[FFmpeg-devel] Fixpoint FFT optimization, with MDCT and IMDCT wrappers for audio optimization

[Michael Niedermayer]

Hi

On Sat, Jul 28, 2007 at 10:17:53PM -0400, Marc Hoffman wrote:
> On 7/27/07, Michael Niedermayer <michaelni at gmx.at> wrote:
> > Hi
> >
> > On Fri, Jul 27, 2007 at 05:40:14PM -0400, mmh wrote:
> > [...]
> >
> > > +static FFTComplex16 *stwids (FFTComplex *w, int n) {
> > > +    int i;
> > > +    FFTComplex16 *v = av_malloc (sizeof (short)*n);
> > > +    for (i = 0; i < n/2; i++) {
> > > +        v[i].re = w[i].re*32767;
> > > +        v[i].im = w[i].im*32767;
> > > +    }
> >
> > this should be 32768 with proper cliping
> >
> 
> Sorry, to make this so much work for you. I guess if you look at this
> what your asking for is reasonable but you do realize that because of
> overflow 32767 doesn't overflow like 32768 would.  If you multiply
> 1*32768 you get -1 here which is not right not sure how proper
> clipping would help either unless you determine the sign prior to the
> computation which is a mess in C.

proper cliping: (there are of course hundreads of other ways to do it)
FFMIN(w[i].re*32768, 32767)


>  So I think it should be left alone
> otherwise this code becomes fairly complex when its not needed.
> 
> So the proper way to do this is to multiply by almost 1 which is
> 0x7fff and not -1.  BTW because of the way I truncate the result
> 0x7fff or 0x8000 produces the identical values except for the case
> 1*0x8000 which is -1.
> 
> Thoughts?

yes fix your code, this is getting annoying
scaling factor is 32768 as you use >>16, if you use 32767 you would have to
divide by 32767 in the innermost loop
and as you mention truncation, please replace this by proper rounding with
lrinf() or equivalent
you do proper rounding in the inner most loop but in the code which is
excuted just once you dont?!

and keep in mind the whole code already uses small coefficients, so this
makes already inacurate code even less accurate

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

I hate to see young programmers poisoned by the kind of thinking
Ulrich Drepper puts forward since it is simply too narrow -- Roman Shaposhnik
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