[FFmpeg-devel] Reordering of GOP Frames

[Michel Bardiaux]

Axel Gallus a ?crit :
> Can please someone comment on my thoughts...
> 
> Lets say we have the following display sequence: I B B P
> Where both B's depend on the following P.
> Therefore the above sequence has be arranged to I P B B in the videofile.
> 
> 1.)    av_read_frame() first reads I in AVPacket struct.
>         avcodec_decode_frame() 

There aint no such animal. I guess you mean avcodec_decode_video.

> decodes AVPacket and has complete frame, 

No.

> because I = Keyframe 
>         frame gets converted and shown ( e.g. via sws_scale() )
> 
> 2.)    av_read_frame() reads P in AVPacket struct.
>         avcodec_decode_frame() decodes AVPacket and signals no frame, because it must'nt show it at this time.
>         So the P-Frame just gets buffered

No, the I-frame comes out.

>        
> 3.)    av_read_frame() reads B in AVPacket struct.
>         avcodec_decode_frame() decodes AVPacket and has complete frame, because P frame already has been decoded
>         frame gets converted and shown ( e.g. via sws_scale() )

Yes (but amended to: because P *and* I ...)

> 
> 4.)    av_read_frame() reads B in AVPacket struct. 
>         avcodec_decode_frame() decodes AVPacket and has complete frame, because P frame already has been decoded
>         frame gets converted and shown ( e.g. via sws_scale() )

Ditto.

> 
> 5.)    av_read_frame() now reads P in AVPacket struct. !??!

Reads *another* P-frame.

>         avcodec_decode_frame() has complete frame, because P frame already has been decoded and buffered
>         frame gets converted and shown ( e.g. via sws_scale() )

The *first* P-frame comes out.

> 
> 
> Are those thought right 5 reads for 4 frames?
> If not, when does the P-frame get shown?
> 
PS: the above assumes the LOW_DELAY flag is not set. If it is set, and 
the stream is I/P only, then each call to avcodec_decode_video returns 
the decoded frame corresponding to the compressed data given it.

I dont know what happens if you use LOW_DELAY with B frames. AFAIK it is 
illegal (not to mention immoral and fattening :-) )

-- 
Michel Bardiaux
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