fix alpha shift during colourspace conversion (#4302)

jcupitt · web-flow · commit 3ba36cedcbec · 2024-12-04T14:44:08.000Z
* fix alpha shift during colourspace conversion The colour functions were not shifting alpha channels, just casting them, so operations which changed depth, like `icc_transform --depth 8` on a 16-bit image, could have an incorrectly scaled alpha. See #4301 Thanks frederikrosenberg * credit in changelog
diff --git a/ChangeLog b/ChangeLog
@@ -9,6 +9,7 @@
 - fix SZI write with openslide4 [goran-hc]
 - heifsave: prevent use of AV1 intra block copy feature [lovell]
 - threadpool: improve cooperative downsizing [kleisauke]
+- fix alpha shift during colourspace conversions [frederikrosenberg]
 
 10/10/24 8.16.0
 
diff --git a/libvips/colour/colour.c b/libvips/colour/colour.c
@@ -371,6 +371,7 @@ vips_colour_build(VipsObject *object)
 			 */
 
 			if (vips_cast(extra_bands[i], &t1, out->BandFmt,
+					"shift", TRUE,
 					NULL)) {
 				g_object_unref(out);
 				return -1;
