[FFmpeg-devel] indeo4/indeo5 inverse quantization

[Michael Niedermayer]

On Mon, May 31, 2010 at 01:18:12AM +0200, Maxim wrote:
> First of all - thank you for your suggestions!
> 
> > [...]
> >
> >   
> >> Index: libavcodec/ivi_common.c
> >> ===================================================================
> >> --- libavcodec/ivi_common.c	(Revision 23369)
> >> +++ libavcodec/ivi_common.c	(Arbeitskopie)
> >> @@ -338,7 +338,8 @@
> >>      RVMapDesc   *rvmap = band->rv_map;
> >>      void (*mc_with_delta_func)(int16_t *buf, const int16_t *ref_buf, uint32_t pitch, int mc_type);
> >>      void (*mc_no_delta_func)  (int16_t *buf, const int16_t *ref_buf, uint32_t pitch, int mc_type);
> >> -    const uint8_t   *base_tab, *scale_tab;
> >> +    const uint16_t  *base_tab;
> >> +    const uint8_t   *scale_tab;
> >>  
> >>      prev_dc = 0; /* init intra prediction for the DC coefficient */
> >>  
> >> @@ -414,7 +415,7 @@
> >>                      if (IVI_DEBUG && !val)
> >>                          av_log(NULL, AV_LOG_ERROR, "Val = 0 encountered!\n");
> >>  
> >> -                    q = (base_tab[pos] * scale_tab[quant]) >> 8;
> >> +                    q = (base_tab[pos] * scale_tab[quant]) >> 9;
> >>                      if (q > 1)
> >>                          val = val * q + FFSIGN(val) * ((q >> 1) - (q & 1));
> >>     
> >
> > ((q >> 1) - (q & 1))
> > could be replaced by
> >
> > ((q^1)-1)>>1
> >   
> 
> What is the advantage of such a replacement? Anyway it looks more
> compactly and friendly to me...
> 
> > also val could be kept in absolute value + sign form and onl be changed
> > to normal c style int afterwards this would avoid the need for FFSIGN()
> >   
> 
> Hmm, I'm not sure that FFSIGN can be completely avoided.
> 
> if (sym == rvmap->esc_sym) { /* Escape - run/val explicitly coded using
> 3 vlc codes */
>     run = get_vlc2(gb, band->blk_vlc.tab->table, IVI_VLC_BITS, 1) + 1;
>     lo  = get_vlc2(gb, band->blk_vlc.tab->table, IVI_VLC_BITS, 1);
>     hi  = get_vlc2(gb, band->blk_vlc.tab->table, IVI_VLC_BITS, 1);
>     val = IVI_TOSIGNED((hi << 6) | lo); /* merge them and convert into
> signed val */
> 
> *************** IVI_TOSIGNED can be rewritten to return the sign separately.
> 
> } else {
>     run = rvmap->runtab[sym];
>     val  = rvmap->valtab[sym];
> 
> *************** Smth like FFSIGN is still needed here.

run  = rvmap->runtab[sym];
val  = rvmap->valtab[sym];
sign = rvmap->sgntab[sym];
(of course only if faster)

?


> }
> 
> > and quant changes per mb not per block thus scale_tab[quant] can be calculated
> > outside the loop
> >   
> 
> This is a good idea in principle. But the main disadvantage of it could
> be the fact that we never know how much coeff were transmitted for a block.
> Consider we have four blocks in a 4-blocks-macroblock for each of them
> only the DC coeff was sent. Then the final amount of calculations will
> look like:
> 
> 64 MULS in order to precalculate 64 Q-values
> -------------------------------------------------------------------------------
>   4 MULS in order to calculate 4 DC coeffs
> 
> which is surely wasteful.

int foobarquant= scale_tab[quant];

will avoid one table lookup and thats what i meant


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I have never wished to cater to the crowd; for what I know they do not
approve, and what they approve I do not know. -- Epicurus
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