videocapture reads the same frame differently on multiple calls

videocapture

asked 2016-05-21 02:46:28 -0600

gaetano
1 ●1

updated 2016-05-22 04:00:54 -0600

pklab

4284 ●7 ●30 ●60

Hi, I am new to the forum. I have started using Opencv with Java a few weeks ago. In my program I use videocapture to read frames from an avi file. Yesterday I have noticed that, over different runs of my progrem, the n-th frame read from the file is different from the same n-th frame read from the same file, in a previous run. I have confirmed this by subtracting two copies of the same frame, after resetting the video capture to the same frame number. Is this expected, or am I doing something wrong?

for (int j = 0; j < 100; j++){
    Mat img = new Mat();
    Mat m1 = new Mat();
    Mat m2 = new Mat();

    VideoCapture vc = new VideoCapture(filename);
    vc.set(Videoio.CAP_PROP_POS_FRAMES, 0);
    for (int i = 0; i < 9; i++) vc.grab();
    vc.read(img);
    m1 = img.clone();
    Imgproc.cvtColor(m1, m1, Imgproc.COLOR_BGR2GRAY);
    vc.release();

    vc = new VideoCapture(filename);
    vc.set(Videoio.CAP_PROP_POS_FRAMES, 0);
    for (int i = 0; i < 9; i++) vc.grab();
    vc.read(img);
    m2 = img.clone();
    Imgproc.cvtColor(m2, m2, Imgproc.COLOR_BGR2GRAY);
    vc.release();

    Mat m3 = new Mat();
    Core.absdiff(m1, m2, m3);

    if (Core.countNonZero(m3) > 0)System.out.println(Core.sumElems(m3));
}

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What if using 2 different capture obj like VideoCapture vc1 = new VideoCapture(filename) and VideoCapture vc2 = new VideoCapture(filename) ?

You should put both grab() and read() in the for loop and check if they returns true.

PS: ~~please provide a bit of your code..~~ please next time update your question instead of add code using comments

pklab ( 2016-05-21 04:05:55 -0600 )edit

In this code I am interested in extracting only the first frame from the video. The "grab" calls are in the two inner loops to make sure to exhaust what it seems to be a frame buffer in the VideoCapture object; the buffer size is 9 in my case, although I am not sure why that is. The outer loop is just to test the overall program multiple times. The output of the System.out is occasionally non-zero, for example 5-6 times over the 100 loop iterations. Instead I would expect it to be always zero, as essentially the program is reading the same first frame twice and, as such, the output of the absdiff should be zero.

gaetano ( 2016-05-23 07:33:09 -0600 )edit

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answered 2016-05-24 09:32:59 -0600

gaetano
1 ●1

It seems I have found a solution to my issue, although I am still not sure if that is the proper way to solve this: I have copied the "opencv_ffmpeg310_64.dll" file from the bin folder to my eclipse project. Now "differences" is indeed zero, always.

When I followed the guide on the Opencv website on how to integrate Opencv in Eclipse, nowhere it is mentioned to include also this dll.

...now I am wondering how did Opencv manage without that dll in the first place, without giving an error nor exception??? I am sure I do not have any other installations of ffmpeg on my pc. Any thoughts on this?

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Did you install opencv using CMake?

LBerger ( 2016-05-24 13:17:16 -0600 )edit

no, in windows, i downloaded and extracted version 3.0 into my programs folder. i did not rebuild the opencv. and then i set up opencv in eclipse as described on the website. i did not get any error and it seemed to work fine for basic examples.

gaetano ( 2016-05-25 02:13:28 -0600 )edit

Do you have opencv in system variable PATH?

LBerger ( 2016-05-25 02:44:38 -0600 )edit

no i haven't. I have set up Opencv in Eclipse as described in http://docs.opencv.org/2.4/doc/tutori.... And the example in the guide works for me, so I assumed everything was fine.

gaetano ( 2016-05-26 04:57:41 -0600 )edit

So In foolder exe example have you got this dll? In folder where you have your exe file have you got this dll

LBerger ( 2016-05-26 09:03:40 -0600 )edit

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answered 2016-05-23 13:17:21 -0600

pklab

4284 ●7 ●30 ●60

updated 2016-05-24 02:36:52 -0600

read() = grab() + retrieve() (check the doc) and I don't know what you get when 1 read() is following N grab() ...

