better rounding behaviour for bicubic

fixed-point bicubic was not rounding to nearest, introducing some noise
in flat areas

ChangeLog

@@ -7,6 +7,7 @@
 - pngsave in interlaced mode make a copy of the whole image
 - vipsthumbnail shrinks to 1/2 window_size
 - vipsthumbnail defaults to bicubic + nosharpen
+- better rounding behaviour for fixed-point bicubic
 
 4/7/14 started 7.40.4
 - fix vips_rawsave_fd(), thanks aferrero2707 

TODO

@@ -6,12 +6,7 @@
 	$ vipsthumbnail Chicago.png --vips-concurrency=4
 	memory: high-water mark 180.57 MB
 
-  shouldn't memuse drop with concurrency 1?
+  shouldn't memuse drop with concurrency 1 ?
-
-- bicubic adds noise to 255/255/255, why? try babe.jpg background
-
-
-
 
 
 

libvips/resample/templates.h

@@ -183,30 +183,35 @@ bicubic_int(
 		(cx[0] * uno_one +
 		 cx[1] * uno_two +
 		 cx[2] * uno_thr +
-		 cx[3] * uno_fou) >> VIPS_INTERPOLATE_SHIFT;
+		 cx[3] * uno_fou +
+		 (VIPS_INTERPOLATE_SCALE >> 1)) >> VIPS_INTERPOLATE_SHIFT;
 
 	const int r1 =
 		(cx[0] * dos_one +
 		 cx[1] * dos_two +
 		 cx[2] * dos_thr +
-		 cx[3] * dos_fou) >> VIPS_INTERPOLATE_SHIFT;
+		 cx[3] * dos_fou +
+		 (VIPS_INTERPOLATE_SCALE >> 1)) >> VIPS_INTERPOLATE_SHIFT;
 
 	const int r2 =
 		(cx[0] * tre_one +
 		 cx[1] * tre_two +
 		 cx[2] * tre_thr +
-		 cx[3] * tre_fou) >> VIPS_INTERPOLATE_SHIFT;
+		 cx[3] * tre_fou +
+		 (VIPS_INTERPOLATE_SCALE >> 1)) >> VIPS_INTERPOLATE_SHIFT;
 
 	const int r3 =
 		(cx[0] * qua_one +
 		 cx[1] * qua_two +
 		 cx[2] * qua_thr +
-		 cx[3] * qua_fou) >> VIPS_INTERPOLATE_SHIFT;
+		 cx[3] * qua_fou +
+		 (VIPS_INTERPOLATE_SCALE >> 1)) >> VIPS_INTERPOLATE_SHIFT;
 
 	return( (cy[0] * r0 +
 		 cy[1] * r1 +
 		 cy[2] * r2 +
-		 cy[3] * r3) >> VIPS_INTERPOLATE_SHIFT );
+		 cy[3] * r3 +
+		 (VIPS_INTERPOLATE_SCALE >> 1)) >> VIPS_INTERPOLATE_SHIFT );
 }
 
 /* Floating-point bicubic, used for int/float/double types.