PolyTree documentation

[JohanDelvaux]

I would like to start with saying thank you for the great library.

I find the polytree structure in clipper2 less transparant in how to use it compared to the original clipperlib. Could two clear examples be added to the documentation? (For example one example a polygon with two holes in it, and another two seperate polygons, one with a hole) Pictures of how it looks internally + code how to iterate over the polys would help.

I am sorry if this is not the right place to ask this question.

[AngusJohnson]

Hi Johan. That's an excellent suggestion. I'll do that (hopefully soonish 😜).

[AngusJohnson]

#include "clipper2/clipper.h"
#include "../../Utils/clipper.svg.utils.h"

using namespace Clipper2Lib;

void PolytreeSample()
{
  //////////////////////////////////////////////
  // make a "subject" polygon with several holes
  // and nested polygons
  //////////////////////////////////////////////
  Paths64 subject;
  // add the outer polygon contour
  subject.push_back(MakePath("0,0 100,0, 100,100, 0,100"));
  // add several holes (note reversed orientation)
  subject.push_back(MakePath("10,10 10,30, 30,30, 30,10"));
  subject.push_back(MakePath("60,10 60,30, 80,30, 80,10"));
  subject.push_back(MakePath("10,60 10,80, 30,80, 30,60"));
  subject.push_back(MakePath("50,50 50,90, 90,90, 90,50"));
  // add a nested outer polygon (inside one of the holes)
  subject.push_back(MakePath("55,55 85,55, 85,75, 55,75"));
  subject.push_back(MakePath("55,80 85,80, 85,85, 55,85"));
  // and 2 deeply nested holes
  subject.push_back(MakePath("60,60, 60,70, 68,70, 68,60"));
  subject.push_back(MakePath("72,60, 72,70, 80,70, 80,60"));

  // draw "subject"
  SvgWriter svg;
  SvgAddSubject(svg, subject, FillRule::NonZero);
  svg.SaveToFile("polytree_sample.svg", 200, 200, 0);
  System("polytree_sample.svg");

  // union "subject" with the result returned in a polytree
  PolyTree64 polytree;
  Clipper64 c64;
  c64.AddSubject(subject);
  c64.Execute(ClipType::Union, FillRule::NonZero, polytree);

  // display polytree's structure
  // (requires latest Beta) 
  std::cout << "Polytree 1:" << std::endl;
  std::cout << polytree << std::endl;

  //and examples of non-recursive ways to access polytree nodes
  //const PolyPath64& first_polygon = *polytree[0];
  //const PolyPath64& first_hole_in_first_polygon = *first_polygon[0];
  //const PolyPath64& a_nested_polygon = *first_hole_in_first_polygon[0];

  /////////////////////////////////////////
  // make another "subject" with 4 polygons 
  /////////////////////////////////////////
  Paths64 subject2;
  // add several outer polygon contours
  subject2.push_back(MakePath("10,10 40,10, 40,40, 10,40"));
  subject2.push_back(MakePath("60,10 90,10, 90,40, 60,40"));
  subject2.push_back(MakePath("10,60 40,60, 40,90, 10,90"));
  subject2.push_back(MakePath("60,60 90,60, 90,90, 60,90"));

  // draw the second "subject"
  SvgWriter svg2;
  SvgAddSubject(svg2, subject2, FillRule::NonZero);
  svg2.SaveToFile("polytree_sample2.svg", 200, 200, 0);
  System("polytree_sample2.svg");

  // union "subject2" with the result in another polytree
  PolyTree64 polytree2;
  Clipper64 c64_2;
  c64_2.AddSubject(subject2);
  c64_2.Execute(ClipType::Union, FillRule::NonZero, polytree2);

  // display polytree's structure
  // (requires latest Beta) 
  std::cout << "Polytree 2:" << std::endl;
  std::cout << polytree2 << std::endl;
}

Polytree 1:
   +- Polygon with 4 nested holes.
      +- Hole with 2 nested polygons.
      |  +- Polygon
      |  +- Polygon with 2 nested holes.
      |     +- Hole
      |     +- Hole
      +- Hole
      +- Hole
      +- Hole

Polytree 2:
   +- Polygon
   +- Polygon
   +- Polygon
   +- Polygon

[JohanDelvaux]

Thank you!
I was actually looking for a C# example : D and how to iterate over the children of a polytree, an the children of the children (how to consume the result). This seems to have changed compared to original clipperlib library and I didnt directly understand how to do it. I will just try it out at a certain point. Thanks for the reaction!

[AngusJohnson]

    private static void ShowPolyPathStructure(PolyPath64 pp, int level)
    {
      string spaces = new string(' ', level * 2);
      string caption = (pp.IsHole ? "Hole " : "Outer ");
      if (pp.Count == 0)
      {
        Console.WriteLine(spaces + caption);
      }
      else
      {
        Console.WriteLine(spaces + caption +string.Format("({0})", pp.Count));
        foreach(PolyPath64 child in pp) { ShowPolyPathStructure(child, level + 1); }
      }
    }

    public static void ShowPolyTreeStructure(PolyTree64 polytree)
    {
      Console.WriteLine("Polytree Root");
      foreach (PolyPath64 child in polytree) { ShowPolyPathStructure(child, 1); }
    }

[JohanDelvaux]

Fantastic! Thanks! Keep up the great work!

[rs0xFFFF]

Hi Angus. I think, it would be helpful if these examples and demos were also archived in github and continuously maintained and expanded.. :-)