[x264-devel] Re: QP related variables

[Laurent Aimar]

On Wed, Jul 21, 2004, Måns Rullgård wrote:
> Laurent Aimar <fenrir at via.ecp.fr> writes:
> 
> > On Thu, Jul 15, 2004, Måns Rullgård wrote:
> >> I've been trying to figure out the meaning of the various QP related
> >> variables used during encoding.  From what I can see the global QP
> >> value is used for all MBs.  Is this the case, or have I overlooked
> >> something?
> >  Yes it's that case, a global qp per frame.
> >
> >>  I also noticed a TODO comment in encoder/analyse.c, line
> >> 850.  What is this referring to?
> >   Implementing a RC that change QP per MB ;)
> >> 
> >> QP adjustments should be done by the rate control, so I need to know
> >> how to properly change the QP used for each MB.  Any hints there?
> >  Yes, the best is just to pass it to x264_macroblock_analyse and to replace
> >   h->mb.qp[h->mb.i_mb_xy] = x264_clip3( h->pps->i_pic_init_qp + h->sh.i_qp_delta + 0, 0, 51 );
> > by
> >   h->mb.qp[h->mb.i_mb_xy] = i_my_qp_for_this_mb;
> >
> > It should work but I never tested if the bitstream is correct.
> What about all the other uses of
 The only one you really need to change is h->mb.qp[h->mb.i_mb_xy].
It will be the QP for the current MB.

 All others "QP variables" are used to create the bitstream and will
help saving a few bits.

 When writing the bistream, only the delta with the last MB (saved by
h->mb.i_last_qp) is written to the bitstream (h->mb.i_last_dqp is also
used in CABAC mode to select a cabac context).
 Now for the first macroblock of a slice, h->mb.i_last_qp is initialized
with h->pps->i_pic_init_qp + h->sh.i_qp_delta (and h->mb.i_last_dqp to 0).
(PPS is constant for the whole stream, while SH could be changed at each slice).

 If you want to save a few bits and
 - if you know the average QP for the whole stream then put it in
h->pps->i_pic_init_qp.
 - if you know the first QP of the slice (before the slice header is written), set
 h->sh.i_qp_delta to i_first_qp - h->pps->i_pic_init_qp.

 Btw, one thing, there is a constraint for the QP variation within a slice, I'm not
sure but I think it is : abs(qp - last_qp) <= 12

-- 
fenrir

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