[FFmpeg-devel] [PATCH] lavc/aacenc_utils: replace sqrtf(Q*sqrtf(Q)) by precomputed value

[Rostislav Pehlivanov]

On 2 March 2016 at 04:04, Ganesh Ajjanagadde <gajjanag at gmail.com> wrote:

> On Tue, Mar 1, 2016 at 7:52 AM, Derek Buitenhuis
> <derek.buitenhuis at gmail.com> wrote:
> > On 3/1/2016 3:21 AM, Ganesh Ajjanagadde wrote:
> >
> > [...]
> >
> >> ---
> >>  libavcodec/aacenc_utils.h | 3 +--
> >>  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > Cool. Looks like an obvious/easy win, assuming it's identical.
>
> They are not precisely identical, and in fact the change results in
> slightly better accuracy wrt the mathematical expression, simply
> because sqrtf(q * sqrtf(q)) is not always a correctly rounded float. I
> vaguely recall negligible ~ 2/3 ulp differences. The table is
> correctly rounded; I tested that while speeding up the tablegen.
>
> Added a small line to this effect in the notes.
>
>
I did test it for a few test tracks and the results had the exact same SHA1
as before. Either the differences are only affecting in very extreme cases
or we've cracked SHA1.