[cairo] Questions and optimizations in cairo-arc

[mental at rydia.net]

Quoting Jason Dorje Short <jdorje at users.sf.net>:

> > See the PostScript Language Reference Manual for a more
> > detailed explanation.
>
> That is very unintuitive since in most cases neither the current
> nor final (x2,y2) point will be on the arc (and of course the
> tangent point (x1,y1) won't be on the arc except in degenerate
> cases).  This at least needs some heavy commenting.

> And couldn't it be generalized as an arc of an oval?  Three
> points define two tangent vectors which define an oval, although
> allowing four points makes it easier to define the two tangent
> vectors.

Another (although not necessarily superior) approach to compare is
SVG's:

http://www.w3.org/TR/SVG/paths.html#PathDataEllipticalArcCommands

-mental