Hi,
Currently in gecko we have code to determine if text being drawn into a
transparent surface has opaque content underneath it. In the case where
it doesn't we ask moz2d/cairo [1] to disable subpixel AA text rendering
for this surface since it can have poor results depending on what the
eventual background color is.
However, this setting was only ever respected for the GDI font rendering
backend with cairo, and the other backends ignored it (unless they do
something internally?). Moz2D matched this until recently, until bug
941095 made the CoreGraphics backend start respecting the value as well.
The two extreme examples of where this might matter are as follows:
- A transparent surface with a very nearly opaque background color (the
OSX content menus uses rgba(255,255,255,0.95) iirc). In this case it's
probably ok to use subpixel AA since we're so close to opaque. The OSX
context menu's used to get subpixel AA text in this case since the value
was ignored, but no longer do (on nightly) since we respect the value.
- A fully transparent surface containing only text. It should be fairly
easy to get this to happen by 3d transforming a <text> element. I
suspect we don't want subpixel AA here, since we have no idea what the
final background color might be (and it could change every frame if the
transform is animated). CG was previously still giving us subpixel AA in
this case, but won't be any more.
I doubt we want to ship the text quality regression for OSX context
menus, so we need to find a solution to this. Preferably something that
is consistent across backends :)
Possible ideas:
- Revert the change to the Moz2D CG backend. This takes us back to
matching our previous behaviour, but not ideal to have Moz2D API
functions that are ignored on some backends.
- Keep the change, and instead #ifdef XP_OSX the callers inside gecko.
This keeps Moz2D cleaner, at the expense of gecko.
- Come up with some definition of 'close enough to opaque to get away
with subpixel AA' and have gecko compute this instead of looking for
completely opaque background. I have no idea what that definition would
be exactly.
- Others?
Does anyone have any suggestions for this, or knowledge about why only
GDI bothered with this previously?
Thanks!
- Matt
[1]
http://mxr.mozilla.org/mozilla-central/source/gfx/thebes/gfxASurface.h#136
or http://mxr.mozilla.org/mozilla-central/source/gfx/2d/2D.h#957
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