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

[Ganesh Ajjanagadde]

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.

>
>>
>> [...]
>> --
>> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>>
>> No great genius has ever existed without some touch of madness. -- Aristotle
>>
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