[skip ci] Try to debug John's reference implementation

kleisauke · kleisauke · commit 55e525a3364b · 2020-05-23T12:53:38.000+02:00
diff --git a/libvips/resample/shrinkh.c b/libvips/resample/shrinkh.c
@@ -237,6 +237,8 @@ vips_shrinkh_build( VipsObject *object )
 		vips_object_local_array( object, 2 );
 
 	VipsImage *in;
+	int width, extra_pixels;
+	double left;
 
 	if( VIPS_OBJECT_CLASS( vips_shrinkh_parent_class )->build( object ) )
 		return( -1 );
@@ -252,11 +254,31 @@ vips_shrinkh_build( VipsObject *object )
 	if( shrink->hshrink == 1 ) 
 		return( vips_image_write( in, resample->out ) );
 
+	/* Output size. We need to always round to nearest, so round(), not
+	 * rint().
+	 */
+	width = VIPS_ROUND_UINT(
+		(double) resample->in->Xsize / shrink->hshrink );
+
+	/* How many pixels we are inventing in the input, -ve for
+	 * discarding.
+	 */
+	extra_pixels =
+		width * shrink->hshrink - resample->in->Xsize;
+
+	/* If we are rounding down, we are not using some input
+	 * pixels. We need to move the origin *inside* the input image
+	 * by half that distance so that we discard pixels equally
+	 * from left and right. 
+	 */
+	left = (1 + extra_pixels) / 2.0;
+	printf( "hshrink: %d, left: %f\n", shrink->hshrink, left );
+
 	/* We need new pixels at the right so that we don't have small chunks
 	 * to average down the right edge.
 	 */
 	if( vips_embed( in, &t[1], 
-		0, 0, 
+		left, 0, 
 		in->Xsize + shrink->hshrink, in->Ysize,
 		"extend", VIPS_EXTEND_COPY,
 		NULL ) )
@@ -274,8 +296,7 @@ vips_shrinkh_build( VipsObject *object )
 	 * example, vipsthumbnail knows the true shrink factor (including the
 	 * fractional part), we just see the integer part here.
 	 */
-	resample->out->Xsize = VIPS_ROUND_UINT( 
-		(double) resample->in->Xsize / shrink->hshrink );
+	resample->out->Xsize = width;
 	if( resample->out->Xsize <= 0 ) { 
 		vips_error( class->nickname, 
 			"%s", _( "image has shrunk to nothing" ) );
diff --git a/libvips/resample/shrinkv.c b/libvips/resample/shrinkv.c
@@ -332,6 +332,8 @@ vips_shrinkv_build( VipsObject *object )
 		vips_object_local_array( object, 4 );
 
 	VipsImage *in;
+	int height, extra_pixels;
+	double top;
 
 	if( VIPS_OBJECT_CLASS( vips_shrinkv_parent_class )->build( object ) )
 		return( -1 );
@@ -347,11 +349,31 @@ vips_shrinkv_build( VipsObject *object )
 	if( shrink->vshrink == 1 )
 		return( vips_image_write( in, resample->out ) );
 
+	/* Output size. We need to always round to nearest, so round(), not
+	 * rint().
+	 */
+	height = VIPS_ROUND_UINT(
+		(double) resample->in->Ysize / shrink->vshrink );
+
+	/* How many pixels we are inventing in the input, -ve for
+	 * discarding.
+	 */
+	extra_pixels =
+		height * shrink->vshrink - resample->in->Ysize;
+
+	/* If we are rounding down, we are not using some input
+	 * pixels. We need to move the origin *inside* the input image
+	 * by half that distance so that we discard pixels equally
+	 * from left and right. 
+	 */
+	top = (1 + extra_pixels) / 2.0;
+	printf( "vshrink: %d, top: %f\n", shrink->vshrink, top );
+
 	/* Make the height a multiple of the shrink factor so we don't need to
 	 * average half pixels.
 	 */
 	if( vips_embed( in, &t[1], 
-		0, 0, 
+		0, top, 
 		in->Xsize, VIPS_ROUND_UP( in->Ysize, shrink->vshrink ), 
 		"extend", VIPS_EXTEND_COPY,
 		NULL ) )
