Building Opencv4 is successful but doesn't download opencv_ffmpeg.dll and gives no error until you try to use video capture

[ferjif]

The Bug is
I recently installed opencv and it built and installed fine. but the problem is the opencv_ffmpeg.dll didn't download. I used both x86 and x64 windows triplets but still the file wasn't downloaded.
Environment

• OS: windows 10
• Compiler: MSVC 19
  To Reproduce
  Steps to reproduce the behavior:

1. ./vcpkg install opencv opencv[ffmpeg]
2. No errors builds just fine

Expected behavior
Opencv builds successfully and you can use it just fine until you want to use cv::VideoCapture(idx, cv::CAP_FFMPEG) which fails to open a capture object

[JackBoosY]

Hi @ferjif, thanks for reporting this issue.
Could you try #11130 changes?

Thanks.

[ferjif]

ok. i'll try it and get back to you.
Thanks

[ferjif]

@JackBoosY sorry i didn't have time to use the pull request but the prebuilt version from opencv even with opencv_ffmpeg.dll failed to use it. so i used cv::CAP_ANY and it worked on both the vcpkg version and the prebuilt version from sourceforge.

[JackBoosY]

@cenit What do you think?

[cenit]

opencv_ffmpeg.dll is a custom ffmpeg build. You cannot find it inside vcpkg, because we use the normal ffmpeg build which is a superset of it (we have aligned windows with Linux and osx, where libopencv_ffmpeg never existed)