Re: [Lib2geom-devel] lib2geom basics: define pathvector, define ellipse, define intersection

[mathog <ma...@ca...>]

On 04-Sep-2012 10:04, Krzysztof Kosiński wrote:
> 2012/9/4 mathog <ma...@ca...>:
>> Thanks for the reply.  The option of having to use livarot, which 
>> "nobody
>> understands" is not
>> terribly appealing, and 2geom has its own issues, notably a broken 
>> boolean
>> shape method.  Is
>> there some reason the inkscape project is creating 2geom instead of 
>> using
>> CGAL?  CGAL seems to do
>> the same things and is in a more advanced state of development.  For
>> instance, here is the documentation
>> for boolean operations:
>>
>> 
>> http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Boolean_set_operations_2/Chapter_main.html
>
> CGAL does not contain boolean operations on paths, only on linear
> polygons and polygons with circular segments.

It sure looks like it does what you say it doesn't.  See the "g"/"m" 
example about 3/4 of the way down the link
above. As near as I can tell the CGAL use of "general polygon" allows 
"traits" to be associated with each face, so that these
may be just a line, or something more complex, like a Bezier curve.

http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Arrangement_on_surface_2/Chapter_main.html#Subsection_32.6.7

So that it can do boolean operations on Bezier bounded shapes (general 
polygons in their parlance).  Quote from the first link, near the g/m 
intersection picture:

   Instantiating the arrangement-traits Arr_traits_2 above with the 
traits class that handle Bézier curves
   Arr_bezier_traits_2, results with the definition of a general-polygon 
set type that can be used to perform
   Boolean set-operations on point sets bounded by Bézier curves.

   The next example computes the intersection of two general polygons 
bounded by Bézier curves read from two
   input files respectively. The default input files our example uses 
(char_g.dat and char_m.dat) define two
   general polygons shaped in the form of the characters g and m in the 
Times New Roman font respectively. Their
   intersection comprises nine simple polygons, as depicted to the 
right.

Granted CGAL is pretty darn complex.  Also the documentation is in a 
technical jargon that I am not at all familiar with,
and where I constantly have the feeling that I am missing the point.  
For instance, they emphasize that the Bezier control points are rational 
numbers, but since floats are represented in the computer by a fixed 
number of bits, these values
could hardly be anything else.  (Well, excluding infinity and NAN.)

Regards,

David Mathog
ma...@ca...
Manager, Sequence Analysis Facility, Biology Division, Caltech