Displaying to the screen with OpenCV and GStreamer

When using opencv and imshow, I find that there is significant delay (I assume due to uplscaling) I would like to avoid imshow if possible. So I am trying to use GStreamer with nveglglessink. My code as is Python:
gst = "appsrc ! video/x-raw,format=RGBA,width=1920,height=1080 ! nvvidconv ! 'video/x-raw(memory:NVMM),format=RGBA,width=1920,height=1080' ! nvegltransform ! nveglglessink -e "
vw = cv2.VideoWriter(gst, cv2.CAP_GSTREAMER, 0, 30, (DISPLAY_WIDTH, DISPLAY_HEIGHT))

Whenever I try to launch this code I get the errror: [ WARN:0] global /usr/local/include/opencv-4.3.0/modules/videoio/src/cap_gstreamer.cpp (1424) open OpenCV | GStreamer warning: error opening writer pipeline: syntax error

I can’t find any problems with the pipeline. Any suggestions?

Python 3.6.9
OpenCV 4.3 with CUDA and GStreamer (I checked)
GStreamer 1.14.5

Hi,
Please refer to the thread:


Generally we save it to a file when calling cv2.VideoWriter(). Not sure if it works if you run with a display sink. May see if others can share suggestion.

You may try to remove the single quotes and ‘-e’ from your pipeline.

@DaneLLL I am aware that this isn’t the normal use case, but I want to give it a shot see if it improves performance over OpenCV imshow.

@Honey_Patouceul I removed those and the stream generation error went away, but now there is a appsrc internal data stream error, and ideas? Thanks!

In case your opencv frames are BGR, you would have to convert in the pipeline.
I am currently unable to try, but this may help:

gst = "appsrc ! queue ! videoconvert ! video/x-raw,format=RGBA ! nvvidconv ! nvegltransform ! nveglglessink "
vw = cv2.VideoWriter(gst, cv2.CAP_GSTREAMER, 0, 30, (FRAME_WIDTH, FRAME_HEIGHT))

Note the width and height (and fps) should be according to the frames you’re pushing into writer.
When it works, you can resize with nvvidconv adding caps with new size:

gst = "appsrc ! queue ! videoconvert ! video/x-raw,format=RGBA ! nvvidconv ! video/x-raw(memory:NVMM), width=DISPLAY_WIDTH, height=DISPLAY_HEIGHT ! nvegltransform ! nveglglessink "
1 Like

@Honey_Patouceul Thanks! That works!

It appears that the stream is faster than using imshow, although still has a bit of latency (still not sure if that is from my code or not). Using imshow the framerate is uneven and slower than I would like, so this is better for my use case.

Hi,
1- How I can use HW accelerate for encoding using gstreamer + opencv in python for writing to file?
2- How I can use HW accelaretor for decoding unsing gstreamer + opencv in python with nvv4l2decoder?
3- All codes of decoding and encoding are same and correctly wotrk for both Nano and Xavier NX?

Hi,
Please refer to the python sample:

import sys
import cv2

def read_cam():
    cap = cv2.VideoCapture("filesrc location=/home/nvidia/sample_1080p_h264.mp4 ! qtdemux ! h264parse ! nvv4l2decoder ! nvvidconv ! video/x-raw, format=BGRx ! videoconvert ! video/x-raw,format=BGR ! appsink  ")

    w = cap.get(cv2.CAP_PROP_FRAME_WIDTH)
    h = cap.get(cv2.CAP_PROP_FRAME_HEIGHT)
    fps = cap.get(cv2.CAP_PROP_FPS)
    print('Src opened, %dx%d @ %d fps' % (w, h, fps))

    gst_out = "appsrc ! video/x-raw, format=BGR ! queue ! videoconvert ! video/x-raw,format=BGRx ! nvvidconv ! nvv4l2h264enc ! h264parse ! matroskamux ! filesink location=test.mkv "
    out = cv2.VideoWriter(gst_out, cv2.CAP_GSTREAMER, 0, float(fps), (int(w), int(h)))
    if not out.isOpened():
        print("Failed to open output")
        exit()

    if cap.isOpened():
        while True:
            ret_val, img = cap.read();
            if not ret_val:
                break;
            out.write(img);
            cv2.waitKey(1)
    else:
     print "pipeline open failed"

    print("successfully exit")
    cap.release()
    out.release()


if __name__ == '__main__':
    read_cam()

The sample follows Honey_Patouceul’s suggestion and works fine. FYR.

@DaneLLL
Thanks a lot.
what’s mean when we use :
video/x-raw, format=BGR and video/x-raw(memory:NVMM),format=(string)NV12
In this program appsrc > video/x-raw, format=BGR that means data in appsrc pushed into video/x-raw, format=BGR? Or this element is for converting?
In my opion appsrc is CPU buffer and both video/x-raw, format=BGR and video/x-raw(memory:NVMM),format=(string)NV12 are GPU buffer?

Hi,
Because OpenCV uses BGR CPU buffers and hardware encoder takes NVMM buffers, need to convert the buffers through videoconvert ! nvvidconv.