Question

Based on How to determine if a list of polygon points are in clockwise order?

I’ve come up with the following code:

bool PointsClockwise(const std::vector<MyPoint>& points)
{  
  double sum = 0.0;
  for(size_t i = 0; i < points.size() - 1; ++i)
    sum += (points[i+1].x()-points[i].x()) * (points[i+1].y()+points[i].y());

  return sum > 0.0;
}

However, this seems to have wrong result in certain cases. Take for example the following ring:

LINESTRING(0 119,0 60,694 70,704 72,712 77,719 83,723 92,725 102,723 111,719 120,712 126,703 130)

It is in counter-clockwise order, but the function returns true.

Thanks!

Answer 1

You missed one of the line segments from your summation – namely the one connecting the last point with the first.
Try that:

bool PointsClockwise(const std::vector<MyPoint>& points)
{  
  double sum = 0.0;
  for(size_t i = 0; i < points.size() - 1; ++i)
    sum += (points[i+1].x()-points[i].x()) * (points[i+1].y()+points[i].y());

  sum += (points[0].x()-points[points.size()-1].x()) * (points[0].y()+points[points.size()-1].y());

  return sum > 0.0;
}