seems to work

add @centre option for downsampling, see

https://github.com/jcupitt/libvips/issues/504

ChangeLog

@@ -43,6 +43,8 @@
   rounding down, thanks ioquatix
 - better support for tile overlaps in google maps mode in dzsave
 - dzsave puts vips-properties.xml in the main dir for gm and zoomify layouts
+- resize and reduce have @centre option for centre convention downsampling
+- vipsthumbnail uses centre convention to better match imagemagick
 
 19/8/16 started 8.3.4
 - better transparency handling in gifload, thanks diegocsandrim

TODO

@@ -1,11 +1,3 @@
-- test possible centre hack
-
-  I think we're moving too far ... this is going to shift the mask by 50%, we
-  just want half a pixel
-
-  put the shift into vips_reduce_make_mask() so it gets picked up by the double
-  path too
-
 - not sure about utf8 error messages on win
 
 - strange: 

libvips/resample/reduce.c

@@ -4,6 +4,8 @@
  * 	- from shrink.c 
  * 15/8/16
  * 	- rename xshrink -> hshrink for greater consistency 
+ * 9/9/16
+ * 	- add @centre option
  */
 
 /*
@@ -75,6 +77,10 @@ typedef struct _VipsReduce {
 	 */
 	VipsKernel kernel;
 
+	/* Use centre rather than corner sampling convention.
+	 */
+	gboolean centre;
+
 } VipsReduce;
 
 typedef VipsResampleClass VipsReduceClass;
@@ -94,9 +100,11 @@ vips_reduce_build( VipsObject *object )
 
 	if( vips_reducev( resample->in, &t[0], reduce->vshrink, 
 		"kernel", reduce->kernel, 
+		"centre", reduce->centre, 
 		NULL ) ||
 		vips_reduceh( t[0], &t[1], reduce->hshrink, 
 			"kernel", reduce->kernel, 
+			"centre", reduce->centre, 
 			NULL ) ||
 		vips_image_write( t[1], resample->out ) )
 		return( -1 );
@@ -143,6 +151,13 @@ vips_reduce_class_init( VipsReduceClass *class )
 		G_STRUCT_OFFSET( VipsReduce, kernel ),
 		VIPS_TYPE_KERNEL, VIPS_KERNEL_LANCZOS3 );
 
+	VIPS_ARG_BOOL( class, "centre", 7, 
+		_( "Centre" ), 
+		_( "Use centre sampling convention" ),
+		VIPS_ARGUMENT_OPTIONAL_INPUT,
+		G_STRUCT_OFFSET( VipsReduce, centre ),
+		FALSE );
+
 	/* The old names .. now use h and v everywhere. 
 	 */
 	VIPS_ARG_DOUBLE( class, "xshrink", 8, 
@@ -178,10 +193,14 @@ vips_reduce_init( VipsReduce *reduce )
  * Optional arguments:
  *
  * * @kernel: #VipsKernel to use to interpolate (default: lanczos3)
+ * * @centre: %gboolean use centre rather than corner sampling convention
  *
  * Reduce @in by a pair of factors with a pair of 1D kernels. This 
  * will not work well for shrink factors greater than three.
  *
+ * Set @centre to use centre rather than corner sampling convention. Centre
+ * convention can be useful to match the behaviour of other systems. 
+ *
  * This is a very low-level operation: see vips_resize() for a more
  * convenient way to resize images. 
  *

libvips/resample/reduceh.cpp

@@ -6,6 +6,8 @@
  * 	- add other kernels
  * 15/8/16
  * 	- rename xshrink as hshrink for consistency
+ * 9/9/16
+ * 	- add @centre option
  */
 
 /*
@@ -321,6 +323,8 @@ vips_reduceh_gen( VipsRegion *out_region, void *seq,
 	s.top = r->top;
 	s.width = r->width * reduceh->hshrink + reduceh->n_point;
 	s.height = r->height;
+	if( reduceh->centre )
+		s.width += 1;
 	if( vips_region_prepare( ir, &s ) )
 		return( -1 );
 
