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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

--- a/sys/src/cmd/resample.c

+++ b/sys/src/cmd/resample.c

@@ -67,10 +67,10 @@

 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 */

@@ -79,8 +79,8 @@

 		}

 		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;

@@ -87,6 +87,7 @@

 			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 @@

 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 */

@@ -107,8 +109,8 @@

 		}

 		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;

@@ -115,6 +117,7 @@

 			if(i >= iny)

 				i = iny-1;

 			v += in[i][off] * K[K2+k];

+			yy += rato10;

 		}

 		out[y][off] = v;

 	}