gEDA-dev: Gschem and Cairo graphics library

[Tomaz Solc]

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Hi everyone

I've been experimenting with the Cairo graphics library.

Cairo is a free 2D graphics library that provides drawing operations
similar to PostScript. It can draw beautifully anti-aliased output to
the screen or to a PostScript or SVG file. It is already integrated into
GTK+ 2.8 and in the future it will also support hardware acceleration
(See http://cairographics.org)

I've made a proof-of-concept patch that uses Cairo instead of plain GDK
to draw solid line objects to the screen. Since solid line seems to be
the most-used object in Gschem (it is used to draw text for example) I
think this is a pretty good demonstration of Cairo's capabilities. You
can find the patch and screenshots at the following address:

http://www.tablix.org/~avian/gschem-cairo

Some notes about the patch:

o Antialiasing helps to make schematics and labels readable at smaller
zoom factors (see screenshots). It also creates some new problems: when
antialiased objects are deleted from the screen (for example: a green
line is overwritten by a black line), a dim outline remains.

o On my computer (Athlon XP 2500+) GSchem becomes too slow to be useful
with this patch. Possible optimizations: Don't create a new Cairo
context when drawing each line (trivial, doesn't help much). Use
optimized Cairo text renderer instead of custom font made with line
segments (complicated).

o Less code duplication: with Cairo it would be possible to use a single
drawing function for PostScript and screen output. It is fairly simple
to convert code that uses GDK to Cairo.

o Currently, all screen coordinates are stored as integers. I had to
modify libgeda to get coordinates as floating point numbers. If Cairo
would be used throughout GSchem, I believe some non-trivial
modifications to libgeda would have to be made.

o Breaks compatibility with older GTK versions.

Best regards
Tomaz Solc
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