$ ffmpeg -init_hw_device cuda=decdev:/dev/dri/renderD129 -init_hw_device vaapi=encdev:/dev/dri/renderD128 -hwaccel cuda -hwaccel_device decdev -hwaccel_output_format cuda -c:v mjpeg_cuvid -f v4l2 -framerate 30 -video_size 1920x1080 -i /dev/video0 -filter_hw_device encdev -vf "hwdownload, format=nv12, hwupload, select='gt(scene,0.04)' -vsync 0 'snapshot.png'" -c:v hevc_vaapi webcam.mp4 -y


ffmpeg version n5.0.1 Copyright (c) 2000-2022 the FFmpeg developers
  built with gcc 12.1.0 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-amf --enable-avisynth --enable-cuda-llvm --enable-lto --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libmfx --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librav1e --enable-librsvg --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-libzimg --enable-nvdec --enable-nvenc --enable-shared --enable-version3
  libavutil      57. 17.100 / 57. 17.100
  libavcodec     59. 18.100 / 59. 18.100
  libavformat    59. 16.100 / 59. 16.100
  libavdevice    59.  4.100 / 59.  4.100
  libavfilter     8. 24.100 /  8. 24.100
  libswscale      6.  4.100 /  6.  4.100
  libswresample   4.  3.100 /  4.  3.100
  libpostproc    56.  3.100 / 56.  3.100
Input #0, video4linux2,v4l2, from '/dev/video0':
  Duration: N/A, start: 7600.422230, bitrate: N/A
  Stream #0:0: Video: mjpeg (Baseline), yuvj422p(pc, bt470bg/unknown/unknown), 1920x1080 [SAR 1:1 DAR 16:9], 30 fps, 30 tbr, 1000k tbn
Stream mapping:
  Stream #0:0 -> #0:0 (mjpeg (mjpeg_cuvid) -> hevc (hevc_vaapi))
Press [q] to stop, [?] for help
[mjpeg_cuvid @ 0x55d3af5d1800] ignoring invalid SAR: 0/0
[Parsed_select_3 @ 0x55d3af79b2c0] [Eval @ 0x7ffef49fabf0] Undefined constant or missing '(' in 'vsync0snapshot.png'
[Parsed_select_3 @ 0x55d3af79b2c0] Error while parsing expression 'gt(scene,0.04) -vsync 0 snapshot.png'
[AVFilterGraph @ 0x55d3af798300] Error initializing filter 'select' with args 'gt(scene,0.04) -vsync 0 snapshot.png'
Error reinitializing filters!
Failed to inject frame into filter network: Invalid argument
Error while processing the decoded data for stream #0:0
[video4linux2,v4l2 @ 0x55d3af61fb40] Some buffers are still owned by the caller on close.
ioctl(VIDIOC_QBUF): Bad file descriptor
Conversion failed!