feat(libvideo2x): use OpenCV to retrieve total frame count

[k4yt3x]

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[mirh]

Why can't you check nb_read_frames?

[k4yt3x]

@mirh Because it only exists in FFprobe, not FFmpeg's C bindings. To do it with just FFmpeg I'll need to decode all frames. I'm also going to need OpenCV anyways for some upcoming features.

[mirh]

Right. But CAP_PROP_FRAME_COUNT still leaves a lot to be desired.
https://stackoverflow.com/questions/31472155/python-opencv-cv2-cv-cv-cap-prop-frame-count-get-wrong-numbers/#47796468
https://pyimagesearch.com/2017/01/09/count-the-total-number-of-frames-in-a-video-with-opencv-and-python/

[k4yt3x]

Yeah I am aware. The goal is to get a rough estimation of the total frames quickly so we can calculate the progress. At this time, I don't see the necessity to have it 100% accurate, since we are only using this information to show the user roughly how much of the video has been processed and how much is left.

I just didn't think it'll be worth decoding every frame in the video just to get an accurate reading. It doesn't matter too much if it's off by a couple of frames.