OpenCV VideoCapture C++：讀取2個幀給出相同的圖片

Question

我正在編寫一個for循環程序，它在for循環的每次迭代中捕獲兩個不同的幀。我的攝像頭針對的是不斷變化的東西。

for循環的單次迭代中的第一幀和第二幀彼此不同，如預期的那樣。然而，很多時候（但不是全部），for循環的第i次迭代的第2幀看起來與for循環的第（i + 1）次迭代的第1幀完全相同。爲什麼會發生？這是我的代碼...

#include "opencv2/core/core.hpp" 
#include "opencv2/features2d/features2d.hpp" 
#include "opencv2/nonfree/features2d.hpp" 
#include "opencv2/calib3d/calib3d.hpp" 
#include "opencv2/highgui/highgui.hpp" 
#include "opencv2/nonfree/nonfree.hpp" 
#include "opencv2/imgproc/imgproc.hpp" 


#include <iostream> 
#include <fstream> 
#include <sstream> 
#include <stdlib.h> 

using namespace cv; 
using namespace std; 


int main(int argc, const char *argv[]) { 
//open webcam to capture video from it 
int choice; 
cout << " 1. Save frames and homography matrices to file.\n"; 
cout << " 2. Read in frames and homography matrices from file.\n"; 
cin >> choice; 
// Choice 1: output 
if (choice == 1){ 
cout << "You picked choice 1!"; 
VideoCapture cap(0); 
if(!cap.isOpened()) 
    return -1; 

Mat frame1c; // will hold the i'th frame, color 
Mat frame1; // ^^, grayscale 
Mat frame2c; // will hold the (i+1)'th frame, color 
Mat frame2; // ^^, grayscale 
std::vector<Mat> theframes; // will hold all the frames 
int myvar; 
string Hmatrices; // will hold all values of all H matrices. 
std::stringstream s; //used for converting the numerical values to a string 

//for(;;){ 
for(int i = 0; i<10; i++){ 
// Capture the two frames 
    cvWaitKey(500); 
    cap.read(frame1c); 
    cvWaitKey(200); 
    cap.read(frame2c); 
    cvWaitKey(200); 

    cvtColor(frame1c,frame1,CV_RGB2GRAY); 
    cvtColor(frame2c,frame2,CV_RGB2GRAY); 

//Detect points on the 2 successive images 
    int minHessian = 400; 
    SurfFeatureDetector detector(minHessian); 
    std::vector<KeyPoint> keypoints1, keypoints2; 
    detector.detect(frame1, keypoints1); 
    detector.detect(frame2, keypoints2); 
// If not enough keypoints are found (e.g. when ThinkPad webcam displays black screen initially), 
// then go to the next iteration of the For loop. 

    if (keypoints1.size() < 4){ 
     cout << " keypoints size <4\n\n"; 
     continue; 
    } 

// If the frames have keypoints, save them to "theframes" 
    cout << "pushback " << i << " f1c\n"; 
    theframes.push_back(frame1c.clone()); 
    cout << "pushback " << i << " f2c\n"; 
    theframes.push_back(frame2c.clone()); 

// Calculate feature-vectors of the points 
    SurfDescriptorExtractor extractor; 
    Mat descriptors1, descriptors2; 
    extractor.compute(frame1, keypoints1, descriptors1); 
    extractor.compute(frame2, keypoints2, descriptors2); 

// Match points on the 2 successive images by comparing feature-vectors 
    FlannBasedMatcher matcher; 
    std::vector<DMatch> matches; 
    matcher.match(descriptors1, descriptors2, matches); 
    cout << " all matches size is " << matches.size(); 

//Eliminate weaker matches 
    double maxdist = 0; 
    double mindist = 100; 
    for (int j = 0; j < descriptors1.rows; j++){ 
     double dist = matches[j].distance; 
     if(dist < mindist) mindist = dist; 
     if(dist > maxdist) maxdist = dist; 
    } 

//build the list of "good" matches 
    std::vector<DMatch> goodmatches; 
    for(int k = 0; k < descriptors1.rows; k++){ 
     if(matches[k].distance <= 3*mindist){ 
      goodmatches.push_back(matches[k]); 
     } 
    } 


//Now compute homography matrix between the stronger matches 

    //-- Localize the object 
    std::vector<Point2f> obj; 
    std::vector<Point2f> scene; 
    cout << "goodmatches size is " << goodmatches.size(); 
    unsigned int i; 
    std::stringstream s; 
    for(int l = 0; l < goodmatches.size(); l++){ 
    //-- Get the keypoints from the good matches 
    obj.push_back(keypoints1[goodmatches[l].queryIdx].pt); 
    scene.push_back(keypoints2[goodmatches[l].trainIdx].pt); 
    } 

