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I want to extract the motion vector from the encoded h264 stream without performing a full decode of the frame.

I'm using the FFmpeg library and am aware of extracting the information using the av_frame_get_side_data. Unfortunately, it does a full decode of the frame before providing the AV_FRAME_DATA_MOTION_VECTORS.

Is it possible to parse the h264 to extract out the motion vectors only, ignoring all the other data.

DavidPostill
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M.Akyuzlu
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2 Answers2

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FFmpeg supplies the example file extract_mvs.c, which needs compiling to be used.

Once compiled, use this command:

./extract_mvs input.mp4 > output.txt

This extracts the motion vectors into a text file.

A ready-made tools is MV-Tractus.

This tool will give you motion vectors in the form of JSON for every frame.

A more recent tool is mv-extractor, described as :

This tool extracts frames, motion vectors, frame types and timestamps from H.264 and MPEG-4 Part 2 encoded videos.

harrymc
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[...] what I'm trying to do is extracting the motion vectors to calculate the motion percentage before decoding the package in order to save some CPU usage,[...]

There is an easier (and probably faster) way to calculate motion percentage from undecoded streams than computing motion vectors and post-processing them:

ffprobe -show_frames file.mp4 | grep pkt_size

The size of the P-frames is directly related to the amount of information contained in those vectors.

I hope this helps!

Further reference can be found here

Rabindranath Andujar
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