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I'm looking for an open-source tool/code or some guidance to extract the motion vectors (MVs) of a H.264 encoded bit sequence. I'm already aware that motion vectors can be visualized using ffmpeg with the following command:

ffplay -flags2 +export_mvs input.mp4 -vf codecview=mv=pf+bf+bb

However, I want to produce a log file where the MVs of P and B frames are listed frame by frame. I checked out the structure of MVs from libavutil/motion_vector.h, but I couldn't find an example which shows how they are extracted and laid over the original sequence by ffplay. I thought that if I can find that out, I could possibly re-arrange the code to extract the MVs to a text file.

I would appreciate any example codes or hints.

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I think the following code can help you:
https://github.com/vadimkantorov/mpegflow
The main idea is that you need to decode frames first and query motion frames by calling av_frame_get_side_data function.

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Try https://github.com/jishnujayakumar/MV-Tractus. This tool will give you motion vectors in the form of JSON for every frame.

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