@@ -335,6 +339,8 @@ vips_reduceh_gen( VipsRegion *out_region, void *seq,
 		q = VIPS_REGION_ADDR( out_region, r->left, r->top + y );
 
 		X = r->left * reduceh->hshrink;
+		if( reduceh->centre )
+			X += 0.5;
 
 		/* We want p0 to be the start (ie. x == 0) of the input 
 		 * scanline we are reading from. We can then calculate the p we
@@ -439,6 +445,7 @@ vips_reduceh_build( VipsObject *object )
 		vips_object_local_array( object, 2 );
 
 	VipsImage *in;
+	int width;
 
 	if( VIPS_OBJECT_CLASS( vips_reduceh_parent_class )->build( object ) )
 		return( -1 );
@@ -465,8 +472,6 @@ vips_reduceh_build( VipsObject *object )
 		return( -1 );
 	}
 	for( int x = 0; x < VIPS_TRANSFORM_SCALE + 1; x++ ) {
-		float fx;
-
 		reduceh->matrixf[x] = 
 			VIPS_ARRAY( object, reduceh->n_point, double ); 
 		reduceh->matrixi[x] = 
@@ -475,18 +480,9 @@ vips_reduceh_build( VipsObject *object )
 			!reduceh->matrixi[x] )
 			return( -1 ); 
 
-		/* Calculate our [0, 1] float displacement. For centre
-		 * convention we must shift by 0.5 and wrap around.
-		 */
-		fx = (float) x / VIPS_TRANSFORM_SCALE;
-		if( reduceh->centre ) {
-			fx += 0.5;
-			if( fx > 1.0 )
-				fx -= 1.0;
-		}
-
 		vips_reduce_make_mask( reduceh->matrixf[x], 
-			reduceh->kernel, reduceh->hshrink, fx ); 
+			reduceh->kernel, reduceh->hshrink, 
+			(float) x / VIPS_TRANSFORM_SCALE );
 
 		for( int i = 0; i < reduceh->n_point; i++ )
 			reduceh->matrixi[x][i] = reduceh->matrixf[x][i] * 
@@ -500,10 +496,15 @@ vips_reduceh_build( VipsObject *object )
 	in = t[0];
 
 	/* Add new pixels around the input so we can interpolate at the edges.
+	 * In centre mode, we read 0.5 pixels more to the right, so we must
+	 * enlarge a little further.
 	 */
+	width = in->Xsize + reduceh->n_point - 1;
+	if( reduceh->centre )
+		width += 1;
 	if( vips_embed( in, &t[1], 
 		reduceh->n_point / 2 - 1, 0, 
-		in->Xsize + reduceh->n_point - 1, in->Ysize,
+		width, in->Ysize,
 		"extend", VIPS_EXTEND_COPY,
 		NULL ) )
 		return( -1 );
@@ -521,7 +522,7 @@ vips_reduceh_build( VipsObject *object )
 	 * fractional part), we just see the integer part here.
 	 */
 	resample->out->Xsize = VIPS_ROUND_UINT( 
-		(in->Xsize - reduceh->n_point + 1) / reduceh->hshrink );
+		resample->in->Xsize / reduceh->hshrink );
 	if( resample->out->Xsize <= 0 ) { 
 		vips_error( object_class->nickname, 
 			"%s", _( "image has shrunk to nothing" ) );

libvips/resample/reducev.cpp

@@ -13,6 +13,8 @@
  * 	- better accuracy with smarter multiplication
  * 15/8/16
  * 	- rename yshrink as vshrink for consistency
+ * 9/9/16
+ * 	- add @centre option
  */
 
 /*
@@ -100,6 +102,10 @@ typedef struct _VipsReducev {
 	 */
 	VipsKernel kernel;
 
+	/* Use centre rather than corner sampling convention.
+	 */
+	gboolean centre;
+
 	/* Number of points in kernel.
 	 */
 	int n_point;
@@ -526,6 +532,8 @@ vips_reducev_gen( VipsRegion *out_region, void *vseq,
 	s.top = r->top * reducev->vshrink;
 	s.width = r->width;
 	s.height = r->height * reducev->vshrink + reducev->n_point;
+	if( reducev->centre )
+		s.height += 1;
 	if( vips_region_prepare( ir, &s ) )
 		return( -1 );
 
