Re: [FFmpeg-devel] [PATCH] Macroblocks modes extraction

[James Almer <jamrial-at-gmail.com@ffmpeg.org>]

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On 7/2/2025 7:57 AM, Timothee wrote:
> Hello,
> 
> I am working on a modification toextract per-macroblock prediction modes 
> (H.264 for now). The goal is to make this information available to 
> vf_codecview and print it in a log file (for now).
> 
> This are what I have added:
> 
> 1.
> 
>     A new H264MBInfostruct holding the prediction modes.

Isn't this what libavutil/video_enc_params.h is for? Or if needed, what 
it could be extended to handle?

> 
> 2.
> 
>     An AVBufferRef *mb_info_refis added to the H264Picturestruct to
>     store this data for each picture.
> 
> 3.
> 
>     This buffer is allocated with av_buffer_allocz()in
>     alloc_picture()and its reference is released with
>     av_buffer_unref()in ff_h264_unref_picture().
> 
> 4.
> 
>     A new function, ff_h264_collect_mb_info(), is called after
>     macroblock decoding to populate the buffer.
> 
> 5.
> 
>     In output_frame(), a new side data (AV_FRAME_DATA_H264_MB_INFO)
>     buffer is allocated, and the contents of srcp->mb_info_ref->dataare
>     copied into it.
> 
> 6. A new function to log the result in vf_codecview.c
> 
> This implementation results in a segmentation fault. I guess it is a 
> race condition, but I am struggling to fix it.
> 
> The patch provided should result in the same error when running `./ 
> ffmpeg -loglevel debug -i input.mp4 -vf codecview=show_modes=1 -f null -`
> 
> Are side data the right thing to use in this context ? Is there a better 
> approach to do it ?
> 
> I am new to FFmpeg and, in general, to large open-source projects, but I 
> am willing to learn so please do not hesitate to correct me.
> 
> Thank you for your guidance.
> 
> Timothée
> 
> 
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