This rewrites into a similar expression avoiding sqrt. Similarity is
assured since sqrt(x^2 + y^2)/(x+y) lies in [1/sqrt(2), 1] for x, y > 0.
Tested on x86-64, Haswell, GNU/Linux.
Command:
ffmpeg -f lavfi -i mandelbrot -f null -
old (draw_mandelbrot):
277625266 decicycles in draw_mandelbrot, 256 runs, 0 skips
304527322 decicycles in draw_mandelbrot, 512 runs, 0 skips
377593582 decicycles in draw_mandelbrot, 1024 runs, 0 skips
338539499 decicycles in draw_mandelbrot, 2048 runs, 0 skips
583630357 decicycles in draw_mandelbrot, 4096 runs, 0 skips
new (draw_mandelbrot):
274635709 decicycles in draw_mandelbrot, 256 runs, 0 skips
300287046 decicycles in draw_mandelbrot, 512 runs, 0 skips
371819935 decicycles in draw_mandelbrot, 1024 runs, 0 skips
336663765 decicycles in draw_mandelbrot, 2048 runs, 0 skips
581851016 decicycles in draw_mandelbrot, 4096 runs, 0 skips
Reviewed-by: Michael Niedermayer <mich...@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanaga...@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 bdcad25..6ba8c59 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*(abs(x-s->w/2) + abs(y-s->h/2))/s->w;
#define Z_Z2_C(outr,outi,inr,ini)\
outr= inr*inr - ini*ini + cr;\
--
2.6.2
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