protocol: mention that buffers with alpha are assumed premultiplied

This turns out to be the default assumption by compositors, OpenGL, and the DRM
framework. Let's say it explicitly to avoid misinterpretation.

See https://lists.freedesktop.org/archives/wayland-devel/2021-September/041990.html.

Signed-off-by: Matt Hoosier <matt.hoosier@garmin.com>

diff --git a/protocol/wayland.xml b/protocol/wayland.xml
index 8c31cdc..029db19 100644 (file)
--- a/protocol/wayland.xml
+++ b/protocol/wayland.xml
@@ -436,6 +436,10 @@
       a height and can be attached to a wl_surface, but the mechanism by which a
       client provides and updates the contents is defined by the buffer factory
       interface.
+
+      If the buffer uses a format that has an alpha channel, the alpha channel
+      is assumed to be premultiplied in the color channels unless otherwise
+      specified.
     </description>
 
     <request name="destroy" type="destructor">