sort out premultiply rules for upsizing

vips_resize() uses vips_affine() for upsizing and vips_reduce() for
downsizing. Affine automaticaly does a vips_premultiply() for images
with an alpha channel, but reduce does not. This meant that we could
sometimes premultiply twice.

This patch adds a "premultiplied" flag for affine which turns automatic
premultiuplication off, vips_resize() uses this to block affine's auto
premul feature, and the resize docs are clarified to stress that the
operation does not do premultiplication for you.

See https://github.com/libvips/libvips/issues/1629

ChangeLog

@@ -20,6 +20,8 @@
 - better handling of unaligned reads in multipage tiffs [petoor]
 - mark old --delete option to vipsthumbnail as deprecated [UweOhse]
 - png save with a bad ICC profile just gives a warning
+- add "premultipled" option to vips_affine(), clarified vips_resize() 
+  behaviour with alpha channels
 
 24/4/20 started 8.9.3
 - better iiif tile naming [IllyaMoskvin]

libvips/resample/affine.c

@@ -88,6 +88,8 @@
  * 	- add "background" parameter
  * 	- better clipping means we have no jaggies on edges
  * 	- premultiply alpha 
+ * 18/5/20
+ * 	- add "premultiplied" flag
  */
 
 /*
@@ -166,6 +168,10 @@ typedef struct _VipsAffine {
 	 */
 	VipsPel *ink;
 
+	/* True if the input is already premultiplied (and we don't need to).
+	 */
+	gboolean premultiplied;
+
 } VipsAffine;
 
 typedef VipsResampleClass VipsAffineClass;
@@ -524,11 +530,13 @@ vips_affine_build( VipsObject *object )
 	affine->trn.idx -= 1;
 	affine->trn.idy -= 1;
 
-	/* If there's an alpha, we have to premultiply before resampling. See
+	/* If there's an alpha and we've not premultiplied, we have to 
+	 * premultiply before resampling. See 
 	 * https://github.com/libvips/libvips/issues/291
 	 */
 	have_premultiplied = FALSE;
-	if( vips_image_hasalpha( in ) ) { 
+	if( vips_image_hasalpha( in ) &&
+		!affine->premultiplied ) { 
 		if( vips_premultiply( in, &t[3], NULL ) ) 
 			return( -1 );
 		have_premultiplied = TRUE;
@@ -680,6 +688,13 @@ vips_affine_class_init( VipsAffineClass *class )
 		G_STRUCT_OFFSET( VipsAffine, background ),
 		VIPS_TYPE_ARRAY_DOUBLE );
 
+	VIPS_ARG_BOOL( class, "premultiplied", 117,
+		_( "Premultiplied" ),
+		_( "Images have premultiplied alpha" ),
+		VIPS_ARGUMENT_OPTIONAL_INPUT,
+		G_STRUCT_OFFSET( VipsAffine, premultiplied ),
+		FALSE );
+
 }
 
 static void
@@ -709,6 +724,7 @@ vips_affine_init( VipsAffine *affine )
  * * @ody: %gdouble, output vertical offset
  * * @extend: #VipsExtend how to generate new pixels 
  * * @background: #VipsArrayDouble colour for new pixels 
+ * * @premultiplied: %gboolean, images are already premultiplied
  *
  * This operator performs an affine transform on an image using @interpolate.
  *
@@ -737,6 +753,10 @@ vips_affine_init( VipsAffine *affine )
  *
  * @idx, @idy, @odx, @ody default to zero.
  *
+ * Image are normally treated as unpremultiplied, so this operation can be used
+ * directly on PNG images. If your images have been through vips_premultiply(),
+ * set @premultiplied. 
+ *
  * This operation does not change xres or yres. The image resolution needs to
  * be updated by the application. 
  *

libvips/resample/resize.c

@@ -289,6 +289,7 @@ vips_resize_build( VipsObject *object )
 				"idx", id, 
 				"idy", id, 
 				"extend", VIPS_EXTEND_COPY, 
+				"premultiplied", TRUE, 
 				NULL ) )  
 				return( -1 );
 			in = t[4];
@@ -300,6 +301,7 @@ vips_resize_build( VipsObject *object )
 				"idx", id, 
 				"idy", id, 
 				"extend", VIPS_EXTEND_COPY, 
+				"premultiplied", TRUE, 
 				NULL ) )  
 				return( -1 );
 			in = t[4];
@@ -311,6 +313,7 @@ vips_resize_build( VipsObject *object )
 				"idx", id, 
 				"idy", id, 
 				"extend", VIPS_EXTEND_COPY, 
+				"premultiplied", TRUE, 
 				NULL ) )  
 				return( -1 );
 			in = t[4];
@@ -444,7 +447,10 @@ vips_resize_init( VipsResize *resize )
  * This operation does not change xres or yres. The image resolution needs to
  * be updated by the application. 
  *
- * See also: vips_shrink(), vips_reduce().
+ * This operation does not premultiply alpha. If your image has an alpha
+ * channel, you should use vips_premultiply() on it first.
+ *
+ * See also: vips_premultiply(), vips_shrink(), vips_reduce().
  *
  * Returns: 0 on success, -1 on error
  */