0.1; // slight penalty for low conf
            }
        }
    }
 
    HungarianAlgorithm HungAlgo;
    assignment.clear();
    HungAlgo.Solve(cost_matrix, assignment);
    // find matches, unmatched_detections and unmatched_predictions
    unmatchedTrajectories.clear();
    unmatchedDetections.clear();
    allItems.clear();
    matchedItems.clear();
 
    if (detNum > trkNum) // there are unmatched detections
    {
        for (unsigned int n = 0; n < detNum; n++)
            allItems.insert(n);
 
        for (unsigned int i = 0; i < trkNum; ++i)
            matchedItems.insert(assignment[i]);
 
        set_difference(allItems.begin(), allItems.end(),
                       matchedItems.begin(), matchedItems.end(),
                       insert_iterator<set<int>>(unmatchedDetections, unmatchedDetections.begin()));
    }
    else if (detNum < trkNum) // there are unmatched trajectory/predictions
    {
        for (unsigned int i = 0; i < trkNum; ++i)
            if (assignment[i] == -1) // unassigned label will be set as -1 in the assignment algorithm
                unmatchedTrajectories.insert(i);
    }
    else
        ;
 
    // filter out matched with low IOU
    matchedPairs.clear();
    for (unsigned int i = 0; i < trkNum; ++i)
    {
        if (assignment[i] == -1) // pass over invalid values
            continue;
        if (cost_matrix[i][assignment[i]] > 1 - _min_iou)
        {
            unmatchedTrajectories.insert(i);
            unmatchedDetections.insert(assignment[i]);
        }
        else
            matchedPairs.push_back(cv::Point(i, assignment[i]));
    }
 
    for (unsigned int i = 0; i < matchedPairs.size(); i++)
    {
        int trkIdx = matchedPairs[i].x;
        int detIdx = matchedPairs[i].y;
        trackers[trkIdx].update(detections[detIdx].box);
        trackers[trkIdx].classId = detections[detIdx].classId;
        trackers[trkIdx].confidence = detections[detIdx].confidence;
    }
 
    // create and initialise new trackers for unmatched detections
    for (auto umd : unmatchedDetections)
    {
        KalmanTracker tracker = KalmanTracker(detections[umd].box, detections[umd].confidence, detections[umd].classId, _next_id++);
        trackers.push_back(tracker);
    }
 
    for (auto it2 = dead_trackers_id.begin(); it2 != dead_trackers_id.end(); it2++)
    {
        for (unsigned int i = 0; i < trackers.size();)
        {
            if (trackers[i].m_id == (*it2))
            {
                trackers.erase(trackers.begin() + i);
                continue;
            }
            i++;
        }
    }
 
    // get trackers' output
    frameTrackingResult.clear();
    for (unsigned int i = 0; i < trackers.size();)
    {
        if ((trackers[i].m_hits >= _min_hits && trackers[i].m_time_since_update <= _max_missed) ||
            frame_count <= _min_hits)
        {
            alive_tracker = true;
            TrackingBox res;
            res.box = trackers[i].get_state();
            res.id = trackers[i].m_id;
            res.frame = frame_count;
            res.classId = trackers[i].classId;
            res.confidence = trackers[i].confidence;
            frameTrackingResult.push_back(res);
        }
 
        // remove dead tracklet
        if (trackers[i].m_time_since_update >= _max_age)
        {
            trackers.erase(trackers.begin() + i);
            continue;
        }
        i++;
    }
}