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resample: improve performance (thanks José Miguel Sánchez García) 

Resample is well known for taking a long time to resize an image. This
patch brings an important performance boost (in my test image, time
was reduced from ~2850ms to ~500ms). It does that by extracting FP
multiplication and division out of the innermost loop of
resamplex/resampley.

The results differ slightly from the current implementation: in my
test: ~0.3% of the bytes had a ±2 difference in their value, which I
attribute to rounding errors. I'm personally not concerned with that
deviation, given the performance gains. However, I recommend testing
it just to be sure I didn't overlook anything.

José Miguel Sánchez García
front
cinap_lenrek 2021-04-25 12:16:40 +02:00
parent b0e2ea4e5f
commit e0cf0261d0
1 changed files with 8 additions and 5 deletions
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13
sys/src/cmd/resample.c
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@ -67,10 +67,10 @@ void
resamplex(uchar *in, int off, int d, int inx, uchar *out, int outx)
{
int i, x, k;
double X, xx, v, rat;
double X, xx, v, rat, rato10;
rat = (double)inx/(double)outx;
rato10 = rat/10.;
for(x=0; x<outx; x++){
if(inx == outx){
/* don't resample if size unchanged */
@ -79,14 +79,15 @@ resamplex(uchar *in, int off, int d, int inx, uchar *out, int outx)
}
v = 0.0;
X = x*rat;
xx = X + rato10*(-K2);
for(k=-K2; k<=K2; k++){
xx = X + rat*k/10.;
i = xx;
if(i < 0)
i = 0;
if(i >= inx)
i = inx-1;
v += in[off+i*d] * K[K2+k];
xx += rato10;
}
out[off+x*d] = v;
}
@ -96,9 +97,10 @@ void
resampley(uchar **in, int off, int iny, uchar **out, int outy)
{
int y, i, k;
double Y, yy, v, rat;
double Y, yy, v, rat, rato10;
rat = (double)iny/(double)outy;
rato10 = rat/10.;
for(y=0; y<outy; y++){
if(iny == outy){
/* don't resample if size unchanged */
@ -107,14 +109,15 @@ resampley(uchar **in, int off, int iny, uchar **out, int outy)
}
v = 0.0;
Y = y*rat;
yy = Y + rato10*(-K2);
for(k=-K2; k<=K2; k++){
yy = Y + rat*k/10.;
i = yy;
if(i < 0)
i = 0;
if(i >= iny)
i = iny-1;
v += in[i][off] * K[K2+k];
yy += rato10;
}
out[y][off] = v;
}