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OZ1SEJ
Asked: 2021-05-15 08:26:53 +0800 CST

Troubleshooting trying to use Canon DSLR as webcam

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I'm following this guide trying to use my Canon EOS 550D as a webcam. It is listed as supported by gphoto2.

So, I connected my Canon to my laptop using a USB-cable, and I had no problems running

$ sudo apt-get install gphoto2 v4l2loopback-utils v4l2loopback-dkms ffmpeg

and

$ sudo modprobe v4l2loopback exclusive_caps=1 max_buffers=2

I can also take a still photo using my Canon using this command

$ gphoto2 --capture-image-and-download

But then, when I try to run

gphoto2 --stdout --capture-movie | ffmpeg -i - -vcodec rawvideo -pix_fmt yuv420p -threads 0 -f v4l2 /dev/video0

I get this output:

$ gphoto2 --stdout --capture-movie | ffmpeg -i - -vcodec rawvideo -pix_fmt yuv420p -threads 0 -f v4l2 /dev/video0
ffmpeg version 4.2.4-1ubuntu0.1 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.3.0-10ubuntu2)
  configuration: --prefix=/usr --extra-version=1ubuntu0.1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-nvenc --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Capturing preview frames as movie to 'stdout'. Press Ctrl-C to abort.
[mjpeg @ 0x55cb3671a780] Format mjpeg detected only with low score of 12, misdetection possible!
Input #0, mjpeg, from 'pipe:':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: mjpeg (Baseline), yuvj422p(pc, bt470bg/unknown/unknown), 1056x704, 25 tbr, 1200k tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (mjpeg (native) -> rawvideo (native))
[swscaler @ 0x55cb3678c5c0] deprecated pixel format used, make sure you did set range correctly

which is all to be expected, but it stops immediately with this error:

[video4linux2,v4l2 @ 0x55cb36720280] ioctl(VIDIOC_G_FMT): Invalid argument
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
Error initializing output stream 0:0 -- 
Conversion failed!

And I really don't know where to go after this. Any help would be appreciated.

webcam

2 Answers

  1. Many device access problems can be resolved through group membership changes.

    Specifically, if ls -l shows that the group permissions (the second "rwx" triplet) is "rw" (e.g."-rw-rw----"), then, adding oneself to the group that owns the device will grant rw access.

    Here's how:

    device="/dev/whatever"
sudo adduser $USER $(stat -c "%G" $device)

    This allows you membership in the group that can rw the device, but there is one more step.

    To make all your processes members of the new group, logout and login. Group memberships are set up at login time.

    To create a single process in the new group (for testing, prior to logout/login):

    newgrp $(stat -c "%G" $device)

    or, just type the group name. See man newgrp.

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  2. You can get this error if you attempt to output to an existing /dev/video* device, such as the webcam integrated into your laptop. So try outputting /dev/video1 or /dev/video2 instead.

    Or choose the device number when you enable v4l2loopback:

    sudo modprobe v4l2loopback video_nr=76

    Now you can output to /dev/video76.

    See v4l2loopback README.md for more info.

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