Cursor: 8 -> 1 bit alpha downsampling improvement

Mouse cursors with 8 bit alpha were downsampled to 1-bit opacity maps by
turning alpha values of 255 into 1 and everything else into 0. This
means that mostly-opaque pixels ended up completely invisible.

This patch changes the behaviour so that only pixels with less than 50%
alpha (0-127) are treated as transparent when converted to 1-bit alpha.

This greatly improves the subjective appearance of anti-aliased mouse
cursors, such as those used by macOS, when using a front-end UI without
support for alpha-blended cursors, such as some VNC clients.

Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20240624101040.82726-1-phil@philjordan.eu>

diff --git a/ui/cursor.c b/ui/cursor.c
index 29717b3ecb7e1da51aa51a3b9b8cb1868944eb53..dd3853320d7659c36667bb49511f44aa6d850520 100644 (file)
--- a/ui/cursor.c
+++ b/ui/cursor.c
@@ -232,7 +232,7 @@ void cursor_get_mono_mask(QEMUCursor *c, int transparent, uint8_t *mask)
     for (y = 0; y < c->height; y++) {
         bit = 0x80;
         for (x = 0; x < c->width; x++, data++) {
-            if ((*data & 0xff000000) != 0xff000000) {
+            if ((*data & 0x80000000) == 0x0) { /* Alpha < 0x80 (128) */
                 if (transparent != 0) {
                     mask[x/8] |= bit;
                 }