@@ -534,7 +542,8 @@ vips_reducev_gen( VipsRegion *out_region, void *vseq,
 	for( int y = 0; y < r->height; y ++ ) { 
 		VipsPel *q = 
 			VIPS_REGION_ADDR( out_region, r->left, r->top + y );
-		const double Y = (r->top + y) * reducev->vshrink; 
+		const double Y = (r->top + y) * reducev->vshrink + 
+			(reducev->centre ? 0.5 : 0.0); 
 		VipsPel *p = VIPS_REGION_ADDR( ir, r->left, (int) Y ); 
 		const int sy = Y * VIPS_TRANSFORM_SCALE * 2;
 		const int siy = sy & (VIPS_TRANSFORM_SCALE * 2 - 1);
@@ -636,6 +645,8 @@ vips_reducev_vector_gen( VipsRegion *out_region, void *vseq,
 	s.top = r->top * reducev->vshrink;
 	s.width = r->width;
 	s.height = r->height * reducev->vshrink + reducev->n_point;
+	if( reducev->centre )
+		s.height += 1;
 	if( vips_region_prepare( ir, &s ) )
 		return( -1 );
 
@@ -653,7 +664,8 @@ vips_reducev_vector_gen( VipsRegion *out_region, void *vseq,
 	for( int y = 0; y < r->height; y ++ ) { 
 		VipsPel *q = 
 			VIPS_REGION_ADDR( out_region, r->left, r->top + y );
-		const double Y = (r->top + y) * reducev->vshrink; 
+		const double Y = (r->top + y) * reducev->vshrink + 
+			(reducev->centre ? 0.5 : 0.0); 
 		const int py = (int) Y; 
 		const int sy = Y * VIPS_TRANSFORM_SCALE * 2;
 		const int siy = sy & (VIPS_TRANSFORM_SCALE * 2 - 1);
@@ -783,7 +795,7 @@ vips_reducev_raw( VipsReducev *reducev, VipsImage *in )
 	 * fractional part), we just see the integer part here.
 	 */
 	resample->out->Ysize = VIPS_ROUND_UINT( 
-		(in->Ysize - reducev->n_point + 1) / reducev->vshrink );
+		resample->in->Ysize / reducev->vshrink );
 	if( resample->out->Ysize <= 0 ) { 
 		vips_error( object_class->nickname, 
 			"%s", _( "image has shrunk to nothing" ) );
@@ -813,6 +825,7 @@ vips_reducev_build( VipsObject *object )
 	VipsImage **t = (VipsImage **) vips_object_local_array( object, 2 );
 
 	VipsImage *in;
+	int height;
 
 	if( VIPS_OBJECT_CLASS( vips_reducev_parent_class )->build( object ) )
 		return( -1 );
@@ -845,9 +858,12 @@ vips_reducev_build( VipsObject *object )
 
 	/* Add new pixels around the input so we can interpolate at the edges.
 	 */
+	height = in->Ysize + reducev->n_point - 1;
+	if( reducev->centre )
+		height += 1;
 	if( vips_embed( in, &t[1], 
 		0, reducev->n_point / 2 - 1, 
-		in->Xsize, in->Ysize + reducev->n_point - 1, 
+		in->Xsize, height, 
 		"extend", VIPS_EXTEND_COPY,
 		NULL ) )
 		return( -1 );
@@ -893,6 +909,13 @@ vips_reducev_class_init( VipsReducevClass *reducev_class )
 		G_STRUCT_OFFSET( VipsReducev, kernel ),
 		VIPS_TYPE_KERNEL, VIPS_KERNEL_LANCZOS3 );
 
