[Libav-user] How to seek to a given frame?

[Philippe Gorley]

On 2019-05-08 10:27 p.m., Mark McKay wrote:
> The docs in the header file say that the AVSEEK_FLAG_BACKWARD is 
> ignored, and the tests I've run seem to work even when I leave it out.

AVSEEK_FLAG_BACKWARD is ignored in avformat_seek_file, not in av_seek_frame.

> 
> Mark
> 
> 
> ------------------------------------------------------------------------
> *Subject:* Re: [Libav-user] How to seek to a given frame?
> *Date:* Wed, 8 May 2019 13:20:17 -0700
> *From:* Michael Armes <michael.armes at gmail.com>
> *To:* "This list is about using libavcodec, libavformat, libavutil, 
> libavdevice and libavfilter." <libav-user at ffmpeg.org>
> *Reply-To:* "This list is about using libavcodec, libavformat, 
> libavutil, libavdevice and libavfilter." <libav-user at ffmpeg.org>
> Use |AVSEEK_FLAG_BACKWARD| 
> <http://ffmpeg.org/doxygen/trunk/avformat_8h.html#ac736f8f4afc930ca1cda0b43638cc678>  to 
> seek to the closest /keyframe/ having a smaller pts than the one you are 
> seeking. You must then iterate through reading frames until you reach 
> your desired position (if you are not seeking to a keyframe).
> --michael
> 
> On Wed, May 8, 2019 at 10:53 AM Mark McKay <mark at kitfox.com 
> <mailto:mark at kitfox.com>> wrote:
> 
>     I'm trying to create a video player with the ability to seek
>     whereever I want.  I'm having some trouble with av_seek_frame(),
>     which seems to keep returning the first keyframe after the timestamp
>     I'm looking for:
> 
> 
>          int timestamp = av_rescale_q(timeMs, AV_TIME_BASE_Q,
> 
>     pFormatCtx->streams[videoStream]->time_base);
> 
>          int err = av_seek_frame(pFormatCtx, videoStream, timestamp, 0);
> 
>     Unfortunately, if it puts me at a position /after/ the frame where
>     I'm seeking, I have already passed the frame I want and can't scan
>     forward to find it.  I've also tried seeking with AVSEEK_FLAG_ANY,
>     but this it not returning frames with valid data.  I'm guessing that
>     you need to jump to the correct keyframe and then scan forward so
>     that the frame buffers are built correctly.
> 
>     I'm also considering scanning the entire video when I first load it
>     to build  an index of packet information. AVSEEK_FLAG_BYTE looks
>     like it could be useful to jump directly to the packet I want -
>     however, I don't know where to find the byte value I would need to
>     pass to the seek function.
> 
>     How can I scan to the frame that I want?
> 
>     Cheers,
>     Mark
> 
>     ---
>     http://www.kitfox.com
> 
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-- 
Philippe Gorley
Free Software Consultant | Montréal, Qc
Savoir-faire Linux