    Mat Hmatrix; 
    cout << "obj size is " << obj.size(); 
    Hmatrix = findHomography(obj, scene, CV_RANSAC); 

    cout << "found the hmatrix\n"; 

      //from http://computer-vision-talks.com/articles/2013-06-07-undocumented-opencv/ 
      //The documentation of cv::findHomography does not state it, but return value of cv::findHomography was always 3x3 matrix of CV_64FC1 type 
      //cv_64fc1 is type double 
      // so can do cout << M.at<double>(0,0), cout << M.at<double>(0,1), etc 

// Save the values of the H matrix into the Hmatrices string 
    s << Hmatrix.at<double>(0,0) << " "; 
    s << Hmatrix.at<double>(0,1) << " "; 
    s << Hmatrix.at<double>(0,2) << " "; 

    s << Hmatrix.at<double>(1,0) << " "; 
    s << Hmatrix.at<double>(1,1) << " "; 
    s << Hmatrix.at<double>(1,2) << " "; 

    s << Hmatrix.at<double>(2,0) << " "; 
    s << Hmatrix.at<double>(2,1) << " "; 
    s << Hmatrix.at<double>(2,2) << "\n"; 
    Hmatrices = Hmatrices + s.str(); 
    /* 
    std::string test = s.str(); 

    cout << "now printing matrix " << test << " and done"; */ 

}//end for loop 

//std::string tosave = s.str(); 
cout << "now printing matrix " << Hmatrices << " and done"; 
ofstream savetofile; 
savetofile.open("hmatrices.txt"); 
Hmatrices.pop_back(); 
savetofile << Hmatrices; 
savetofile.close(); 

// Save theframes to jpeg files: 
for(int m = 0; m<theframes.size(); m++){ 
    cout << "saving to jpg" << m << "num\n"; 
    imwrite("frame" + std::to_string((long long)m) + ".jpg", theframes[m]); 
    cout << "made it after theframes i \n"; 
    cout << "inforloop theframes size " << theframes.size() << " that\n"; 
} 
int theint; 
cin >> theint; 

return 0;} 

//Choice 2: Read in 
else{ 
string filename; 
cout << "You picked choice 2!"; 
cout << "Enter name of file with the homography matrices: "; 
cin >> filename; 
cout << "\nYou entered the filename " << filename << "\n"; 

// Read in homography matrices 
ifstream readh; 
readh.open(filename); 
vector <double[3]> hs; 
int counter = 0; 
int i = 0; 
cout << "made it this far"; 
vector<Mat> thematrices; 
double dataforMat [3][3]; 
while(readh>>dataforMat[0][0]>>dataforMat[0][1]>>dataforMat[0][2]>>dataforMat[1][0]>>dataforMat[1][1]>>dataforMat[1][2]>>dataforMat[2][0]>>dataforMat[2][1]>>dataforMat[2][2]){ 
    cout << "made it to whileloop iteration number " << i; 
    thematrices.push_back(Mat(3,3,CV_64FC1,dataforMat)); 
    cout << "now printing matrix in mat form..\n"; 
    cout << thematrices[i]; 
    cout << "\nfinished printing mat matrix\n"; 
    i++; 
} 

int numh = thematrices.size(); 
vector<Mat> theimages; 

// Read in image files, named i.jpg for i = 1,2,3,..... 
for(int n = 0; n < numh; n++){ 
    theimages.push_back(imread("frame" + std::to_string((long long)n) + ".jpg")); 
} 

    imshow("title",theimages[0]); 
    waitKey(1000); 

    imshow("title",theimages[1]); 
    waitKey(1000); 

    imshow("title",theimages[2]); 
    waitKey(1000); 

    imshow("title",theimages[3]); 
    waitKey(1000); 

/* 
int theint; 
cin >> theint;*/ 

return 0;} 
return 0; 
} 

Answer 1

什麼是你的相機的fps？很可能，在循環中，您正在讀取來自相機的最後一幀，並且當您嘗試再次讀取時（第二次循環），則沒有新幀，因此相機會給出最新幀。
嘗試檢查幀是否相等，如果是這樣，請閱讀下一幀，直到獲得「新的東西」。如果你想確保圖像是相同的，使用subtract函數比較這兩個圖像，然後調用countNonZero。如果結果爲0，則圖像相等。