[FFmpeg-devel] New seeking API

[Ivan Schreter]

Vladimir Pantelic wrote:
> Ivan Schreter wrote:
>
>   
>> BTW, I take it this new API will implicitly assume 
>> "AVSEEK_FLAG_BACKWARD", since I cannot really imagine a reason why 
>> someone would like to get a key frame _after_ the given timestamp/position.
>>     
>
> Actually, I would. Given the timestamp "t_c" of the currently displayed 
> video frame, going to the next key frame (seeking the minimum possible 
> amount forward) would be done by
> 	
> 	...seek( ..., t_c, AVSEEK_FLAG_FORWARD );
>
> Otherwise, I would have to use:
>
> 	...seek( ..., t_c + t_d, AVSEEK_FLAG_BACKWARD );
>
> but how would I know how to chose t_d so that I do neither skip over one 
> key frame nor seek back before t_c.
>   
Seeking is relatively expensive.

Instead, you could set loop filter to skip non-reference frames and 
decode only reference frames (i.e., usually key frames and P frames in 
MPEG).

Additionally, in some codecs (namely H.264), key frames are not 
necessarily frames without references, i.e., decoding a random key frame 
doesn't necessarily produce usable output image and/or following frames 
don't necessarily decode to a usable output image. Instead, one has to 
pay attention to convergence duration, which will tell you, how much 
further you need to decode to get a usable picture. This is still under 
development (API how to tell the user,  convergence_duration, is 
currently somewhat broken).

What is the (end user) use case you'd want to address by seeking to the 
next key frame?

Regards,

Ivan