openCV doc

オプティカルフロー(calcOpticalFlowFarneback)

Gallery

src

dst

what process?

密なオプティカルフローを使って前後の差分から物体の移動量を計算しています。 Gunnar Farneback のアルゴリズムを使っています。 ※密なオプティカルフローとは、画像全体から前後のフレーム間での差分をもとに物体、またはコーナーの移動量を計測する手法です。

cook code C++

#include <iostream>
#include <opencv2/core.hpp>
#include <opencv2/core/utility.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/videoio.hpp>
#include <opencv2/video.hpp>

/* ベクトルを描画 */
void draw_flow(cv::Mat& img, cv::Mat flow, int step){
    for(int y = 0; y < img.rows; y += step){
        for(int x = 0; x < img.cols; x += step)
        {
            const cv::Point2f& fxy = flow.at<cv::Point2f>(y, x);
            cv::line(img, cv::Point(x,y), cv::Point(cvRound(x+fxy.x), cvRound(y+fxy.y)), cv::Scalar(0,0,255));
            cv::circle(img, cv::Point(x,y), 1, cv::Scalar(0,0,255), -1);
        }
    }
}

int main(int argc, const char* argv[])
{
    // ./out -src=[image path]
    cv::String keys = "{src||}";
    cv::CommandLineParser parser(argc, argv, keys);
    cv::String src_path = parser.get<cv::String>("src");

cv::VideoCapture capture(src_path);
    if (!capture.isOpened()){
        //error in opening the video input
        std::cerr << "Unable to open file!" << std::endl;
        return 0;
    }

/* 初期設定 */
    int cap_width = int(capture.get(cv::CAP_PROP_FRAME_WIDTH));
    int cap_height = int(capture.get(cv::CAP_PROP_FRAME_HEIGHT));
    int fps = int(capture.get(cv::CAP_PROP_FPS));
    int fourcc = cv::VideoWriter::fourcc('m','p','4','v');
    cv::Size video_size = cv::Size(cap_width,cap_height);

cv::VideoWriter writer("output.mp4", fourcc, fps, video_size);

cv::Mat frame1, prvs;
    capture >> frame1;
    cv::cvtColor(frame1, prvs, cv::COLOR_BGR2GRAY);

while(true){
        cv::Mat frame2, next;
        capture >> frame2;
        if (frame2.empty())
            break;
        cv::cvtColor(frame2, next, cv::COLOR_BGR2GRAY);

cv::Mat flow(prvs.size(), CV_32FC2);
        cv::calcOpticalFlowFarneback(prvs, next, flow, 0.5, 3, 15, 3, 5, 1.2, 0);

draw_flow(frame2,flow,16);
        writer << frame2;
        int keyboard = cv::waitKey(30);
        if (keyboard == 'q'){
            break;
        }

prvs = next;
    }

capture.release();
    writer.release();

return 0;
}

cook code Python

import numpy as np
import cv2
import argparse

#ベクトルを描画
def draw_flow(img, flow, step=16):
    h, w = img.shape[:2]
    y, x = np.mgrid[step / 2:h:step, step /
        2:w:step].reshape(2, -1).astype(int)
    fx, fy = flow[y, x].T
    lines = np.vstack([x, y, x + fx, y + fy]).T.reshape(-1, 2, 2)
    lines = np.int32(lines + 0.5)
    cv2.polylines(img, lines, 0, (0, 0,255))
    for (x1, y1), (_x2, _y2) in lines:
        cv2.circle(img, (x1, y1), 1, (0, 0,255), -1)
    return img
    
def main():
    parser = argparse.ArgumentParser()
    parser.add_argument('--src')
    args = parser.parse_args()

cap = cv2.VideoCapture(args.src)

cap_width = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))
    cap_height = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))
    fps = cap.get(cv2.CAP_PROP_FPS)

fmt = cv2.VideoWriter_fourcc('m', 'p', '4', 'v')
    writer = cv2.VideoWriter('output.mp4', fmt, fps, (cap_width,cap_height))

_ret, prev = cap.read()
    prevgray = cv2.cvtColor(prev, cv2.COLOR_BGR2GRAY)

while cap.isOpened():
        _ret, img = cap.read()
        
        if img is None:
        	break

if cv2.waitKey(30) & 0xFF == ord('q'):
        	break

gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
        flow = cv2.calcOpticalFlowFarneback(prevgray, gray, None, 0.5, 3, 15, 3, 5, 1.2, 0)
        prevgray = gray

frame = draw_flow(img, flow)
        writer.write(frame)

cap.release()
    writer.release()

if __name__ == '__main__':
    main()

Reference

https://github.com/opencv/opencv/blob/4.4.0/samples/cpp/dis_opticalflow.cpp
https://github.com/opencv/opencv/blob/4.4.0/samples/python/opt_flow.py
https://stackoverflow.com/questions/34750415/optical-flow-ignore-sparse-motions
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