[FFmpeg-devel] [PATCH] Fix for inverted sign in ffvorbis?audio?output (resubmit)

[Michael Niedermayer]

On Thu, May 14, 2009 at 12:09:31PM +0300, Siarhei Siamashka wrote:
> On Thu, May 14, 2009 at 11:29 AM, Reimar D?ffinger wrote:
> > On Thu, May 14, 2009 at 11:06:21AM +0300, Siarhei Siamashka wrote:
> >> And what about this variant?
> >>
> >> + ? ?alpha = (scale < 0 ? 0.5 * M_PI : 0) + 0.25 * M_PI / n;
> >> + ? ?scale = -sqrt(fabs(scale));
> >> ? ? ?for(i=0;i<n4;i++) {
> >> - ? ? ? ?alpha = 2 * M_PI * (i + 1.0 / 8.0) / n;
> >> - ? ? ? ?s->tcos[i] = -cos(alpha);
> >> - ? ? ? ?s->tsin[i] = -sin(alpha);
> >> + ? ? ? ?s->tcos[i] = cos(alpha) * scale;
> >> + ? ? ? ?s->tsin[i] = sin(alpha) * scale;
> >> + ? ? ? ?alpha += 2.0 * M_PI / n;
> >
> > Rounding errors accumulate like that, IMO you really should never do
> > that unless a) you know the value you add can be represented exactly
> > or b) you absolutely can't avoid it for performance reasons.
> 
> Everything is calculated in double precision floating point, with the
> final results stored in single precision. Additionally, transcendental
> functions generally have their precision unspecified unless I'm
> mistaken. There are plenty of extra bits of precision in intermediate
> calculations. Regression test does not detect any problems, this
> probably means that accumulated rounding errors never had any real
> impact and the final result of double->float conversion was always the
> same (of course it could affect the results theoretically, but this
> probability is rather low).

We did spend enough time debuging cos/sin rounding errors, in the past already
iam not going to take any chances when theres no benefit from it at all

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

Everything should be made as simple as possible, but not simpler.
-- Albert Einstein
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