From ac4e5ce05908d3354e25d866d626de16a289f576 Mon Sep 17 00:00:00 2001
From: John Cupitt <jcupitt@gmail.com>
Date: Mon, 14 Mar 2016 14:58:02 +0000
Subject: [PATCH] return of shrink-less-affine-more

though a bit less extreme
---
 libvips/resample/resize.c | 21 ++++++++++++++++-----
 tools/vipsthumbnail.c     | 16 +++++++++++++---
 2 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/libvips/resample/resize.c b/libvips/resample/resize.c
index fc9651d5..c2e39c0f 100644
--- a/libvips/resample/resize.c
+++ b/libvips/resample/resize.c
@@ -115,11 +115,22 @@ vips_resize_build( VipsObject *object )
 	/* If the factor is > 1.0, we need to zoom rather than shrink.
 	 * Just set the int part to 1 in this case.
 	 */
-	int_hshrink = resize->scale > 1.0 ? 
-		1 : VIPS_FLOOR( 1.0 / resize->scale );
-	if( vips_object_argument_isset( object, "vscale" ) ) 
-		int_vshrink = resize->vscale > 1.0 ? 
-			1 : VIPS_FLOOR( 1.0 / resize->vscale );
+
+	/* We want the int part of the shrink to leave a bit to do with
+	 * blur/reduce/sharpen, or we'll see strange changes in aliasing on int
+	 * shrink boundaries as we resize.
+	 */
+
+	if( resize->scale > 1.0 )
+		int_hshrink = 1;
+	else
+		int_hshrink = VIPS_FLOOR( 1.0 / (resize->scale * 1.1) );
+	if( vips_object_argument_isset( object, "vscale" ) ) {
+		if( resize->vscale > 1.0 )
+			int_vshrink = 1;
+		else
+			int_vshrink = VIPS_FLOOR( 1.0 / (resize->vscale * 1.1) );
+	}
 	else
 		int_vshrink = int_hshrink;
 
diff --git a/tools/vipsthumbnail.c b/tools/vipsthumbnail.c
index a507d568..0461a678 100644
--- a/tools/vipsthumbnail.c
+++ b/tools/vipsthumbnail.c
@@ -223,11 +223,21 @@ thumbnail_find_jpegshrink( VipsImage *im )
 	if( linear_processing )
 		return( 1 ); 
 
-	if( shrink >= 8 )
+	/* Shrink-on-load is a simple block shrink and will add quite a bit of
+	 * extra sharpness to the image. We want to block shrink to a size a
+	 * bit above our target, then anti-alias/downsample/sharpen to the
+	 * target. 
+	 *
+	 * For example, consider making a 400-pixel-across image from an
+	 * 800-pixel image. If we load at 1/2 size, we could find ourselves
+	 * doing no further processing, which would make a 400-px version look
+	 * very different from a 450-px version.
+	 */
+	if( shrink >= 8.5 )
 		return( 8 );
-	else if( shrink >= 4 )
+	else if( shrink >= 4.5 )
 		return( 4 );
-	else if( shrink >= 2 )
+	else if( shrink >= 2.5 )
 		return( 2 );
 	else 
 		return( 1 );