On Jan 20, 2014, at 5:48 PM, Matt Woodrow <m...@mozilla.com> wrote: > 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 suspect Safari will drop the sub pixel AA in this case and we should to. > 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. I’d suggest we do this for now. OS X supports drawing subpixel-AA to partially transparent surfaces. I believe the amount of sub pixel AA depends on the alpha of the surface. (Imagine drawing sub-pixel aa text to a transparency group to get an idea of how this would work) On OS X we really only want to be disabling subpixel-AA when were drawing to surface that currently or will be not pixel aligned. -Jeff _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
