From 0dd6b095aa1366e20709e29b64da928299b63a6c Mon Sep 17 00:00:00 2001
From: John Cupitt <jcupitt@gmail.com>
Date: Tue, 3 Apr 2018 14:36:43 +0100
Subject: [PATCH] more conservative hasalpha

The result of hasalpha is used to turn on things like
premultiplication, so we should be rather conservative
about when we signal this. We don't want to premultiply
things that should not be premultiplied.

Check Type as well as bands.

See: https://github.com/jcupitt/libvips/issues/918
---
 ChangeLog               |  1 +
 libvips/iofuncs/image.c | 45 ++++++++++++++++++++++++++++++++++-------
 2 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index ff184274..83949abd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -10,6 +10,7 @@
 - svgload was missing is_a [lovell]
 - better header sniffing for small files
 - drop incompatible ICC profiles before save
+- better hasalpha rules
 
 12/3/18 started 8.6.4
 - better fitting of fonts with overhanging edges, thanks Adrià 
diff --git a/libvips/iofuncs/image.c b/libvips/iofuncs/image.c
index 411e0b15..9045449f 100644
--- a/libvips/iofuncs/image.c
+++ b/libvips/iofuncs/image.c
@@ -12,8 +12,10 @@
  * 	- vips_image_write() does not ref input for non-partial images
  * 29/10/16
  * 	- add vips_image_hasalpha()
- * 11/10/1
+ * 11/10/17
  * 	- more severing for vips_image_write()
+ * 3/4/18
+ * 	- better rules for hasalpha
  */
 
 /*
@@ -2866,18 +2868,47 @@ vips_image_ispartial( VipsImage *image )
  * vips_image_hasalpha: (method)
  * @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. 
+ * Look at an image's interpretation and see if it has extra alpha bands. For
+ * example, a 4-band #VIPS_INTERPRETATION_RGB would, but a six-band 
+ * #VIPS_INTERPRETATION_MULTIBAND would not. 
  *
  * 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 );
+	/* The result of hasalpha is used to turn on things like
+	 * premultiplication, so we are rather conservative about when we
+	 * signal this. We don't want to premultiply things that should not be
+	 * premultiplied.
+	 */
+	switch( image->Type ) { 
+	case VIPS_INTERPRETATION_B_W:
+	case VIPS_INTERPRETATION_GREY16:
+		return( image->Bands > 1 ); 
+
+	case VIPS_INTERPRETATION_RGB:
+	case VIPS_INTERPRETATION_CMC:
+	case VIPS_INTERPRETATION_LCH:
+	case VIPS_INTERPRETATION_LABS:
+	case VIPS_INTERPRETATION_sRGB:
+	case VIPS_INTERPRETATION_YXY:
+	case VIPS_INTERPRETATION_XYZ:
+	case VIPS_INTERPRETATION_LAB:
+	case VIPS_INTERPRETATION_RGB16:
+	case VIPS_INTERPRETATION_scRGB:
+	case VIPS_INTERPRETATION_HSV:
+		return( image->Bands > 3 ); 
+
+	case VIPS_INTERPRETATION_CMYK:
+		return( image->Bands > 4 ); 
+
+	default:
+		/* We can't really infer anything about bands from things like
+		 * HISTOGRAM or FOURIER.
+		 */
+		return( FALSE ); 
+	}
 }
 
 /**