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

[Marc Hoffman]

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.  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?

Thanks
Marc