void build_skeleton(const char* fname)
{
typedef typename Kernel::Point_2 Point_2;
typedef CGAL::Polygon_2<Kernel> Polygon_2;
typedef CGAL::Straight_skeleton_builder_traits_2<Kernel> SsBuilderTraits;
typedef CGAL::Straight_skeleton_2<Kernel> Ss;
typedef CGAL::Straight_skeleton_builder_2<SsBuilderTraits,Ss> SsBuilder;
Polygon_2 pgn;
std::ifstream input(fname);
FT x,y;
while(input)
{
input >> x;
if (!input) break;
input >> y;
if (!input) break;
pgn.push_back( Point_2( typename Kernel::FT(x), typename Kernel::FT(y) ) );
}
input.close();
std::cout << "Polygon has " << pgn.size() << " points\n";
if(!pgn.is_counterclockwise_oriented()) {
std::cerr << "Polygon is not CCW Oriented" << std::endl;
}
if(!pgn.is_simple()) {
std::cerr << "Polygon is not simple" << std::endl;
}
CGAL::Timer time;
time.start();
SsBuilder ssb;
ssb.enter_contour(pgn.vertices_begin(), pgn.vertices_end());
boost::shared_ptr<Ss> straight_ske = ssb.construct_skeleton();
time.stop();
std::cout << "Time spent to build skeleton " << time.time() << "\n";
if(!straight_ske->is_valid()) {
std::cerr << "Straight skeleton is not valid" << std::endl;
}
std::cerr.precision(60);
print_straight_skeleton(*straight_ske);
}
int main()
{
// A start-shaped polygon, oriented counter-clockwise as required for outer contours.
Point_2 pts[] = { Point_2(-1,-1)
, Point_2(0,-12)
, Point_2(1,-1)
, Point_2(12,0)
, Point_2(1,1)
, Point_2(0,12)
, Point_2(-1,1)
, Point_2(-12,0)
} ;
std::vector<Point_2> star(pts,pts+8);
// We want an offset contour in the outside.
// Since the package doesn't support that operation directly, we use the following trick:
// (1) Place the polygon as a hole of a big outer frame.
// (2) Construct the skeleton on the interior of that frame (with the polygon as a hole)
// (3) Construc the offset contours
// (4) Identify the offset contour that corresponds to the frame and remove it from the result
double offset = 3 ; // The offset distance
// First we need to determine the proper separation between the polygon and the frame.
// We use this helper function provided in the package.
boost::optional<double> margin = CGAL::compute_outer_frame_margin(star.begin(),star.end(),offset);
// Proceed only if the margin was computed (an extremely sharp corner might cause overflow)
if ( margin )
{
// Get the bbox of the polygon
CGAL::Bbox_2 bbox = CGAL::bbox_2(star.begin(),star.end());
// Compute the boundaries of the frame
double fxmin = bbox.xmin() - *margin ;
double fxmax = bbox.xmax() + *margin ;
double fymin = bbox.ymin() - *margin ;
double fymax = bbox.ymax() + *margin ;
// Create the rectangular frame
Point_2 frame[4]= { Point_2(fxmin,fymin)
, Point_2(fxmax,fymin)
, Point_2(fxmax,fymax)
, Point_2(fxmin,fymax)
} ;
// Instantiate the skeleton builder
SsBuilder ssb ;
// Enter the frame
ssb.enter_contour(frame,frame+4);
// Enter the polygon as a hole of the frame (NOTE: as it is a hole we insert it in the opposite orientation)
ssb.enter_contour(star.rbegin(),star.rend());
// Construct the skeleton
boost::shared_ptr<Ss> ss = ssb.construct_skeleton();
// Proceed only if the skeleton was correctly constructed.
if ( ss )
{
print_straight_skeleton(*ss);
// Instantiate the container of offset contours
ContourSequence offset_contours ;
// Instantiate the offset builder with the skeleton
OffsetBuilder ob(*ss);
// Obtain the offset contours
ob.construct_offset_contours(offset, std::back_inserter(offset_contours));
// Locate the offset contour that corresponds to the frame
// That must be the outmost offset contour, which in turn must be the one
// with the largetst unsigned area.
