[openbeos] Re: GSoC: Sub-pixel antialiasing for font rendering
- From: Stephan Assmus <superstippi@xxxxxx>
- To: openbeos@xxxxxxxxxxxxx
- Date: Thu, 03 Apr 2008 17:38:11 +0200
Hi Andrej,
Andrej Spielmann wrote:
> Hello everybody,
>
> I would like to submit a proposal for the sub-pixel antialiasing project
> for GSoC. I know the basics of how sub-pixel antialiasing works and I
> think that the most important part of this project would actually be to
> get a complex understanding of how text-rendering in Haiku works. Could
> you please suggest any materials that would help me familiarize myself
> with the structure of Haiku's app_server and font rendering. I would like
> to get a more precise idea of what needs to be done before writing the
> application.
This is a nice project, thanks for considering it! :-)
All the code that is involved is at src/servers/app/drawing/Painter/. Some
AGG code might need to be adjusted at src/libs/agg and headers/libs/agg.
(The rasterizer clipping could interfere when trying to generate scanlines
with triple X resolution.)
Two things happen for text rendering: The text either is or is not
transformed (not scaled or rotated). For the untransformed case, a cache of
bitmaps for each glyph is used, generated by Freetype, otherwise a cache of
vector glyphs is used.
In both cases, the AGG backend is used to generate so called "scanlines of
coverage values". These scanlines are passed to the different drawing mode
implementations (see src/servers/app/drawing/Painter/drawing_modes) and
contain one coverage value on [0..255] per pixel. The DrawingModes apply a
drawing mode (copy, over, add, blend, alpha, etc) between a high color and
a low color. The coverage values define a smooth transition of the result.
The cleanest way to implement sub-pixel anti-aliasing would be to support
it for any and all vector drawing at once, not only text rendering. Then
you would have to change every DrawingMode implementation to interprete the
coverage values as triples for individual red, green and blue subpixels. Or
better yet, write a complete alternative set of DrawingModes, so the
feature could be turned on/off.
The last thing to do would be to change the code path which generates the
glyph bitmaps directly to use the Freetype subpixel version. And whereever
Painter uses the AGG rasterizer, it would need to triple the X resolution.
I know that there is some filtering involved to avoid strong color
fringing. You need to define where this happens. I assume Freetype already
does it, but for the normal vector pipelines, you would need to do it
yourself "somewhere".
Also see my old article on Painter and AGG here:
<http://www.haiku-os.org/documents/dev/painter_and_how_agg_works>
If you have any further questions, please don't hesitate to post them here!
Best regards,
-Stephan
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