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I'm trying to record some videos using a USB camera, but I'm having some issues when using ffmpeg.

If I run ffmpeg -f video4linux2 -t 00:00:10 -i /dev/video0 out.mpg, the program tries to record at 640x480 resolution and ffmpeg hangs.

However, if I add the -s to ffmpeg and record at lower resolutions than 640x480 (e.g., 320x240), the video is recorded successfully.

After a hang, if I hit CTRL+C, ffmpeg resumes, yielding a file of 0 kB in size. Using strace I can see that an ioctl call to the device keeps returns -EINVAL and subsequent ioctls return -EAGAIN.

ioctl(3, VIDIOC_G_STD, 0xbe84dfb0)      = -1 EINVAL (Invalid argument)
ioctl(3, VIDIOC_DQBUF, {type=V4L2_BUF_TYPE_VIDEO_CAPTURE}) = -1 EAGAIN (Resource temporarily unavailable)

Any ideas why this happens?

I'm using ffmpeg version 2.8.7, built through busybox 1.25. The host architecture is an ARM processor running kernel 3.2.

I also tried compiling the most recent version from source, and the problem persists...

Guilherme Costa
  • 324

1 Answers1

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Figured out the reason: transcoding and raw data volume.

Using the command line mentioned in my question I was reading from a raw format (yuv422), and transcoding it to mpeg-1, which was the default encoding for my version of ffmpeg. The data amount being streamed from the camera was simply too much for the processor, causing the hang. The camera I was using was capable of streaming in a compressed format as well (mjpeg). By switching to this format, ffmpeg no longer hanged, and was capable of recording at 15 fps.

However, there was a transcoding step, from mjpeg to mpeg-1. I was able to reach a higher fps count by telling ffmpeg to copy the stream, removing the last transcoding step.

Guilherme Costa
  • 324