背景差分法是一种很常用而且广泛传感的技术，主要用于背景不动的情况下提取前景。它主要的原理是在当前帧和背景做减法，然后使用threshold进行二值化得到前景掩码。下面是背景减法的示意图。 背景差分法主要包含以下两个步骤： 1.背景的建立 2.背景的更新 两个关键点，第一个就是如何选择背景，第二个就是什么时候更新背景，这样可以适应背景变化的情况，更新背景的快慢也很重要，如果更新慢了，快速运动的物体如果没有重叠就认为是两个物体了，如果更新快了，慢速运动的物体会被认为是背景，这样就造成漏检。 这里学习一下在opencv中如何使用背景差分法。
使用了两个方法来产生前景掩码： 1. cv::bgsegm::BackgroundSubtractorMog 2. cv::BackgroundSubtractorMog2
//opencv #include "opencv2/imgcodecs.hpp" #include "opencv2/imgproc.hpp" #include "opencv2/videoio.hpp" #include <opencv2/highgui.hpp> #include <opencv2/video.hpp> //C #include <stdio.h> //C++ #include <iostream> #include <sstream> using namespace cv; using namespace std; // Global variables Mat frame; //current frame Mat fgMaskMOG2; //fg mask fg mask generated by MOG2 method Ptr<BackgroundSubtractor> pMOG2; //MOG2 Background subtractor char keyboard; //input from keyboard void help(); void processVideo(char* videoFilename); void processImages(char* firstFrameFilename); void help() { cout << "--------------------------------------------------------------------------" << endl << "This program shows how to use background subtraction methods provided by " << endl << " OpenCV. You can process both videos (-vid) and images (-img)." << endl << endl << "Usage:" << endl << "./bg_sub {-vid <video filename>|-img <image filename>}" << endl << "for example: ./bg_sub -vid video.avi" << endl << "or: ./bg_sub -img /data/images/1.png" << endl << "--------------------------------------------------------------------------" << endl << endl; } int main(int argc, char* argv[]) { //print help information help(); //check for the input parameter correctness if(argc != 3) { cerr <<"Incorret input list" << endl; cerr <<"exiting..." << endl; return EXIT_FAILURE; } //create GUI windows namedWindow("Frame"); namedWindow("FG Mask MOG 2"); //create Background Subtractor objects pMOG2 = createBackgroundSubtractorMOG2(); //MOG2 approach if(strcmp(argv[1], "-vid") == 0) { //input data coming from a video processVideo(argv[2]); } else if(strcmp(argv[1], "-img") == 0) { //input data coming from a sequence of images processImages(argv[2]); } else { //error in reading input parameters cerr <<"Please, check the input parameters." << endl; cerr <<"Exiting..." << endl; return EXIT_FAILURE; } //destroy GUI windows destroyAllWindows(); return EXIT_SUCCESS; } void processVideo(char* videoFilename) { //create the capture object VideoCapture capture(videoFilename); if(!capture.isOpened()){ //error in opening the video input cerr << "Unable to open video file: " << videoFilename << endl; exit(EXIT_FAILURE); } //read input data. ESC or 'q' for quitting keyboard = 0; while( keyboard != 'q' && keyboard != 27 ){ //read the current frame if(!capture.read(frame)) { cerr << "Unable to read next frame." << endl; cerr << "Exiting..." << endl; exit(EXIT_FAILURE); } //update the background model pMOG2->apply(frame, fgMaskMOG2); //get the frame number and write it on the current frame stringstream ss; rectangle(frame, cv::Point(10, 2), cv::Point(100,20), cv::Scalar(255,255,255), -1); ss << capture.get(CAP_PROP_POS_FRAMES); string frameNumberString = ss.str(); putText(frame, frameNumberString.c_str(), cv::Point(15, 15), FONT_HERSHEY_SIMPLEX, 0.5 , cv::Scalar(0,0,0)); //show the current frame and the fg masks imshow("Frame", frame); imshow("FG Mask MOG 2", fgMaskMOG2); //get the input from the keyboard keyboard = (char)waitKey( 30 ); } //delete capture object capture.release(); } void processImages(char* fistFrameFilename) { //read the first file of the sequence frame = imread(fistFrameFilename); if(frame.empty()){ //error in opening the first image cerr << "Unable to open first image frame: " << fistFrameFilename << endl; exit(EXIT_FAILURE); } //current image filename string fn(fistFrameFilename); //read input data. ESC or 'q' for quitting keyboard = 0; while( keyboard != 'q' && keyboard != 27 ){ //update the background model pMOG2->apply(frame, fgMaskMOG2); //get the frame number and write it on the current frame size_t index = fn.find_last_of("/"); if(index == string::npos) { index = fn.find_last_of("\\"); } size_t index2 = fn.find_last_of("."); string prefix = fn.substr(0,index+1); string suffix = fn.substr(index2); string frameNumberString = fn.substr(index+1, index2-index-1); istringstream iss(frameNumberString); int frameNumber = 0; iss >> frameNumber; rectangle(frame, cv::Point(10, 2), cv::Point(100,20), cv::Scalar(255,255,255), -1); putText(frame, frameNumberString.c_str(), cv::Point(15, 15), FONT_HERSHEY_SIMPLEX, 0.5 , cv::Scalar(0,0,0)); //show the current frame and the fg masks imshow("Frame", frame); imshow("FG Mask MOG 2", fgMaskMOG2); //get the input from the keyboard keyboard = (char)waitKey( 30 ); //search for the next image in the sequence ostringstream oss; oss << (frameNumber + 1); string nextFrameNumberString = oss.str(); string nextFrameFilename = prefix + nextFrameNumberString + suffix; //read the next frame frame = imread(nextFrameFilename); if(frame.empty()){ //error in opening the next image in the sequence cerr << "Unable to open image frame: " << nextFrameFilename << endl; exit(EXIT_FAILURE); } //update the path of the current frame fn.assign(nextFrameFilename); } }结果显示如果人停下了，那么就检测不到了。
保存使用cv::imread就可以了，代码实现很简单
string imageToSave = "output_MOG_" + frameNumberString + ".png"; bool saved = imwrite(imageToSave, fgMaskMOG); if(!saved) { cerr << "Unable to save " << imageToSave << endl; }