what are the video plugins?

edit

Installed OpenCV v4 on Ubuntu 20.04 using vcpkg:

 ./vcpkg install --triplet x64-linux opencv[core,dnn,ffmpeg,jpeg,opengl,openmp,png,tiff,webp]

My C++ code looks like this:

cv::VideoCapture input_video;
input_video.setExceptionMode(true);
input_video.open("set_07/2020-10-02-082129.m4v");

The call into cv::VideoCapture::open() results in a bunch of errors about "Found 0 plugin(s) for ..." before it throws a C++ exception. Debug info from OpenCV looks like this:

[ INFO:0] global /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/videoio_registry.cpp (187) VideoBackendRegistry VIDEOIO: Enabled backends(7, sorted by priority): FFMPEG(1000); GSTREAMER(990); INTEL_MFX(980); V4L2(970); CV_IMAGES(960); CV_MJPEG(950); FIREWIRE(940)
[ INFO:0] global /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/backend_plugin.cpp (342) getPluginCandidates VideoIO pluigin (FFMPEG): glob is 'libopencv_videoio_ffmpeg*.so', 1 location(s)
[ INFO:0] global /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/backend_plugin.cpp (349) getPluginCandidates     - /home/stephane/src/DarkHelp/build/src-tool: 0
[ INFO:0] global /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/backend_plugin.cpp (353) getPluginCandidates Found 0 plugin(s) for FFMPEG
[ INFO:0] global /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/backend_plugin.cpp (342) getPluginCandidates VideoIO pluigin (GSTREAMER): glob is 'libopencv_videoio_gstreamer*.so', 1 location(s)
[ INFO:0] global /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/backend_plugin.cpp (349) getPluginCandidates     - /home/stephane/src/DarkHelp/build/src-tool: 0
[ INFO:0] global /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/backend_plugin.cpp (353) getPluginCandidates Found 0 plugin(s) for GSTREAMER
[ INFO:0] global /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/backend_plugin.cpp (342) getPluginCandidates VideoIO pluigin (INTEL_MFX): glob is 'libopencv_videoio_intel_mfx*.so', 1 location(s)
[ INFO:0] global /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/backend_plugin.cpp (349) getPluginCandidates     - /home/stephane/src/DarkHelp/build/src-tool: 0
[ INFO:0] global /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/backend_plugin.cpp (353) getPluginCandidates Found 0 plugin(s) for INTEL_MFX
[ INFO:0] global /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/cap_images.cpp (282) icvExtractPattern Pattern: set_07/%04d-10-02-082129.m4v @ 2020
[ INFO:0] global /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/cap_images.cpp (313) open CAP_IMAGES: Stop scanning. Can't read image file: set_07/2020-10-02-082129.m4v
OpenCV(4.3.0) Error: Unspecified error (could not open 'set_07/2020-10-02-082129.m4v') in open, file /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/cap.cpp, line 177
Caught exception: OpenCV(4.3.0) /home/stephane/src/vcpkg/buildtrees/opencv4/src/4.3.0-0c6047baf6.clean/modules/videoio/src/cap.cpp:177: error: (-2:Unspecified error) could not open 'set_07/2020-10-02-082129.m4v' in function 'open'

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