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video frame is different frome imgs?

asked 2013-03-17 00:07:51 -0500

wdxz6547 gravatar image

updated 2013-03-17 23:34:41 -0500

int main(int argc, char** argv)
{
    if (argc != 2) {
    help(argv);
    return 1;
    }
    std::string arg = argv[1];
    VideoCapture capture(arg); 
    if (!capture.isOpened()){ 
    capture.open(atoi(arg.c_str()));
    }
    if (!capture.isOpened()) {
    cerr << "Failed to open a video device or video file!\n" << endl;
    help(argv);
    return 1;
    }

    char filename[200];
    Mat frame1;
    Mat frame2;
    Mat frame3;
    Mat frame4;

    capture >> frame1;
    sprintf(filename,"frame%.3d.jpg",1);
    imwrite(filename,frame1);
    frame3 = imread(filename);

    capture.set(CV_CAP_PROP_POS_FRAMES, 100);

    capture >> frame2;
    sprintf(filename,"frame%.3d.jpg",2);
    imwrite(filename,frame2);
    frame4 = imread(filename);

    cout << (double)cmpHistOfHSV(frame1, frame2) << endl;
    cout << (double)partOfCmpHist(frame1, frame2) << endl;

    cout << cmpHistOfHSV(frame3, frame4) << endl;
    cout << partOfCmpHist(frame3, frame4) << endl;
    return 0;
}

the output:

0
0
4074.85
6790.25

why, the video frame is different the img?

Appendix:

double cmpHistOfHSV(Mat src_base1, Mat src_base2)
{
    const int HL_RANGE = 0;
    const int HR_RANGE = 256;
    const int SL_RANGE = 0;
    const int SR_RANGE = 180;

    const int H_BINS = 12;//50
    const int S_BINS = 6;//60

    Mat hsv_base1, hsv_base2;

    // Convert to HSV
    cvtColor(src_base1, hsv_base1, CV_BGR2HSV);
    cvtColor(src_base2, hsv_base2, CV_BGR2HSV);


    //30 bins for Hue, 32 for Saturation
    int h_bins = H_BINS;
    int s_bins = S_BINS;
    int histSize[] = {h_bins, s_bins};

    //hue varies from 0 to 256, saturation from 0 to 180
    float h_ranges[] = {HL_RANGE, HR_RANGE};
    float s_ranges[] = {SL_RANGE, SR_RANGE};
    const float *ranges[] = {h_ranges, s_ranges};

    //Use the 0th and 1th channels
    int channels[] = {0, 1};

    //Histogram
    MatND hist_base1;
    MatND hist_base2;

    //Calculate the histogram for HSV images
    calcHist(&hsv_base1, 1, channels, Mat(), hist_base1, 2, histSize, ranges, true, false);
    normalize(hist_base1, hist_base1, 0, 1, NORM_MINMAX, -1, Mat());

    calcHist(&hsv_base2, 1, channels, Mat(), hist_base2, 2, histSize, ranges, true, false);
    normalize(hist_base2, hist_base2, 0, 1, NORM_MINMAX, -1, Mat());

    double base_base = compareHist(hist_base1, hist_base2, 1);
    return base_base;
}
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2 answers

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answered 2013-03-17 03:53:06 -0500

  1. I do not find cmpHistOfHSV and partOfCmpHist functions in OpenCV. What do they do?
  2. Jpeg is lossy image format. It means that frame1 differs from frame3.
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Comments

yeah, it is written by myself which is used for compare two images or frames. what's the different between Jpeg and frame?

wdxz6547 gravatar imagewdxz6547 ( 2013-03-17 20:57:20 -0500 )edit
0

answered 2013-03-18 02:55:52 -0500

updated 2013-03-18 02:57:51 -0500

In OpenCV 1.X, it is not allowed to modify the returned image buffer from capture. According to the doc, it seems not relevant with OpenCV 2.X, but usually, I made deep copy of images returned by VideoCapture object, did you try to copy these images (frame 1&2)? Maybe they return 0 because, the pointers are pointing to the same (internal) image? Try it and let us know.

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the docs are wrong there !

it does not matter, if you're using the 1.0 or the 2.0 api, a frame from a camera points to Driver memory, so you need a "deep copy" if

  1. you try to manipulate the image ( draw into it, or such).

  2. it goes out of scope

  3. you're trying to store it in a vector or such

berak gravatar imageberak ( 2013-03-18 14:15:50 -0500 )edit

Hey, I wanted to ask something regarding what you said. If I have a VideoCapture class capturing video frames, I can't store those frames in a vector directly? How come?

Uni gravatar imageUni ( 2013-08-01 15:18:25 -0500 )edit
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Asked: 2013-03-17 00:07:51 -0500

Seen: 154 times

Last updated: Mar 18 '13