Does VideoWriter have a lower boundary to the FPS?

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thanks. seeing that code confirms that you aren’t using the default/builtin .AVI file writer. that one rounds its FPS values. .mp4 requires OpenCV to use some library, which is often FFMPEG but I guess OpenCvSharp might prefer Windows media APIs (dshow/MSMF). I haven’t checked those backends’ code for FPS rounding issues.

since you’re using OpenCvSharp, consider contacting the developers of that wrapper. they should be the first contact for any issues.

I just did a quick test. CAP_FFMPEG and CAP_DSHOW manage to create an mp4 file with 1.7 fps, but CAP_MSMF does not, it rounds down to 1.0 fps.

OpenCvSharp does support the apiPreference argument:

http://shimat.github.io/opencvsharp/api/OpenCvSharp.VideoWriter.html#OpenCvSharp_VideoWriter__ctor_System_String_OpenCvSharp_VideoCaptureAPIs_OpenCvSharp_FourCC_System_Double_OpenCvSharp_Size_System_Boolean_