NN now rounds down rather than to nearest

nearest-neighbour interpolation now strictly rounds down rather than
rounding to nearest

round to nearest caused strange half-pixels along top/left image edges
during enlargement, and extra 1.5 sized pixels everywhere else

instead we should fix the thing that round to nearest was supposed to fix by
adding an extra pair of params to the affine transform which displace in
input space (dx/dy displace in output space)

ChangeLog

@@ -22,6 +22,8 @@
 - vipsthumbnail -o allows absolute file names
 - much better exif handling for jpg images (thanks Gary)
 - preserve jpeg app13 (photoshop ipct)
+- nearest neighbour goes back to round down ... round to nearest caused a
+  range of annoying problems, such as strange half-pixels along edges
 
 14/11/12 started 7.30.6
 - capture tiff warnings earlier

TODO

@@ -1,3 +1,8 @@
+- now we've removed round-to-nearest from NN, we need something extra in the
+  affine transform to displace the input cods
+
+  dx/dy displace output
+
 - check libtool version number, should be binary-compat with 7.30 
 
 - quadratic doesn't work for order 3

libvips/resample/interpolate.c

@@ -333,10 +333,8 @@ vips_interpolate_nearest_interpolate( VipsInterpolate *interpolate,
 {
 	const int ps = IM_IMAGE_SIZEOF_PEL( in->im );
 
-	/* We know x/y are always positive, so we can just (int) them.
-	 */
-	const int xi = (int) (x + 0.5);
-	const int yi = (int) (y + 0.5);
+	const int xi = (int) x;
+	const int yi = (int) y;
 
 	const VipsPel *p = IM_REGION_ADDR( in, xi, yi );
 	VipsPel *q = (VipsPel *) out;