supporting antialiased text in gdk

[Alexander Larsson <alla lysator liu se>]

The linux-fb port uses anitialiased text rendered by freetype, and since
other ports may start using antialiased text rendering in the future i
think we need to discuss some issues that has appeared due to this.

There are two ways to do antialiased rendering, both with their own
problems:
A) Render each pixel as gc_foreground*alpha + drawable_content*(1-alpha)
B) Render each pixel as gc_foreground*alpha + gc_background*(1-alpha)

(GtkFB currently uses A, but that might change)

Now, what are the problems with these to with respect to the
semantics of gdk/gtk? 

The problem with A is that rendering the same text multiple time at
the same place produces different results, since the output from the
previously drawn text is used in the AA calculation. This is easily
visible by i.e. scrolling a GtkCList horizontally. Some text is repainted
twice, producing very bad looking "fat" AA text.

The problem with B is that you must specify a correct background color for
every GC that is used to draw text. This is not currently done.

Which, if any, methods do we want to support in gtk? What does other
systems use (win32, beos)?

Supporting either method in gdk needs changes in gtk. Method A is clearly
more powerful, as you can draw AA text over any pixmap, but method B is
much more efficient to implement, since the AA calculations between fg and
bg can be done in advance, rendering only becomes a table lookup. Also,
most text in real apps is rendered over clean widget backgrounds.

/ Alex