try to improve vips_resize() quality

small improvements to vips_resize() quality:

* turn down the anti-alias filter a little so we don't smudge out texture
* don't do the final sharpening pass if we skipped the anti-alias filter
* fix a >/>= mixup which meant we didn't sharpen for small resizes

ChangeLog

@@ -12,6 +12,7 @@
 - sizealike / formatalike / bandsalike elide completely if they can for
   a x2 saving in C stack use in many cases
 - added vips_mapim() ... resample with an index image, plus test
+- try to improve vips_resize() quality a little more
 
 7/5/15 started 8.1.1
 - oop, vips-8.0 wrapper script should be vips-8.1, thanks Danilo

TODO

@@ -6,9 +6,6 @@
 
 
 
-- vips_resize() should not use the anti-alias filter if vips_shrink() has
-  not been called, ie. for shrinks < 2 or so
-
 - still not happy about float->int mask conversion in im_vips2mask.c
 
 - looks like we have a race in tiled threadcache? see 

libvips/resample/resize.c

@@ -75,6 +75,7 @@ G_DEFINE_TYPE( VipsResize, vips_resize, VIPS_TYPE_RESAMPLE );
 static int
 vips_resize_build( VipsObject *object )
 {
+	VipsObjectClass *class = VIPS_OBJECT_GET_CLASS( object );
 	VipsResample *resample = VIPS_RESAMPLE( object );
 	VipsResize *resize = (VipsResize *) object;
 
@@ -87,6 +88,7 @@ vips_resize_build( VipsObject *object )
 	int int_shrink_width;
 	double residual;
 	double sigma;
+	gboolean anti_alias;
 
 	if( VIPS_OBJECT_CLASS( vips_resize_parent_class )->build( object ) )
 		return( -1 );
@@ -129,6 +131,7 @@ vips_resize_build( VipsObject *object )
 
 	/* A copy for enlarge resize.
 	 */
+	vips_info( class->nickname, "box shrink by x %d", int_shrink );
 	if( vips_shrink( in, &t[0], int_shrink, int_shrink, NULL ) )
 		return( -1 );
 	in = t[0];
@@ -189,14 +192,19 @@ vips_resize_build( VipsObject *object )
 	 * Don't blur for very small shrinks, blur with radius 1 for x1.5
 	 * shrinks, blur radius 2 for x2.5 shrinks and above, etc.
 	 */
-	sigma = ((1.0 / residual) - 0.5) / 1.5;
+	sigma = ((1.0 / residual) - 0.5) / 2.0;
-	if( residual < 1.0 &&
+	anti_alias = residual < 1.0 && sigma > 0.1;
-		sigma > 0.1 ) { 
+	if( anti_alias ) { 
+		vips_info( class->nickname, "anti-alias sigma %g", sigma );
 		if( vips_gaussblur( in, &t[2], sigma, NULL ) )
 			return( -1 );
 		in = t[2];
 	}
 
+	vips_info( class->nickname, "residual affine %g", residual );
+	vips_info( class->nickname, "%s interpolation", 
+		VIPS_OBJECT_GET_CLASS( resize->interpolate )->nickname );
+
 	if( vips_affine( in, &t[3], residual, 0, 0, residual, 
 		"interpolate", resize->interpolate,
 		"idx", resize->idx,
@@ -205,9 +213,12 @@ vips_resize_build( VipsObject *object )
 		return( -1 );
 	in = t[3];
 
-	/* If we are upsampling, don't sharpen.
+	/* If we are upsampling, don't sharpen. Also don't sharpen if we
+	 * skipped the anti-alias filter. 
 	 */
-	if( int_shrink > 1 ) { 
+	if( int_shrink >= 1 && 
+		anti_alias ) { 
+		vips_info( class->nickname, "final sharpen" );
 		t[5] = vips_image_new_matrixv( 3, 3,
 			-1.0, -1.0, -1.0,
 			-1.0, 32.0, -1.0,

tools/vipsthumbnail.c

@@ -439,10 +439,6 @@ thumbnail_shrink( VipsObject *process, VipsImage *in,
 
 	shrink = calculate_shrink( in );
 
-	vips_info( "vipsthumbnail", "shrink by %g", shrink );
-	vips_info( "vipsthumbnail", "%s interpolation", 
-		VIPS_OBJECT_GET_CLASS( interp )->nickname );
-
 	if( vips_resize( in, &t[4], 1.0 / shrink, 
 		"interpolate", interp,
 		NULL ) )