I am capturing and processing video frames with OpenCV, and I would like to write them as a h265 video file. I am struggling to get a proper Gstreamer pipeline to work from OpenCV.

Gstreamer works fine by itself. In particular, I am able to run this command, which encodes video very quickly (thanks to GPU acceleration) and saves it to a mkv file:

gst-launch-1.0 videotestsrc num-buffers=90 ! 'video/x-raw, format=(string)I420, width=(int)640, height=(int)480' ! omxh265enc ! matroskamux ! filesink location=test.mkv
Now I would like to do the same thing from within my OpenCV application. My code is something like:
Mat img_vid = Mat(1024, 1024, CV_8UC3);
VideoWriter video;
video.open("appsrc ! autovideoconvert ! omxh265enc ! matroskamux ! filesink location=test.mkv", 0, (double)25, cv::Size(1024, 1024), true);
if (!video.isOpened()) {
   printf("can't create writer\n");
   return -1;
while ( ... ) {
    // Capture frame into img_vid => That works fine
    video.write(img_vid);
At first sight, this seems to work, but what it does is it creates file named “appsrc ! autovideoconvert ! omxh265enc ! matroskamux ! filesink location=test.mkv” and fills it with uncompressed video frames, completely ignoring the fact that this is a Gstreamer pipeline.
I have tried other pipelines, but they result in a variety of errors:
video.open("appsrc ! autovideoconvert ! omxh264enc ! 'video/x-h264, streamformat=(string)byte-stream' ! h264parse ! qtmux ! filesink location=test.mp4 -e", 0, (double)25, cv::Size(1024, 1024), true);
Which results in:
(Test:5533): GStreamer-CRITICAL **: gst_element_make_from_uri: assertion 'gst_uri_is_valid (uri)' failed
OpenCV Error: Unspecified error (GStreamer: cannot find appsrc in manual pipeline
) in CvVideoWriter_GStreamer::open, file /home/ubuntu/opencv/modules/videoio/src/cap_gstreamer.cpp, line 1363
VIDEOIO(cvCreateVideoWriter_GStreamer(filename, fourcc, fps, frameSize, is_color)): raised OpenCV exception:
/home/ubuntu/opencv/modules/videoio/src/cap_gstreamer.cpp:1363: error: (-2) GStreamer: cannot find appsrc in manual pipeline
 in function CvVideoWriter_GStreamer::open
I also tried the simple:
video.open("appsrc ! autovideosink", 0, (double)25, cv::Size(1024, 1024), true);
which yields:
GStreamer Plugin: Embedded video playback halted; module appsrc0 reported: Internal data flow error.
I am using OpenCV 3.1 with Gstreamer support.

I am running L4T R24.2.1.
              
I got the answer from Stack Overflow:
https://stackoverflow.com/questions/43412797/opening-a-gstreamer-pipeline-from-opencv-with-videowriter
Just add a space at the end of the pipeline string, and it works!
"appsrc ! autovideoconvert ! omxh265enc ! matroskamux ! filesink location=test.mkv "
i developed this code on TK1
cv::VideoCapture cap(0); 
std::string pipeline = "appsrc ! autovideoconvert ! omxh265enc ! matroskamux ! filesink    
    location=test.mkv ";
	cv::VideoWriter writer;//(pipeline);
    writer.open(pipeline,0, (double)30, cv::Size(640, 480), true);
	 if (!writer.isOpened()) {
        printf("=ERR= can't create video writer\n");
        return -1;
	if(cap.isOpened()){
	  cv::Mat frame;   
      while(1){
        cap >> frame;
        if (!frame.empty()){
		    cv::imshow( "Frame", frame );
			writer<<frame;
           char c=(char)cv::waitKey(40);
           if(c==27)
             break;
	cap.release();
but i still got this error
GStreamer Plugin: Embedded video playback halted; module appsrc0 reported: Internal data flow error.
Best Regards

Akrita