add vips_interpolate_get_window_offset()

ChangeLog

@@ -35,6 +35,7 @@
 - im_max()/im_min() are now convenience functions
 - im_maxpos_avg() handles complex and multi-band images
 - added im_point(), rewrite im_point_bilinear() in terms of this
+- added vips_interpolate_get_window_offset()
 
 25/3/09 started 7.18.0
 - revised version numbers

libvips/include/vips/interpolate.h

@@ -78,6 +78,11 @@ typedef struct _VipsInterpolateClass {
 	/* Or just set this if you want a constant.
 	 */
 	int window_size;
+
+	/* Stencils are offset by this much. Default to window_size / 2 
+	 * (centering) if undefined. 
+	 */
+	int (*get_window_offset)( VipsInterpolate * );
 } VipsInterpolateClass;
 
 GType vips_interpolate_get_type( void );
@@ -85,6 +90,7 @@ void vips_interpolate( VipsInterpolate *interpolate,
 	PEL *out, REGION *in, double x, double y );
 VipsInterpolateMethod vips_interpolate_get_method( VipsInterpolate * );
 int vips_interpolate_get_window_size( VipsInterpolate *interpolate );
+int vips_interpolate_get_window_offset( VipsInterpolate *interpolate );
 
 /* How many bits of precision we keep for transformations, ie. how many
  * pre-computed matricies we have.

libvips/resample/im_affine.c

@@ -186,9 +186,8 @@ affinei_gen( REGION *or, void *seq, void *a, void *b )
 	REGION *ir = (REGION *) seq;
 	const IMAGE *in = (IMAGE *) a;
 	const Affine *affine = (Affine *) b;
-	const int window_size = 
-		vips_interpolate_get_window_size( affine->interpolate );
-	const int half_window_size = window_size / 2;
+	const int window_offset = 
+		vips_interpolate_get_window_offset( affine->interpolate );
 	const VipsInterpolateMethod interpolate = 
 		vips_interpolate_get_method( affine->interpolate );
 
@@ -233,7 +232,7 @@ affinei_gen( REGION *or, void *seq, void *a, void *b )
 
 	/* Add a border for interpolation. Plus one for rounding errors.
 	 */
-	im_rect_marginadjust( &need, half_window_size + 1 );
+	im_rect_marginadjust( &need, window_offset + 1 );
 
 	/* Clip against the size of (2).
 	 */
@@ -401,22 +400,25 @@ im__affinei( IMAGE *in, IMAGE *out,
 	VipsInterpolate *interpolate, Transformation *trn )
 {
 	IMAGE *t3 = im_open_local( out, "im_affine:3", "p" );
-	const int window_size = vips_interpolate_get_window_size( interpolate );
+	const int window_size = 
+		vips_interpolate_get_window_size( interpolate );
+	const int window_offset = 
+		vips_interpolate_get_window_offset( interpolate );
 	Transformation trn2;
 
 	/* Add new pixels around the input so we can interpolate at the edges.
 	 */
 	if( !t3 ||
 		im_embed( in, t3, 1, 
-			window_size / 2, window_size / 2, 
+			window_offset, window_offset, 
 			in->Xsize + window_size, in->Ysize + window_size ) )
 		return( -1 );
 
 	/* Set iarea so we know what part of the input we can take.
 	 */
 	trn2 = *trn;
-	trn2.iarea.left += window_size / 2;
-	trn2.iarea.top += window_size / 2;
+	trn2.iarea.left += window_offset;
+	trn2.iarea.top += window_offset;
 
 #ifdef DEBUG_GEOMETRY
 	printf( "im__affinei: %s\n", in->filename );

libvips/resample/interpolate.c

@@ -94,6 +94,12 @@ vips_interpolate_real_get_window_size( VipsInterpolate *interpolate )
 	return( class->window_size );
 }
 
+static int
+vips_interpolate_real_get_window_offset( VipsInterpolate *interpolate )
+{
+	return( vips_interpolate_get_window_size( interpolate ) / 2 );
+}
+
 static void
 vips_interpolate_class_init( VipsInterpolateClass *class )
 {
@@ -106,6 +112,7 @@ vips_interpolate_class_init( VipsInterpolateClass *class )
 #endif /*DEBUG*/
 	class->interpolate = NULL;
 	class->get_window_size = vips_interpolate_real_get_window_size;
+	class->get_window_size = vips_interpolate_real_get_window_offset;
 	class->window_size = -1;
 }
 
@@ -152,9 +159,22 @@ vips_interpolate_get_window_size( VipsInterpolate *interpolate )
 	VipsInterpolateClass *class = VIPS_INTERPOLATE_GET_CLASS( interpolate );
 
 	g_assert( class->get_window_size );
+
 	return( class->get_window_size( interpolate ) );
 }
 
+/* Get this interpolator's required window offset.
+ */
+int
+vips_interpolate_get_window_offset( VipsInterpolate *interpolate )
+{
+	VipsInterpolateClass *class = VIPS_INTERPOLATE_GET_CLASS( interpolate );
+
+	g_assert( class->get_window_offset );
+
+	return( class->get_window_offset( interpolate ) );
+}
+
 /* VipsInterpolateNearest class
  */