From 646960077da70f5b8161a87de0fb1ab9545cd847 Mon Sep 17 00:00:00 2001
From: John Cupitt <jcupitt@gmail.com>
Date: Thu, 9 Feb 2017 09:10:03 +0000
Subject: [PATCH] fix cache hint for shrink

shrink had UNBUFFERED as a cache hint, but for non-int shrinks it used
affine, so you could get out of order reads

set regular SEQ as the cache hint, and switch to reduce for the final
resize in the non-int case

see https://github.com/jcupitt/libvips/issues/596
---
 libvips/resample/shrink.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/libvips/resample/shrink.c b/libvips/resample/shrink.c
index 19eff243..e97fb3c1 100644
--- a/libvips/resample/shrink.c
+++ b/libvips/resample/shrink.c
@@ -7,6 +7,9 @@
  * 15/8/16
  * 	- more accurate resize
  * 	- rename xshrink -> hshrink for greater consistency 
+ * 9/2/17
+ * 	- use reduce, not affine, for any residual shrink
+ * 	- expand cache hint
  */
 
 /*
@@ -86,17 +89,16 @@ vips_shrink_build( VipsObject *object )
 
 	if( hshrink_int != shrink->hshrink || 
 		vshrink_int != shrink->vshrink ) {
-		/* Shrink by int factors, affine to final size.
+		/* Shrink by int factors, reduce to final size.
 		 */
-		double xresidual = hshrink_int / shrink->hshrink; 
-		double yresidual = vshrink_int / shrink->vshrink;
+		double xresidual = shrink->hshrink / hshrink_int; 
+		double yresidual = shrink->vshrink / vshrink_int;
 
 		if( vips_shrinkv( resample->in, &t[0], vshrink_int, NULL ) ||
 			vips_shrinkh( t[0], &t[1], hshrink_int, NULL ) )
 			return( -1 ); 
 
-		if( vips_affine( t[1], &t[2], 
-			xresidual, 0.0, 0.0, yresidual, NULL ) ||
+		if( vips_reduce( t[1], &t[2], xresidual, yresidual, NULL ) ||
 			vips_image_write( t[2], resample->out ) )
 			return( -1 );
 	}
@@ -126,7 +128,10 @@ vips_shrink_class_init( VipsShrinkClass *class )
 	vobject_class->description = _( "shrink an image" );
 	vobject_class->build = vips_shrink_build;
 
-	operation_class->flags = VIPS_OPERATION_SEQUENTIAL_UNBUFFERED;
+	/* You'd think UNBUFFERED would work, but we will use reduce for non-int
+	 * shrinks, so it has to be straight SEQ.
+	 */
+	operation_class->flags = VIPS_OPERATION_SEQUENTIAL;
 
 	VIPS_ARG_DOUBLE( class, "vshrink", 9, 
 		_( "Vshrink" ), 
@@ -175,7 +180,7 @@ vips_shrink_init( VipsShrink *shrink )
  *
  * Shrink @in by a pair of factors with a simple box filter. For non-integer
  * factors, vips_shrink() will first shrink by the integer part with a box
- * filter, then use vips_affine() plus bilinear interpolation to shrink by the
+ * filter, then use vips_reduce() to shrink by the
  * remaining fractional part. 
  *
  * This is a very low-level operation: see vips_resize() for a more
@@ -184,7 +189,7 @@ vips_shrink_init( VipsShrink *shrink )
  * This operation does not change xres or yres. The image resolution needs to
  * be updated by the application. 
  *
- * See also: vips_resize(), vips_affine().
+ * See also: vips_resize(), vips_reduce().
  *
  * Returns: 0 on success, -1 on error
  */