[PATCH RFC xserver] dix: Work around non-premultiplied ARGB cursor data

[Michel Dänzer]

From: Michel Dänzer <michel.daenzer at amd.com>

Some games incorrectly use non-premultiplied ARGB cursor data, presumably
because that's what Windows uses. On some hardware (and with SWcursor),
this breaks areas of the cursor which are supposed to be transparent
(and presumably also translucent areas, but that's less noticeable).

This change checks for pixels with alpha == 0 and any non-alpha component
!= 0. If any such pixel is found, the data is assumed to be
non-premultiplied and fixed up by multiplying the RGB components with the
alpha component.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92309
Signed-off-by: Michel Dänzer <michel.daenzer at amd.com>
---

I'm on the fence about whether this is a good or bad idea.

Pro:
* Allows users with affected setups to play broken games
* Less overhead than a corresponding workaround in the driver

Con:
* Makes the problem completely invisible to game developers, so once
  it's in we can probably never remove it again

Opinions?

 dix/cursor.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/dix/cursor.c b/dix/cursor.c
index e459456..25d6767 100644
--- a/dix/cursor.c
+++ b/dix/cursor.c
@@ -288,6 +288,29 @@ AllocARGBCursor(unsigned char *psrcbits, unsigned char *pmaskbits,
         goto error;
 
     *ppCurs = pCurs;
+
+    if (argb) {
+        size_t i, size = bits->width * bits->height;
+
+        for (i = 0; i < size; i++) {
+            if ((argb[i] & 0xff000000) == 0 && (argb[i] & 0xffffff) != 0) {
+                /* ARGB data doesn't seem pre-multiplied, fix it */
+                for (i = 0; i < size; i++) {
+                    CARD32 a, ar, ag, ab;
+
+                    a = argb[i] >> 24;
+                    ar = a * ((argb[i] >> 16) & 0xff) / 0xff;
+                    ag = a * ((argb[i] >> 8) & 0xff) / 0xff;
+                    ab = a * (argb[i] & 0xff) / 0xff;
+
+                    argb[i] = a << 24 | ar << 16 | ag << 8 | ab;
+                }
+
+                break;
+            }
+        }
+    }
+
     return Success;
 
  error:
-- 
2.8.1