Videocapture fails to open video in OpenCV 4.3.0 on ARMV7

[rokopi-byte]

System information (version)

• OpenCV => 4.3.0
• Operating System / Platform => Raspbian Buster/ARMV7
• Compiler => GCC 8.3

Detailed description

The following sample code to read a video fails with the following error. Opencv has been crosscompiled with inference engine using a docker, as the documentation says.

[ WARN:0] global /opencv/modules/videoio/src/cap_gstreamer.cpp (1759) handleMessage OpenCV | GStreamer warning: Embedded video playback halted; module matroskademux0 reported: Internal data stream error.
Error opening video stream or file
[ WARN:0] global /opencv/modules/videoio/src/cap_gstreamer.cpp (888) open OpenCV | GStreamer warning: unable to start pipeline
[ WARN:0] global /opencv/modules/videoio/src/cap_gstreamer.cpp (480) isPipelinePlaying OpenCV | GStreamer warning: GStreamer: pipeline have not been created

Steps to reproduce

#include "opencv2/opencv.hpp"
#include <iostream>

using namespace std;
using namespace cv;

int main(){

  // Create a VideoCapture object and open the input file
  // If the input is the web camera, pass 0 instead of the video file name
  VideoCapture cap("../trackingexample/faceocc2.webm");

  // Check if camera opened successfully
  if(!cap.isOpened()){
    cout << "Error opening video stream or file" << endl;
    return -1;
  }

  while(1){

    Mat frame;
    // Capture frame-by-frame
    cap >> frame;

    // If the frame is empty, break immediately
    if (frame.empty())
      break;

    // Display the resulting frame
    imshow( "Frame", frame );

    // Press  ESC on keyboard to exit
    char c=(char)waitKey(25);
    if(c==27)
      break;
  }

  // When everything done, release the video capture object
  cap.release();

  // Closes all the frames
  destroyAllWindows();

  return 0;
}

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  answers.opencv.org, Stack Overflow, etc and have not found solution
• I updated to latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc

[alalek]

GStreamer warning: Embedded video playback halted; module matroskademux0 reported: Internal data stream error.

This can't be fixed in OpenCV.
Ensure that GStreamer works on your system with used video stream.

[rokopi-byte]

I'm sorry, it's related to webm format, matroska plugin is not working on raspbian buster.

[PatrickNa]

@roccopietrini how did you come to this conclusion? I am facing the same error on a Jetson Xavier. There I installed OpenCV 4.4.0 with Python 3.8 by means of pip.

[rokopi-byte]

@PatrickNa Honesly I don't know precisely, I just followed this to get opencv installed on the raspberry, and matroska was not working, while other formats were working. I didn't manage to make it work, as it is something related to gstreamer, which I'm not familiar with.. I remember I tried installing gstreamer plugin packages without any luck. If you find out something, please write here. Thanks

[PatrickNa]

@roccopietrini I made it work.

This is what I have learned: When creating a VideoCapture instance one can, but does not have to, specify a 'Capture API'. In case it is not specified the system falls back to what is available. In my case, I have installed OpenCV with pip and the only capture API that was there was GStreamer, which expects a different syntax as with what I was familiar with. You can check which capture APIs are available by running the following lines in a python shell. The Video I/O section lists the information you are looking for.

import cv2
print(cv2.getBuildInformation())

Since I am more familiar with FFMPEG I compiled OpenCV including this API and was able to use VideoCapture as I knew it.
Hope this helps!