Problem: VideoWriter writer.open() will return false if it fails to open, while writer.isOpened() will also return false.

Solution: The runtime environment lacks opencv_ffmpeg***_64.dll and opencv_ffmpeg***.dll underlying call libraries, just copy them over.

opencv version, 2.x only supports avi format, 3.x supports mp4 (here all refer to the official native version, not the self-compiled version)

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