[Ffmpeg-devel] av_seek_frame() units

[Andy Parkins]

On Tuesday 2005 July 05 06:24, Steve Willis wrote:

> I would also very much like some more information on this. I asked a
> similar question last week, and am very grateful for the response. I am
> a newcomer to avcodec/avformat, and could really use some tips or
> example code on the best way to seek to a particular frame given its
> time, even if that frame is not a keyframe. Could someone explain the
> HURRY_UP mode that was suggested for quickly skipping from the nearest
> keyframe to the actual frame I am seeking?
>
> Like Ian, I need some help understanding how to map a timestamp to a
> framenumber as well.

I don't know for sure is my answer.  However, I am having success treating the 
units to av_seek_frame() as being in AV_TIME_BASE units (at present this is a 
64 bit integer representing time in microseconds passed 
AVFormatContext.start_time)

As to seeking to a particular (non-key) frame, I believe the suggestion from 
Rich Felker was that you seek to the nearest key frame using av_seek_frame() 
then enable hurry_up mode until you hit the frame you want. e.g. (error 
checking and initialisation ommitted)

av_seek_frame( fmtCtx, -1, TARGET_PTS, AVSEEK_FLAG_BACKWARD );
CodecCtx->hurry_up = 1;
do {
    av_read_frame( fmtCtx, &Packet );
    // should really be checking that this is a video packet
    MyPts = Packet.pts / Packet.duration *
	AV_TIME_BASE / av_q2d( Stream->r_frame_rate);
    if( MyPts > TARGET_PTS )
        break;
    avcodec_decode_video( CodecCtx, pFrame, &gotFrame, Packet.data,
        Packet.size );
    av_free_packet( &Packet );
} while(1);
CodecCtx->hurry_up = 0;
// Now near TARGET_PTS with Packet ready for decode (and free)

The calculation of MyPts is probably the answer to your question about frame 
numbers and timestamp.  I am sure that I am not doing this in the correct 
way, but it's working for me :-)

Hope that's helped.


Andy

-- 
Dr Andrew Parkins, M Eng (hons), AMIEE

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