Clarify alpha-map loop prevention (#23581)

Signed-off-by: Adam Jackson <ajax@redhat.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/renderproto.txt b/renderproto.txt
index a736ba7..70bf260 100644
--- a/renderproto.txt
+++ b/renderproto.txt
@@ -615,7 +615,10 @@ CreatePicture
 	Rendering is additionally clipped by the geometry and clip mask of
 	alpha-map.  Exposures to the window do not affect the contents of
 	alpha-map.  Alpha-map must refer to a picture containing a Pixmap,
-	not a Window (or a Match error results).
+	not a Window (or a Match error results).  If the alpha-map picture
+	itself already has an alpha-map, that alpha-map is used instead.  If
+	the alpha-map picture points to the same drawable as the picture
+	itself, None is used instead.
 
 	The clip-mask restricts reads and writes to drawable.  Only pixels
 	where the clip-mask has bits set to 1 are read or written.  Pixels