cv::VideoCapture::open() cannot open any video files #18500
Description
64-bit Ubuntu 20.04. Installed OpenCV like this:
`./vcpkg install --triplet x64-linux opencv[core,dnn,ffmpeg,jpeg,opengl,openmp,png,tiff,webp]`
This gives me OpenCV4 4.3.0#1 (4.3.0-0c6047baf6).
Most of OpenCV is working fine. I'm using it to read/write images, cv::Mat, etc. But when I attempt to use cv::VideoCapture, it fails to open any video file and throws an exception.
I recompiled the app using debug mode so I could see the messages in the console. This code:
cv::VideoCapture input_video;
input_video.setExceptionMode(true);
input_video.open("set_07/2020-10-02-082129.m4v");
Results in the following errors:
[ 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'
How can I get more information on why this is failing, or what I need to do so OpenCV can find whatever plugins it is expecting to find?