ffmpeg can't access to usb webcam from python subprocess.Popen in windows

Question

If I run the following ffmpeg command from windows cmd it works

ffmpeg -f dshow -i video="USB Video Device" -s 1280x720 -f rawvideo -pix_fmt rgb24 pipe:

I then tried to execute the same in python
Python 3.7.4 (tags/v3.7.4:e09359112e, Jul 8 2019, 20:34:20) [MSC v.1916 64 bit (AMD64)] on win32

args = [
    'ffmpeg', '-f', 'dshow', '-i', 'video=USB Video Device', '-s', '1280x720',
    '-f', 'rawvideo', '-pix_fmt','rgb24', 'pipe:'
]
ffmpeg_proc = subprocess.Popen(args, stdout=subprocess.PIPE)

but I get this error

D:\Projects\Applications\VideoCapture\Virtualenv\Scripts\python.exe D:/Projects/Applications/VideoCapture/Sources/Application/video_capture.py "USB Video Device" 1280 720
ffmpeg version 4.4-full_build-www.gyan.dev Copyright (c) 2000-2021 the FFmpeg developers
  built with gcc 10.2.0 (Rev6, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libglslang --enable-vulkan --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  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
Input #0, dshow, from 'video=USB Video Device':
  Duration: N/A, bitrate: N/A
  Stream #0:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 640x480, 30 fps, 30 tbr, 10000k tbn, 10000k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> rawvideo (native))
Press [q] to stop, [?] for help
video=USB Video Device: I/O error
Output #0, rawvideo, to 'pipe:':
  Metadata:
    encoder         : Lavf58.76.100
  Stream #0:0: Video: rawvideo (RGB[24] / 0x18424752), rgb24, 1280x720, q=2-31, 663552 kb/s, 30 fps, 30 tbn
    Metadata:
      encoder         : Lavc58.134.100 rawvideo
frame=    0 fps=0.0 q=0.0 Lsize=       0kB time=00:00:00.00 bitrate=N/A speed=   0x    
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)

The inportant row is USB Video Device: I/O error
I've tried also to add shell=True to the Popen but with the same result.

Doing some tests I found that using pycharm debug mode or simply lunching the same code inside "Pycharm Python Console" no more I/O error and all seems to work. I think something changes in the environment created for the subrocess but I wasn't able to understand what PyCharm is doing under the hood.

Here the output inside PyCharm Python Console

D:\Projects\Applications\VideoCapture\Virtualenv\Scripts\python.exe "C:\Program Files\JetBrains\PyCharm Community Edition 2019.1.1\plugins\python-ce\helpers\pydev\pydevconsole.py" --mode=client --port=65058
import sys; print('Python %s on %s' % (sys.version, sys.platform))
sys.path.extend(['D:\\Projects\\Applications\\VideoCapture', 'D:\\Projects\\Applications\\VideoCapture\\Sources\\Application', 'D:\\Projects\\Applications\\VideoCapture\\Sources\\Compiler', 'D:/Projects/Applications/VideoCapture'])
PyDev console: starting.
Python 3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 20:34:20) [MSC v.1916 64 bit (AMD64)] on win32
runfile('D:/Projects/Applications/VideoCapture/Sources/Application/video_capture.py', args=['USB Video Device', '1280', '720'], wdir='D:/Projects/Applications/VideoCapture/Sources/Application')
ffmpeg version 4.4-full_build-www.gyan.dev Copyright (c) 2000-2021 the FFmpeg developers
  built with gcc 10.2.0 (Rev6, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libglslang --enable-vulkan --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  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
Input #0, dshow, from 'video=USB Video Device':
  Duration: N/A, start: 111984.462000, bitrate: N/A
  Stream #0:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 640x480, 30 fps, 30 tbr, 10000k tbn, 10000k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> rawvideo (native))
Press [q] to stop, [?] for help
Output #0, rawvideo, to 'pipe:':
  Metadata:
    encoder         : Lavf58.76.100
  Stream #0:0: Video: rawvideo (RGB[24] / 0x18424752), rgb24(pc, progressive), 1280x720, q=2-31, 663552 kb/s, 30 fps, 30 tbn
    Metadata:
      encoder         : Lavc58.134.100 rawvideo
av_interleaved_write_frame(): Invalid argument
    Last message repeated 1 times
Error writing trailer of pipe:: Invalid argument
frame=  118 fps= 30 q=-0.0 Lsize=  313200kB time=00:00:03.93 bitrate=652306.1kbits/s dup=53 drop=0 speed=   1x

Does anyone now how to fix this problem or have any knowledge on how PyCharm Python Console works?

Answer 1

The quotes are important!

The argument should be 'video="USB Video Device"' instead of 'video=USB Video Device'

args = [
    'ffmpeg', '-f', 'dshow', '-i', 'video="USB Video Device"', '-s', '1280x720',
    '-f', 'rawvideo', '-pix_fmt','rgb24', 'pipe:'
]

ffmpeg_proc = subprocess.Popen(args, stdout=subprocess.PIPE)