[Ffmpeg-devel] [Ffmpeg-devel-old] how to get the previous frame

[Michel Bardiaux]

Claude Joseph-Angelique wrote:
> Michel Bardiaux wrote:
> 
>> Thomas Koninckx wrote:
>>
>>> On Tue, 2005-04-26 at 11:38 +0200, Claude Joseph-Angelique wrote:
>>>
>>>> Hi,
>>>> I'm trying to make a backward frame-by-frame on a video stream. But 
>>>> av_read_frame() works sequencialy. How must I use av_seek_frame to 
>>>> get the last frame without image degradation.
>>>
>>>
>>>
>>> I needed to do the same a while ago. What I did --but it might be that
>>> there is a much better solution-- is to make a small jump backward
>>> (av_seek_frame current pts-1 sec), and then read frames until I'm back
>>> at the current pts. I store always 2 frames in a small buffer. This way
>>> I always got the previous frame, undistorted also if it is not a key
>>> frame.
>>> If somebody has a better solution.
>>>
>>> Regards,
>>> Thomas
>>>
>> It is probably the only solution, at least with MPEG-something codecs. 
>> But it will have to be even more complicated:
>>
>> (1) You have to check to be sure you encounter a key frame.
>>
>> (2) In MPEG-4 you can have very large GOPs, of several seconds, so a 
>> jump of -1 sec is not enough to be before the key frame
>>
>> HaND,
> 
> 
> Ok,
> First thanks for your explanations. I'll try to seek to the last key 
> frame in order to read frame from it to the current pts (always having 
> the previous frame). But how can I know if a frame is  a key frame? The 
> "key_frame" field of the "AVFrame" structure seems to be always set to 0.

It's pkt->flags & PKT_FLAG_KEY where pkt is the AVPacket argument of 
av_read_frame.

> 
> Thanks
> 
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-- 
Michel Bardiaux
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Tel : +32 2 790.29.41