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

[Michael Niedermayer]

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

and mandelbrot is difficult to benchmark due to the variable number
of iterations per pixel, the skip code from START/STOP timer should
possibly be disabled for this


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

Old school: Use the lowest level language in which you can solve the problem
            conveniently.
New school: Use the highest level language in which the latest supercomputer
            can solve the problem without the user falling asleep waiting.
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