Re: [Lib2geom-devel] [Inkscape-devel] Boolops progress

[Nathan H. <nj...@nj...>]

On Mon, Apr 20, 2015 at 02:18:36PM +0200, Krzysztof Kosiński wrote:
> 2015-04-20 7:15 GMT+02:00 Nathan Hurst <nj...@nj...>:
> >> - Linear and Bezier segments of arbitrary degree
> >> - Code is fairly clean and understandable
> >> - Reasonably good performance, even though there are very few
> >> optimizations
> >
> > This is almost certainly an improvement on what we have, to gain
> > confidence can you also write some unit tests that capture the various
> > edge cases as per that svg (and anything else you can think of).
> 
> The results will be slightly different than in that SVG, because the
> algorithm I used does not require uncrossing as a preprocessing step.
> I think this is a nice feature, since it preserves self-intersections
> in places where they don't affect the result. I wrote a short summary
> of the underlying algorithm on Wikipedia recently:
> https://en.wikipedia.org/wiki/Greiner%E2%80%93Hormann_clipping_algorithm

Nice!

> For the arc-Bezier case, my thesis advisor suggested to convert
> elliptical arcs to four rational quadratic Beziers, each representing
> 1/4 of the ellipse between its conjugate diameters and intersect that
> using well known algorithms, then go back to the elliptical arc
> representation by using the fact that the weight of the middle control
> point is related to the time on the elliptical arc (the angle in the
> untransformed unit circle). I'm not sure about this. I'll first try
> the conic equation route, since it's the simplest and requires minimal
> new code.

Yeah, I think that is wise.  I'd forgotten about rational beziers
representing conics.

> The conic code implements some interesting stuff that I could use, but
> it needs to be refactored first. I'm pretty certain that If I hadn't
> spent substantial time on refactoring, writing unit tests for existing
> stuff and fixing the uncovered bugs, I wouldn't have managed to get
> this far.

Yes, this work has desperately need to be done.  I didn't grok the
value of unit tests when I wrote the initial code.  Now I do.  Please
refactor and unit test, it will make the library so much more
trustworthy.

njh