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McEs, A Hacker Life
Tuesday, November 25, 2008
 Text-on-path with cairo

Cairotwisted is the name a piece of code I wrote using pangocairo to lay text on a path. Over the years many people have found it useful, so here is a public post to make sure it's easier to find on the interwebs.

cairotwisted
The code is shipped in Pango tarball under examples/cairotwisted.c and can be browsed here. Click on the thumbnail to see the full size image output.

At the core of it there is a function to map one path onto another one. This is done by parameterizing the second path and computing it's gradient (see the code).

Parameterizing a Bezier curve uniformly is not easy though. I currently flatten curves to lines and use that, but that has its own down sides as the error introduced in the gradient can magnify out of bound. Any ideas?

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Comments:
I bow to your awesomeness
 
Ok, now your done with that, where is our UCS4-based API?

Just kidding. Awesome work on Pango. This AbiWord developer bows for you. - uwog
 
hells to the hell yes!
 
I've been a developer on lib2geom (geometry backend for inkscape) for a couple years. One of our main focuses is _not_ doing that conversion to line segments. In order to do this we introduce a path representation known as s-power-basis, which is closed under complex algebraic and calculus operations.

Here's basically how we do path-along-path:

PwD2 uskeleton = arc_length_parametrization(skeleton,2,.1)
PwD2 n = force_continuity(rot90(derivative(uskeleton));
PwD2 output = compose(uskeleton, path[X]) + path[Y] * compose(n, path[X]);

(PwD2 is actually a template type I can't write without blogger's html tag stuff getting pissy)

And that's it. Converting sbasis to / from beziers is very efficient - we do all our rendering with cairo.
 
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