[Image-SIG] downscaling by area averaging

[Fredrik Lundh]

Douglas Bagnall wrote:

> > + I'm beginning to wonder if there's not an off-by-something error
> >   lurking somewhere in stretch.  hmm...
>
> In my tests, all the resize and stretch filters appear to shift the
> image down and to the right by about half an output pixel.  Assuming
> my controls images are right...

I've attached a (preliminary) patch.

</F>

Index: libImaging/Antialias.c
===================================================================
--- libImaging/Antialias.c      (revision 2394)
+++ libImaging/Antialias.c      (working copy)
@@ -145,7 +145,7 @@
     if (imIn->xsize == imOut->xsize) {
         /* vertical stretch */
         for (yy = 0; yy < imOut->ysize; yy++) {
-            center = yy * scale;
+            center = (yy + 0.5) * scale;
             ww = 0.0;
             ss = 1.0 / filterscale;
             /* calculate filter weights */
@@ -222,7 +222,7 @@
     } else {
         /* horizontal stretch */
         for (xx = 0; xx < imOut->xsize; xx++) {
-            center = xx * scale;
+            center = (xx + 0.5) * scale;
             ww = 0.0;
             ss = 1.0 / filterscale;
             xmin = floor(center - support);