rc cameras, section 'video conferencing'. ffmpeg/v4l2 options currently still valid?

Hi to all,

first, the obligatory Thank You for gphoto!!

using a command from the wiki, I encountered an error message though, 
aiming to use my Nikon D7100 as a webcam.

When running
$ gphoto2 --stdout --capture-movie | ffmpeg -i - -vcodec rawvideo 
-pix_fmt yuv420p -threads 0 -f v4l2 /dev/video0
the camera's mirror opens but then gphoto aborts with the following 
error message from ffmpeg/v4l2


[error message]
gphoto2 --stdout --capture-movie | ffmpeg -i - -vcodec rawvideo -pix_fmt 
yuv420p -threads 0 -f v4l2 /dev/video0
ffmpeg version 4.4-6ubuntu5 Copyright (c) 2000-2021 the FFmpeg developers
   built with gcc 11 (Ubuntu 11.2.0-7ubuntu1)
   configuration: --prefix=/usr --extra-version=6ubuntu5 
--toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu 
--incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl 
--disable-stripping --enable-gnutls --enable-ladspa --enable-libaom 
--enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca 
--enable-libcdio --enable-libcodec2 --enable-libdav1d --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-librabbitmq 
--enable-librubberband --enable-libshine --enable-libsnappy 
--enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh 
--enable-libtheora --enable-libtwolame --enable-libvidstab 
--enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx265 
--enable-libxml2 --enable-libxvid --enable-libzimg --enable-libzmq 
--enable-libzvbi --enable-lv2 --enable-omx --enable-openal 
--enable-opencl --enable-opengl --enable-sdl2 --enable-pocketsphinx 
--enable-librsvg --enable-libmfx --enable-libdc1394 --enable-libdrm 
--enable-libiec61883 --enable-nvenc --enable-chromaprint --enable-frei0r 
--enable-libx264 --enable-shared
   libavutil      56. 70.100 / 56. 70.100
   libavcodec     58.134.100 / 58.134.100
   libavformat    58. 76.100 / 58. 76.100
   libavdevice    58. 13.100 / 58. 13.100
   libavfilter     7.110.100 /  7.110.100
   libswscale      5.  9.100 /  5.  9.100
   libswresample   3.  9.100 /  3.  9.100
   libpostproc    55.  9.100 / 55.  9.100
Nehme Vorschaubilder als Film in 'stdout' auf. Drücke Ctrl-C zum abbrechen.
[mjpeg @ 0x5633a608f440] Format mjpeg detected only with low score of 
25, misdetection possible!
Input #0, mjpeg, from 'pipe:':
   Duration: N/A, bitrate: N/A
   Stream #0:0: Video: mjpeg (Baseline), yuvj422p(pc, 
bt470bg/unknown/unknown), 640x360, 25 tbr, 1200k tbn, 25 tbc
Stream mapping:
   Stream #0:0 -> #0:0 (mjpeg (native) -> rawvideo (native))
[swscaler @ 0x5633a60f7b80] deprecated pixel format used, make sure you 
did set range correctly
[video4linux2,v4l2 @ 0x5633a6092f40] 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!
[/error message]


Help is appreciated.
Kind regards
Ulrich



Running Ubuntu 21.10 with Kernel 5.13.0-22
v4l2loopback kernel module is installed and loaded alongside videodev
$ gphoto2 --summary displays the camera infos correctly
$ gphoto2 --capture-image-and-download and other commands work as expected

Versions:
This version of gphoto2 is using the following software versions and 
options:
gphoto2         2.5.27         gcc, popt(m), exif, cdk, aa, jpeg, readline
libgphoto2      2.5.27         standard camlibs, gcc, ltdl, EXIF
libgphoto2_port 0.12.0         iolibs: disk ptpip serial usb1 
usbdiskdirect usbscsi, gcc, ltdl, EXIF, USB, serial without locking



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