OpenGL Screen Tearing Problem

[reboot]

I have been using OBS-Studio on Linux for some time now and always noticed some strange stutter in the streamed video and in saved files. This is not visible in the preview windows of OBS.

You can see this problem in the YouTube Video (at 1:00 and at 2:38) and in this Sequence of Frames from the Video.

Since this is not visible in the preview window I investigated how the frames are encoded and found that it basically works like this:

output_frame()
    gs_entercontext();
    download_frame(..., ..., &frame);
        stagesurface_map(..., &frame->data[0], ...)
            *frame->data[0] = glMapBuffer(GL_PIXEL_PACK_BUFFER, GL_READ_ONLY);
    gs_leavecontext();
    output_video_data(..., &frame, ...);

Which means the pointer that is returned by OpenGL is passed to the encoder after the GL context is detached from the turrent thread. I could not find any information in the OpenGL documentation how long the pointer is valid and may be used, but I wrote the work-a-round reboot/obs-studio@c8b54a7 that copies the frame data into a buffer allocated in memory before it is passed to the encoder. The seems to fix the problem and I did not notice any stutter in streams or video files anymore. The work-a-round is not really a fix, but it demonstrates that OpenGL seems to reuse the buffer memory for the new image, before the encoder finished encoding the frame.

Hardware and Driver Info:

OpenGL vendor string: NVIDIA Corporation
OpenGL renderer string: GeForce GTS 250/PCIe/SSE2
OpenGL version string: 3.3.0 NVIDIA 331.67

[jp9000]

Hm, thank you for pointing it out, I'll investigate.