add vips_image_hasalpha()

ChangeLog

@@ -3,6 +3,7 @@
 - added tiff save to buffer
 - added dzsave save to buffer (zip only)
 - revise header get/set functions
+- added vips_image_hasalpha()
 
 18/10/16 started 8.4.3
 - fix error detection in gif_close, thanks aaron42net

libvips/include/vips/image.h

@@ -473,6 +473,7 @@ int vips_image_encode( VipsImage *in, VipsImage **out, VipsCoding coding );
 gboolean vips_image_isMSBfirst( VipsImage *image );
 gboolean vips_image_isfile( VipsImage *image );
 gboolean vips_image_ispartial( VipsImage *image );
+gboolean vips_image_hasalpha( VipsImage *image );
 
 VipsImage *vips_image_copy_memory( VipsImage *image );
 int vips_image_wio_input( VipsImage *image );

libvips/iofuncs/image.c

@@ -10,6 +10,8 @@
  * 	- add vips_image_new_from_memory_copy()
  * 10/6/16
  * 	- vips_image_write() does not ref input for non-partial images
+ * 29/10/16
+ * 	- add vips_image_hasalpha()
  */
 
 /*
@@ -2775,6 +2777,24 @@ vips_image_ispartial( VipsImage *image )
 		return( 0 );
 }
 
+/**
+ * vips_image_hasalpha:
+ * @image: image to check
+ *
+ * libvips assumes an image has an alpha if it has two bands (ie. it is a
+ * monochrome image with an extra band), if it has four bands (unless it's been
+ * tagged as CMYK), or if it has more than four bands. 
+ *
+ * Return %TRUE if @image has an alpha channel.
+ */
+gboolean
+vips_image_hasalpha( VipsImage *image )
+{
+	return( image->Bands == 2 ||
+		(image->Bands == 4 && image->Type != VIPS_INTERPRETATION_CMYK) ||
+		image->Bands > 4 );
+}
+
 /**
  * vips_image_write_prepare:
  * @image: image to prepare

tools/vipsthumbnail.c

@@ -432,9 +432,7 @@ thumbnail_shrink( VipsObject *process, VipsImage *in )
 	 * https://github.com/jcupitt/libvips/issues/291
 	 */
 	have_premultiplied = FALSE;
-	if( in->Bands == 2 ||
+	if( vips_image_hasalpha( in ) ) { 
-		(in->Bands == 4 && in->Type != VIPS_INTERPRETATION_CMYK) ||
-		in->Bands == 5 ) {
 		vips_info( "vipsthumbnail", "premultiplying alpha" ); 
 		if( vips_premultiply( in, &t[3], NULL ) ) 
 			return( NULL );