ContourSequence::iterator f = offset_contours.end();
double lLargestArea = 0.0 ;
for (ContourSequence::iterator i = offset_contours.begin(); i != offset_contours.end(); ++ i )
{
double lArea = CGAL_NTS abs( (*i)->area() ) ; //Take abs() as Polygon_2::area() is signed.
if ( lArea > lLargestArea )
{
f = i ;
lLargestArea = lArea ;
}
}
// Remove the offset contour that corresponds to the frame.
offset_contours.erase(f);
print_polygons(offset_contours);
}
}
return 0;
}
PwhPtr CstmCGAL::applyOffset(double offset, const Polygon_with_holes_2& poly) {
// This code is inspired from the CGAL example Straight_skeleton_2/Low_level_API
// As the offset can only produce an interior polygon, we need to produce a frame
// that encloses the polygon and is big enough so that the offset of the contour
// does not interfere with the one ot the polygon. See CGAL doc page for more info
boost::optional<double> margin = CGAL::compute_outer_frame_margin(
poly.outer_boundary().vertices_begin(),poly.outer_boundary().vertices_end(),offset);
if ( margin ) {
CGAL::Bbox_2 bbox = CGAL::bbox_2(poly.outer_boundary().vertices_begin(),poly.outer_boundary().vertices_end());
double fxmin = bbox.xmin() - *margin ;
double fxmax = bbox.xmax() + *margin ;
double fymin = bbox.ymin() - *margin ;
double fymax = bbox.ymax() + *margin ;
// Create the rectangular frame
Point_2 frame[4]= { Point_2(fxmin,fymin)
, Point_2(fxmax,fymin)
, Point_2(fxmax,fymax)
, Point_2(fxmin,fymax)
} ;
SsBuilder ssb ;
ssb.enter_contour(frame,frame+4);
// We have to revert the orientation of the polygon
std::vector<Point_2> outerBoundary = std::vector<Point_2>(
poly.outer_boundary().vertices_begin(),poly.outer_boundary().vertices_end());
ssb.enter_contour(outerBoundary.rbegin(), outerBoundary.rend());
SsPtr ss = ssb.construct_skeleton();
if ( ss ) {
std::vector<Polygon_2Ptr> offset_contours ;
OffsetBuilder ob(*ss);
ob.construct_offset_contours(offset, std::back_inserter(offset_contours));
// Locate the offset contour that corresponds to the frame
// That must be the outmost offset contour, which in turn must be the one
// with the largest unsigned area.
std::vector<Polygon_2Ptr>::iterator f = offset_contours.end();
double lLargestArea = 0.0 ;
for (std::vector<Polygon_2Ptr>::iterator i = offset_contours.begin(); i != offset_contours.end(); ++ i) {
double lArea = CGAL_NTS abs( (*i)->area() ) ; //Take abs() as Polygon_2::area() is signed.
if ( lArea > lLargestArea ) {
f = i ;
lLargestArea = lArea ;
}
}
offset_contours.erase(f);
// Construct result polygon
std::vector<Point_2> newOuterBoundary = std::vector<Point_2>(
offset_contours.front()->vertices_begin(), offset_contours.front()->vertices_end());
Polygon_with_holes_2 result = Polygon_with_holes_2(Polygon_2(newOuterBoundary.rbegin(), newOuterBoundary.rend()));
// We have to handle the holes separately
for (auto it = poly.holes_begin() ; it != poly.holes_end() ; it++) {
std::vector<Point_2> hole = std::vector<Point_2>(it->vertices_begin(),it->vertices_end());
std::vector<PwhPtr> holeOffsets =
CGAL::create_interior_skeleton_and_offset_polygons_with_holes_2(offset,
Polygon_with_holes_2(Polygon_2(hole.begin(), hole.end())));
for (auto it2 = holeOffsets.begin() ; it2 != holeOffsets.end() ; it++) {
std::vector<Point_2> revertNewHoles = std::vector<Point_2>(
(*it2)->outer_boundary().vertices_begin(),(*it2)->outer_boundary().vertices_end());
result.add_hole(Polygon_2(revertNewHoles.rbegin(), revertNewHoles.rend()));
}
}
return boost::make_shared<Polygon_with_holes_2>(result);
}
}
return NULL;
}