Region.IsVisible(PointF) has very slow performance for large floating point values

Question

I have run into a strange performance issue, and it would be great with an explanation to the behavior I\'m experiencing.

I\'m using System.Drawing.Region.IsVisible

Answer 1

We've have been struggling with the same issue: with a 50k points polygon it tooks 45 seconds to build the rectangles behind the IsVisible() method. We tried to cache this data, but finally we had over 1 millions rectangles; with multiple regions we had hundreds megabytes of data to cache.

Finally we moved to the PNPoly algorithm, that takes just few ms:

https://wrf.ecse.rpi.edu/Research/Short_Notes/pnpoly.html

Here the c# version:

    public bool IsVisible(Point p, List points)
    {
        int i, j = points.Count - 1;
        bool isVisible = false;
        for (i = 0; i < points.Count; i++)
        {
            if (points[i].Y < p.Y && points[j].Y >= p.Y 
                || points[j].Y < p.Y && points[i].Y >= p.Y)
            {
                if (points[i].X + (p.Y - points[i].Y) / (points[j].Y - points[i].Y) 
                    * (points[j].X - points[i].X) < p.X)
                {
                    isVisible = !isVisible;
                }
            }
            j = i;
        }
        return isVisible;
    }