[FFmpeg-devel] [PATCH] avfilter/vsrc_mandelbrot: avoid sqrt for epsilon calculation

[Ganesh Ajjanagadde]

This rewrites into the mathematically equivalent expression avoiding sqrt,
and results in a very minor speedup.

Tested on x86-64, Haswell, GNU/Linux.
Command:
ffmpeg -v error -f lavfi -i mandelbrot -f null -

old (draw_mandelbrot):
3982389425 decicycles in draw_mandelbrot,     256 runs,      0 skips
7634221782 decicycles in draw_mandelbrot,     512 runs,      0 skips
20576449397 decicycles in draw_mandelbrot,    1024 runs,      0 skips
12949998655 decicycles in draw_mandelbrot,    2048 runs,      0 skips

new (draw_mandelbrot):
3966406060 decicycles in draw_mandelbrot,     256 runs,      0 skips
7553322112 decicycles in draw_mandelbrot,     512 runs,      0 skips
20454169970 decicycles in draw_mandelbrot,    1024 runs,      0 skips
12822228615 decicycles in draw_mandelbrot,    2048 runs,      0 skips

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
---
 libavfilter/vsrc_mandelbrot.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libavfilter/vsrc_mandelbrot.c b/libavfilter/vsrc_mandelbrot.c
index 950c5c8..20da8ae 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= SQR(scale/s->w)*(SQR(x-s->w/2) + SQR(y-s->h/2));
 
 #define Z_Z2_C(outr,outi,inr,ini)\
             outr= inr*inr - ini*ini + cr;\
@@ -300,7 +300,7 @@ static void draw_mandelbrot(AVFilterContext *ctx, uint32_t *color, int linesize,
 #define Z_Z2_C_ZYKLUS(outr,outi,inr,ini, Z)\
             Z_Z2_C(outr,outi,inr,ini)\
             if(use_zyklus){\
-                if(Z && fabs(s->zyklus[i>>1][0]-outr)+fabs(s->zyklus[i>>1][1]-outi) <= epsilon)\
+                if(Z && SQR(fabs(s->zyklus[i>>1][0]-outr)+fabs(s->zyklus[i>>1][1]-outi)) <= epsilon)\
                     break;\
             }\
             s->zyklus[i][0]= outr;\
@@ -358,7 +358,7 @@ static void draw_mandelbrot(AVFilterContext *ctx, uint32_t *color, int linesize,
                 if(s->inner==PERIOD){
                     int j;
                     for(j=i-1; j; j--)
-                        if(SQR(s->zyklus[j][0]-zr) + SQR(s->zyklus[j][1]-zi) < epsilon*epsilon*10)
+                        if(SQR(s->zyklus[j][0]-zr) + SQR(s->zyklus[j][1]-zi) < epsilon*10)
                             break;
                     if(j){
                         c= i-j;
-- 
2.6.2