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

diff --git a/sys/src/cmd/resample.c b/sys/src/cmd/resample.c
index d11e299603a2879a2b12284f994f6192456b1ce9..3e4c20f4623af7b473529eac39bda918c6996c73 100644 (file)
--- a/sys/src/cmd/resample.c
+++ b/sys/src/cmd/resample.c
@@ -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;
        }