+	VIPS_ARG_BOOL( reducev_class, "centre", 7, 
+		_( "Centre" ), 
+		_( "Use centre sampling convention" ),
+		VIPS_ARGUMENT_OPTIONAL_INPUT,
+		G_STRUCT_OFFSET( VipsReducev, centre ),
+		FALSE );
+
 	/* Old name.
 	 */
 	VIPS_ARG_DOUBLE( reducev_class, "yshrink", 3, 
@@ -920,9 +943,13 @@ vips_reducev_init( VipsReducev *reducev )
  * Optional arguments:
  *
  * * @kernel: #VipsKernel to use to interpolate (default: lanczos3)
+ * * @centre: %gboolean use centre rather than corner sampling convention
  *
  * Reduce @in vertically by a float factor. The pixels in @out are
- * interpolated with a 1D mask. 
+ * interpolated with a 1D mask generated by @kernel.
+ *
+ * Set @centre to use centre rather than corner sampling convention. Centre
+ * convention can be useful to match the behaviour of other systems. 
  *
  * This is a very low-level operation: see vips_resize() for a more
  * convenient way to resize images. 

libvips/resample/resize.c

@@ -20,6 +20,8 @@
  * 	- faster and better upsizing
  * 15/8/16
  * 	- more accurate resizing
+ * 9/9/16
+ * 	- add @centre option
  */
 
 /*
@@ -78,6 +80,7 @@ typedef struct _VipsResize {
 	double scale;
 	double vscale;
 	VipsKernel kernel;
+	gboolean centre;
 
 	/* Deprecated.
 	 */
@@ -252,6 +255,7 @@ vips_resize_build( VipsObject *object )
 			vscale );
 		if( vips_reducev( in, &t[2], 1.0 / vscale, 
 			"kernel", resize->kernel, 
+			"centre", resize->centre, 
 			NULL ) )  
 			return( -1 );
 		in = t[2];
@@ -262,6 +266,7 @@ vips_resize_build( VipsObject *object )
 			hscale );
 		if( vips_reduceh( in, &t[3], 1.0 / hscale, 
 			"kernel", resize->kernel, 
+			"centre", resize->centre, 
 			NULL ) )  
 			return( -1 );
 		in = t[3];
@@ -354,6 +359,13 @@ vips_resize_class_init( VipsResizeClass *class )
 		G_STRUCT_OFFSET( VipsResize, kernel ),
 		VIPS_TYPE_KERNEL, VIPS_KERNEL_LANCZOS3 );
 
+	VIPS_ARG_BOOL( class, "centre", 7, 
+		_( "Centre" ), 
+		_( "Use centre sampling convention" ),
+		VIPS_ARGUMENT_OPTIONAL_INPUT,
+		G_STRUCT_OFFSET( VipsResize, centre ),
+		FALSE );
+
 	/* We used to let people set the input offset so you could pick centre
 	 * or corner interpolation, but it's not clear this was useful. 
 	 */
@@ -398,6 +410,7 @@ vips_resize_init( VipsResize *resize )
  *
  * * @vscale: %gdouble vertical scale factor
  * * @kernel: #VipsKernel to reduce with 
+ * * @centre: %gboolean use centre rather than corner sampling convention
  *
  * Resize an image. 
  *
@@ -410,6 +423,9 @@ vips_resize_init( VipsResize *resize )
  * vips_resize() normally uses #VIPS_KERNEL_LANCZOS3 for the final reduce, you
  * can change this with @kernel.
  *
+ * Set @centre to use centre rather than corner sampling convention. Centre
+ * convention can be useful to match the behaviour of other systems. 
+ *
  * When upsizing (@scale > 1), the operation uses vips_affine() with
  * a #VipsInterpolate selected depending on @kernel. It will use
  * #VipsInterpolateBicubic for #VIPS_KERNEL_CUBIC and above.

tools/vipsthumbnail.c

@@ -450,7 +450,7 @@ thumbnail_shrink( VipsObject *process, VipsImage *in )
 
 	shrink = calculate_shrink( in );
 
-	if( vips_resize( in, &t[4], 1.0 / shrink, NULL ) ) 
+	if( vips_resize( in, &t[4], 1.0 / shrink, "centre", TRUE, NULL ) ) 
 		return( NULL );
 	in = t[4];