Sorry but what happens using simple read() like below:

boolean ok1,ok2;

VideoCapture vc1 = new VideoCapture(filename);
// Thread.sleep(500L);    // file is loading, wait a bit
ok1 = vc1.read(m1);
vc1.release();

Thread.sleep(1000L);      // wait 1 one second to be sure the file has been released

VideoCapture vc2 = new VideoCapture(filename);
// Thread.sleep(500L);    // file is loading, wait a bit
ok2 =  vc2.read(m2);
vc2.release();

if(ok1 && ok2) {
    Mat m3 = new Mat();
    Core.absdiff(m1, m2, m3);
    int differences = Core.countNonZero(m3);
    System.out.println("Total differences: "+differences);
}
else System.out.println("Unable to load both frames");

ok1 (and/or ok2) can be false if 1st frame isn't ready as soon as the file has been opened. In this case try some sleep just before read()

EDIT: I think you are doing some mistake with loops and functions ... I don't have a java env ready but the C++ code below matches N frames with 0 difference on C++/x64/VS2013/Win7/OpenCV 3.1dev

Mat m1, m2, m3,m4;
bool ok1, ok2;
string filename = "C:/Users/xyz/Desktop/AVSS_AB_Easy_Divx.avi";
VideoCapture vc1(filename);
VideoCapture vc2(filename);
int N = 100;
for (int i = 0; i < N; i++) {
    ok1 = vc1.read(m1);
    ok2 = vc2.read(m2);
    if (ok1 && ok2) {
        absdiff(m1, m2, m3);
        cvtColor(m3, m4, CV_BGR2GRAY);
        int differences = countNonZero(m4);
        cout << "Total differences: " << differences << endl;
    }
    else cout << "Unable to load both frames" << endl;
}

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i have tried your code (with small midifications: countNonZero gives me an error for a 3-channel image, so had to convert to grayscale): when i read the first frame, the difference is zero. But if i have 2 reads, i.e. i read first and second frame, than ok1 and ok2 are both true, but differences is non zero. I have tried to increase the sleep time, but it does not change anything. The video I am using now can be downloaded from ftp://motinas.elec.qmul.ac.uk/pub/iLids/ (the second in the list). It is like as if the decoding process of the frames introduce small artefacts. For example, in one run I get: Total differences: 23856 For a 720x576 video, this is an average of ~6% difference. So very small differences betwee the two images. I have never experienced the same problem in Matlab.

gaetano ( 2016-05-23 16:29:43 -0600 )edit

check edit in my answer

pklab ( 2016-05-24 02:37:35 -0600 )edit

Thank you for your comments. Here is my java version of your suggested code:

    Mat m1 = new Mat(), m2 = new Mat(), m3 = new Mat(), m4 = new Mat();
    boolean ok1, ok2;

    String name = "avss.avi";
    String filename = System.getProperty("user.home") + "\\MyDataset Modified\\Abandoned-Removed\\TIC_littering\\" + name;

    VideoCapture vc1 = new VideoCapture(filename);
    VideoCapture vc2 = new VideoCapture(filename);

    int N = 100;

    for (int i = 0; i < N; i++){
        ok1 = vc1.read(m1);
        ok2 = vc2.read(m2);

        if (ok1 && ok2){
            Core.absdiff(m1, m2, m3);
            Imgproc.cvtColor(m3, m4, Imgproc.COLOR_BGR2GRAY);
            int differences = Core.countNonZero(m4);
            System.out.println("Total differences: " + differences);
        }           
        else System.out.println("Unable to load both frames");
    }

gaetano ( 2016-05-24 06:10:35 -0600 )edit

Here the output:

Total differences: 0 Total differences: 129845 Total differences: 129402 Total differences: 130983 ....

The output is indeed zero for the first frame, but then it is non zero on multiple occasions.

gaetano ( 2016-05-24 06:11:43 -0600 )edit

I don't know. The Java code should work fine like as mine in C++. Maybe is some OS and/or codec issue. Some other user who has a Java ready installation should test your code.

pklab ( 2016-05-24 09:12:20 -0600 )edit

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