如何给使升压无限边缘到边缘有限？

Question

我发现，在升压库，Voronoi图，一些边缘数据是无限的。根据该指令，它必须被裁剪。但我不能找到如何做到这一点。有谁可以给​​我一个示例代码，请？

谢谢

Answer 1

这是一个相当陈旧的问题，但我发现它试图解决相同问题的时候。这是唯一公平分享我突发奇想所以接下来的可怜虫没有弄明白自己的解决方案。

我不知道这是否是做事情特别好的方式，我想，如果你开始使用弧形细胞会挣扎，但它的工作确定为我的目的。

最根本的问题是，你只有无限的边缘一个顶点，所以你要自己计算的方向向量。使用方向是垂直于由边缘分开的两个点之间的矢量。

#include <vector>
#include <boost/polygon/voronoi.hpp>
using boost::polygon::voronoi_builder;
using boost::polygon::voronoi_diagram;

typedef boost::polygon::point_data<int> point;
typedef voronoi_diagram<double>::cell_type cell_type;
typedef voronoi_diagram<double>::edge_type edge_type;
typedef voronoi_diagram<double>::vertex_type vertex_type;

int main(int argc, char *argv[])
{
   std::vector<point> points;

   // Populate with random points
   for (int i = 0; i < 50; i++)
   {
      points.push_back(point(60 + rand() % 500, 60 + rand() % 500));
   }

   voronoi_diagram<double> vd;
   construct_voronoi(points.begin(), points.end(), &vd);

   // vd now contains the voronoi diagram. Let's visualise it
   // pseudocode 'draw_line(x1, y1, x2, y2)'

   for (voronoi_diagram<double>::const_cell_iterator it = vd.cells().begin();
        it != vd.cells().end(); ++it)
   {
      const cell_type& cell = *it;
      const edge_type* edge = cell.incident_edge();

      do
      {
         if (edge->is_primary())
         {
            // Easy finite edge case
            if (edge->is_finite())
            {
               // Without this check each edge would be drawn twice
               // as they are really half-edges
               if (edge->cell()->source_index() < 
                   edge->twin()->cell()->source_index())
               {
                  draw_line(edge->vertex0()->x(), edge->vertex0()->y(),
                            edge->vertex1()->x(), edge->vertex1()->y());
               }
            }
            else
            {
               // This covers the infinite edge case in question.
               const vertex_type* v0 = edge->vertex0();
               // Again, only consider half the half-edges, ignore edge->vertex1()
               // to avoid overdrawing the lines
               if (v0)
               {
                  // Direction of infinite edge if perpendicular to direction
                  // between the points owning the two half edges. 
                  // Take the rotated right vector and multiply by a large 
                  // enough number to reach your bounding box
                  point p1 = points[edge->cell()->source_index()];
                  point p2 = points[edge->twin()->cell()->source_index()];
                  int end_x = (p1.y() - p2.y()) * 640;
                  int end_y = (p1.x() - p2.x()) * -640;

                  draw_line(v0->x(), v0->y(),
                            end_x, end_y);
               }
            }
         }
         edge = edge->next();
      } while (edge != cell.incident_edge());
   }
}

Answer 2

我在这里发现这个代码段：http://www.boost.org/doc/libs/1_55_0/libs/polygon/example/voronoi_visualizer.cpp

void clip_infinite_edge(
      const edge_type& edge, std::vector<point_type>* clipped_edge) {
    const cell_type& cell1 = *edge.cell();
    const cell_type& cell2 = *edge.twin()->cell();
    point_type origin, direction;
    // Infinite edges could not be created by two segment sites.
    if (cell1.contains_point() && cell2.contains_point()) {
      point_type p1 = retrieve_point(cell1);
      point_type p2 = retrieve_point(cell2);
      origin.x((p1.x() + p2.x()) * 0.5);
      origin.y((p1.y() + p2.y()) * 0.5);
      direction.x(p1.y() - p2.y());
      direction.y(p2.x() - p1.x());
    } else {
      origin = cell1.contains_segment() ?
          retrieve_point(cell2) :
          retrieve_point(cell1);
      segment_type segment = cell1.contains_segment() ?
          retrieve_segment(cell1) :
          retrieve_segment(cell2);
      coordinate_type dx = high(segment).x() - low(segment).x();
      coordinate_type dy = high(segment).y() - low(segment).y();
      if ((low(segment) == origin) ^ cell1.contains_point()) {
        direction.x(dy);
        direction.y(-dx);
      } else {
        direction.x(-dy);
        direction.y(dx);
      }
    }
    coordinate_type side = xh(brect_) - xl(brect_);
    coordinate_type koef =
        side / (std::max)(fabs(direction.x()), fabs(direction.y()));
    if (edge.vertex0() == NULL) {
      clipped_edge->push_back(point_type(
          origin.x() - direction.x() * koef,
          origin.y() - direction.y() * koef));
    } else {
      clipped_edge->push_back(
          point_type(edge.vertex0()->x(), edge.vertex0()->y()));
    }
    if (edge.vertex1() == NULL) {
      clipped_edge->push_back(point_type(
          origin.x() + direction.x() * koef,
          origin.y() + direction.y() * koef));
    } else {
      clipped_edge->push_back(
          point_type(edge.vertex1()->x(), edge.vertex1()->y()));
    }
  }

它可能会丢失一些类变量或方法，但逻辑是这里重要的。

Answer 3

我写了一个程序，以便边缘的矩形边界框内部局限于夹无限边缘Voronoi单元。该代码是在https://github.com/datajaguar/jaguardb/blob/master/src/JagCGAL.cc

getIntersectionPointWithBox（）方法基本上检查边界框的每一侧（总共4个侧面），并获取交叉点，如果有一个。 fillInCorners（）方法检查周围是否缺少边框点，并添加顶点坐标的边缘。为了使每个小区的多边形，它增加了第一点，在年底使多边形闭合。