[FFmpeg-devel] [PATCH 09/10] avfilter/vsrc_mandelbrot: use hypot()

Michael Niedermayer michaelni at gmx.at
Mon Nov 23 22:49:13 CET 2015


On Mon, Nov 23, 2015 at 03:48:40PM -0500, Ganesh Ajjanagadde wrote:
> On Mon, Nov 23, 2015 at 2:13 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> > On Mon, Nov 23, 2015 at 01:57:24PM -0500, Ganesh Ajjanagadde wrote:
> >> On Mon, Nov 23, 2015 at 1:02 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> >> > On Mon, Nov 23, 2015 at 12:43:52PM -0500, Ganesh Ajjanagadde wrote:
> >> >> On Sun, Nov 22, 2015 at 3:56 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> >> >> > On Sun, Nov 22, 2015 at 3:07 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> >> >> >> On Sun, Nov 22, 2015 at 12:05:49PM -0500, Ganesh Ajjanagadde wrote:
> >> >> >>> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> >> >> >>> ---
> >> >> >>>  libavfilter/vsrc_mandelbrot.c | 2 +-
> >> >> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >> >> >>>
> >> >> >>> diff --git a/libavfilter/vsrc_mandelbrot.c b/libavfilter/vsrc_mandelbrot.c
> >> >> >>> index 950c5c8..a0c101e 100644
> >> >> >>> --- a/libavfilter/vsrc_mandelbrot.c
> >> >> >>> +++ b/libavfilter/vsrc_mandelbrot.c
> >> >> >>> @@ -291,7 +291,7 @@ static void draw_mandelbrot(AVFilterContext *ctx, uint32_t *color, int linesize,
> >> >> >>>
> >> >> >>>              use_zyklus= (x==0 || s->inner!=BLACK ||color[x-1 + y*linesize] == 0xFF000000);
> >> >> >>>              if(use_zyklus)
> >> >> >>> -                epsilon= scale*1*sqrt(SQR(x-s->w/2) + SQR(y-s->h/2))/s->w;
> >> >> >>> +                epsilon= scale*hypot(x-s->w/2, y-s->h/2)/s->w;
> >> >> >>
> >> >> >> old:
> >> >> >>  704 decicycles in hypo, 1048570 runs,      6 skips
> >> >> >>
> >> >> >> new:
> >> >> >> 1075 decicycles in hypo, 1048566 runs,     10 skips
> >> >> >>
> >> >> >> that is from START/STOP_TIMER over hypot()
> >> >> >>
> >> >> >> the code is speed relevant as its executed per pixel
> >> >> >
> >> >> > Thanks for testing. Looking more closely, I see no reason for
> >> >> > expensive sqrt calls anyway: one can simply square both sides; it
> >> >> > should be cheaper. Will rework, post benchmark if it is indeed faster
> >> >> > and does not suffer from floating point overflow, else will simply
> >> >> > push a trivial removal of the "1".
> >> >>
> >> >> It seems like getting rid of the sqrt altogether has a very slight
> >> >> positive impact (if any at all). I can post the patch, but would like
> >> >> to know what to benchmark. There are numerous choices, e.g
> >> >> draw_mandelbrot as a whole, the outer loop, or the inner loop.
> >> >> I personally think the inner x loop (lines 268-388) is a good place to
> >> >> look at, since the difference is very small anyway, and further
> >> >> localization is impossible.
> >> >
> >

> >> > please post the patch

please post the patch

[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

No human being will ever know the Truth, for even if they happen to say it
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