v4l2 output encoder does not support h264 (only raw)

[rogerbrent]

Using tools such as v4l2loopback it's useful to create a virtual v4l2 device and write h264 video to it. Unfortunately this isn't supported by ffmpeg, this gives an error:

ffmpeg -i X -c:v h264 -f v4l2 /dev/video0

Note this is possible with other software like v4l2compress_h264.

[Carl Eugen Hoyos]

Would you like to tell us about the error?

Please test current FFmpeg git head and provide your command line together with the complete, uncut console output to make this a valid ticket.

[rogerbrent]

pi@raspberrypi:~$ ./ffmpeg -f v4l2 -i /dev/video0 -c:v h264 -f v4l2 /dev/video1
ffmpeg version git-2017-03-12-ff17c76 Copyright (c) 2000-2017 the FFmpeg developers

built with gcc 4.9.2 (Raspbian 4.9.2-10)
configuration: --enable-gpl --enable-nonfree --enable-mmal --enable-omx --enable-omx-rpi
libavutil 55. 48.100 / 55. 48.100
libavcodec 57. 83.100 / 57. 83.100
libavformat 57. 66.104 / 57. 66.104
libavdevice 57. 3.100 / 57. 3.100
libavfilter 6. 76.100 / 6. 76.100
libswscale 4. 3.101 / 4. 3.101
libswresample 2. 4.100 / 2. 4.100
libpostproc 54. 2.100 / 54. 2.100

Input #0, video4linux2,v4l2, from '/dev/video0':

Duration: N/A, start: 208455.331127, bitrate: 24576 kb/s

Stream #0:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 640x480, 24576 kb/s, 5 fps, 5 tbr, 1000k tbn, 1000k tbc

Stream mapping:

Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (h264_omx))

Press [q] to stop, ? for help
[h264_omx @ 0x1ee6310] Using OMX.broadcom.video_encode
[v4l2 @ 0x1ee4f20] V4L2 output device supports only a single raw video stream
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
Error initializing output stream 0:0 -- �?ھ m?
Conversion failed!

[Carl Eugen Hoyos]

Replying to rogerbrent:

pi@raspberrypi:~$ ./ffmpeg -f v4l2 -i /dev/video0 -c:v h264 -f v4l2 /dev/video1

What is this command line supposed to do?
You send h264 frames to the v4l2 device - what is it supposed to do with them?
Does your v4l2 device contain a h264 decoder?

Error initializing output stream 0:0 -- �?ھ m?

This awkward error (reading uninitialized memory) is hopefully fixed.

[rogerbrent]

video1 Is a loopback (virtual) device and another program will read back the h264 stream and process it.