[FFmpeg-devel] [PATCH 08/11] avfilter/avf_showvolume: use log10 instead of log()/M_LN10

[Ganesh Ajjanagadde]

On Oct 29, 2015 9:26 AM, "Ronald S. Bultje" <rsbultje at gmail.com> wrote:
>
> Hi,
>
> On Thu, Oct 29, 2015 at 8:47 AM, Ganesh Ajjanagadde <gajjanag at mit.edu>
> wrote:
>
> > On Thu, Oct 29, 2015 at 8:22 AM, Paul B Mahol <onemda at gmail.com> wrote:
> > > On 10/29/15, Ganesh Ajjanagadde <gajjanagadde at gmail.com> wrote:
> > >> This is likely more precise and conveys the intent better.
> > >>
> > >> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> > >> ---
> > >>  libavfilter/avf_showvolume.c | 2 +-
> > >>  1 file changed, 1 insertion(+), 1 deletion(-)
> > >>
> > >> diff --git a/libavfilter/avf_showvolume.c
b/libavfilter/avf_showvolume.c
> > >> index 95b5388..395375a 100644
> > >> --- a/libavfilter/avf_showvolume.c
> > >> +++ b/libavfilter/avf_showvolume.c
> > >> @@ -197,7 +197,7 @@ static int filter_frame(AVFilterLink *inlink,
> > AVFrame
> > >> *insamples)
> > >>              max = FFMAX(max, src[i]);
> > >>
> > >>          max = av_clipf(max, 0, 1);
> > >> -        values[VAR_VOLUME] = 20.0 * log(max) / M_LN10;
> > >> +        values[VAR_VOLUME] = 20.0 * log10(max);
> > >>          values[VAR_CHANNEL] = c;
> > >>          color = av_expr_eval(s->c_expr, values, NULL);
> > >>
> > >> --
> > >> 2.6.2
> > >
> > > Have you checked which one is faster?
> > > I really have no opinion on this but gain is neglible.
> >
> > No, I have not. I suspect, but have not confirmed that log10 is
> > slightly slower:
> >
> >
https://stackoverflow.com/questions/10810105/explanation-wanted-log10-faster-than-log-and-log2-but-only-with-o2-and-greater
> > (janneb's comment). Keep in mind their benchmark did not include the
> > cost of a division.
>
>
> A good compiler (including yours) will merge the division into the
multiply:
> a * func(x) / c
> is the same as:
> (a / c) * func(x)
>
> and the compiler can pre-calculate the output of a/c into a single
constant.

Not necessarily, floating point is not associative.

>
> Ronald
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