[matroska-devel] Re: b-frames

Steve Lhomme steve.lhomme at free.fr
Sun Jun 29 10:33:26 CEST 2003


>>Yes and no. You can also have P frames with prio of
>>0 because they are
>>used the same as I frames in MPEG (for references).
> 
> 
> One day someone told me I should read and follow the
> specs instead of what he was saying (/me whistle).
> So to remind you the specs :
> 
> "ReferencePriority : This frame is referenced and has
> the specified cache priority. In cache only a frame of
> the same or higher priority can replace this frame. A
> value of 0 means the frame is not referenced."
 >
> So what does that mean ? Somewhat the contrary of what
> both of you are saying here :p

OK, you're right. The principle is the same only change a <= with a >= :)

> First that B frames should have a priority of 0 since
> they aren't referenced (it is the case in avs2matroska
> Chris ;)), and that I and P frames would have a
> priority of 1 or 2.

No, that just proves what I said before : I and P frames need the same 
prio number, as it's how it's done in MPEG.
Whoever wrote these specs, I love him ! (I spent the afternoon at the 
Gay Pride, so... ;)

> Currently when there are only I and P frames nobody
> uses priorities (why ? well maybe because using the
> same priority number for each frame would be enough
> since in this case we only need the previous decoded
> frame - whatever it may be - to be able to decode the
> next one if it isn't a new I frame, so setting all
> frames to a priority of 1 would just be a waste of
> space).

Yes, it's not a problem, especially since they should have the same prio 
of 1. Theoretically a value of 0 for MinCache and prio of 0 would be OK. 
But not according to the specs. In a system where the caching is done 
out of the codec (as matroska is done) these 0 values may lead to a pb. 
None exist at the moment, but maybe one day...

> There is still the question of what priorities to use.
> Actually avs2matroska uses priorities of 0 for
> B-frames (right, since they aren't referenced), and 2
> for the other frames).
> Is this good ?

Yes.

> __________________________________
> Do you Yahoo!?

NO !



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