[FFmpeg-devel] Understanding motion vectors from ff_print_debug_info()

[Pravin Bhat]

Hey All-

A while back Paolo suggested that people looking to extract motion 
vectors in ffmpeg should look at
the method ff_print_debug_info() in libavcodec/mpegvideo.c.
This advice turned out to be very useful in my case. Thx Paolo.

However, I have a couple of questions about the constants being used 
ff_print_debug_info() that would
really help noobs like myself really understand what is going on. Please 
consider posting a reply if you
are familiar with this part of ffmpeg.

# It looks like you have to divide by 2 (or a higher power of 2) the 
values in the motion_val array
to obtain the true motion vectors. Why is this so?

For example ff_print_debug_info() processes the motion_val vectors in 
the following manner:
motion_val[direction][xy][0]>>shift
where shift = 1 + s->quarter_sample.
The 'quarter_sample' variable makes sense. It's accounting for some sort 
of mode where the video is
sub-sampled by 2 in the X and Y axis. But I don't understand why the '1' 
is added to 'shift' in effect
dividing all motion vectors by 2. 

# For computing the 'mv_stride' variable avcodec.h suggests the 
following formula:
mv_stride= (mb_width << mv_sample_log2) + 1
While I don't understand why that '1' is added mv_stride.

What worries me is that the addition of that '1' is conditional and I 
don't understand when not to add that '1'. For example 
ff_print_debug_info() uses the following formula:
(s->mb_width << mv_sample_log2) + (s->codec_id == CODEC_ID_H264 ? 0 : 1)

# Finally, is there any way to populate the AVFrame.motion_val array 
with the motion vectors without having the motion vectors visualized on 
the decoded video frame? I'm setting AVCodecContext.debug_mv = 1 to 
obtain the motion vectors but as a side effect that flag also draws the 
motion vectors. It looks like ff_print_debug_info() always draws the 
motion vectors when the debug_mv flag is set. It seems wasteful to have 
to decode the video twice in order to obtain the motion vectors and the 
video frames without the visualization.

Thank you for your time.
-pro