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

[Claude Joseph-Angelique]

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.

Thanks