CGAL users discussion list

[Shriramana Sharma <>]

Hello. I'm new to CGAL. I'm thinking of using CGAL for doing boolops
on glyph outlines which are polygons outlined by Bezier curves.
Thankfully, there is a help section directly devoted to this:
http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Boolean_set_operations_2/Chapter_main.html#Subsection_19.4.3

However I'm not an expert programmer and this generic programming
stuff is somewhat intimidating. Basically I think I'll be writing some
simple wrapper to the exact CGAL functionality I require in my
project. As this is my initial post (regarding actual programming) I
have quite a number of questions. My sincere thanks in advance for
your kindness and patience in reading and replying!

My questions:

1. HANDLING OF SELF-INTERSECTIONS

a. I'd like some clarification whether the boolops code works on
self-intersecting polygons or not. The post:
http://cgal-discuss.949826.n4.nabble.com/self-intersecting-beziers-and-arrangement-algorithms-td1576662.html
seems to indicate that they are handled properly. Therein, EfiF says:
"I have a feeling that self intersections are handled correctly after
all as opposed to the statement in the manual. I'll try to
investigate." but I do not see any update in that thread after that,
and the manual still says that the polygon being simple is a condition
for the boolop code to work.

b. Just a clarification of the above: if the answer to the above is
yes, then it means that self-intersecting polygons are simplified by
the boolop algorithm and the output will no longer contain
self-intersections, right?

c. This would further mean that if I perform a union on a single
self-intersecting polygon it would produce a simple or relatively
simple polygon that is graphically equal to the input polygon, right?

2.

Compiling CGAL-4.1/examples/Boolean_set_operations_2/bezier_traits_adapter.cpp
which is the example of boolops on font glyphs takes 24 seconds on my
Lenovo Z570 laptop (i5-2410M @ 2.3 GHz running with hyperthreading on,
3 GB RAM, 640 GB HDD) running Kubuntu Precise 64-bit with 1 GB RAM
free.

I suspect most of the time is taken up by the numerous template
instantiations in CGAL. Is it possible for me to do some pre-compiled
headers trick or something to reduce this compile time?

3.

I was wondering about the content of char_g.dat related to the boolops
example: especially whether those curves are really degree 6 -- and I
wrote the attached (GPLed) Python program (using PyQt) to help me
visualize the glyph formed by the data. (I removed the /100000000 from
char_g.dat for my programming convenience.) Surely the original font
glyph doesn't contain 6th degree curves -- is this then just to
demonstrate that such higher-degree curves are also handled?

4.

I tried to use the pretty print utilities with the glyph intersection
example, and just added (apart from the #include "print_utils.h") the
following lines before the "Intersection took x seconds" line:

  int i = 1 ;
  for (rit = R.begin(); rit != R.end(); ++rit)
  {
    std::cout << "Polygon #" << i << ":" << std::endl;
    print_polygon_with_holes ( *rit ) ;
    ++i;
  }

but I'm getting an error:

/home/samjnaa/bn/cpp-cgal/boolean-test.cpp: In function ‘int main(int, 
char**)’:
/home/samjnaa/bn/cpp-cgal/boolean-test.cpp:168:37: error: no matching
function for call to ‘print_polygon_with_holes(const
CGAL::General_polygon_with_holes_2<CGAL::General_polygon_2<CGAL::Arr_Bezier_curve_traits_2<CGAL::Cartesian<CORE::BigRat>,
CGAL::Cartesian<CORE::Expr>, CGAL::CORE_algebraic_number_traits> >
>&)’
/home/samjnaa/bn/cpp-cgal/boolean-test.cpp:168:37: note: candidate is:
/home/samjnaa/bn/cpp-cgal/print_utils.h:27:6: note: template<class
Kernel, class Container> void print_polygon_with_holes(const
CGAL::Polygon_with_holes_2<Kernel_, Container_>&)
make[2]: *** [CMakeFiles/boolean-test.dir/boolean-test.cpp.o] Error 1
make[1]: *** [CMakeFiles/boolean-test.dir/all] Error 2
make: *** [all] Error 2

I'm not sure what I'm doing wrong. I'm using GCC 4.6.3 which comes
with my Kubuntu Precise 64-bit OS. Can you please help me solve the
above problem?

Thanking you very much once again for your kindness and patience,

-- 
Shriramana Sharma

Attachment: n-degree-bezier-display.zip
Description: Zip archive