CODEWITHSUNDEEP
c#comautocad
Steve M
Steve M

Reputation: 9784

Algorithm for points inside a polyline inexplicably fails (AutoCAD)

I'm using the .NET API for Autocad, I have an algorithm (which I did not write) for determining if a point lies within a polygon (straight lines only).

I have been testing my command on the same 51 polygons repeatedly. 99% it will work perfectly. Every once in a while it will fail on 1 or more of the polygons, returning false for over 2000 points I am creating inside the bounding box of the polyline. I have seen it fail when the polyline isa simple rectangle and all of the points lie distributed in a grid within the polyline. It should have returned true over 2000 times in that case. It will never fail for just 1 of the points, it will fail all of them. I have confirmed that the points are being correctly created where I expect them to be and that the vertices of the polygon are where I expect them to be. When it fails, the last angle variable for the last point is at exactly double PI.

I am not doing any multi-threading. The only possibly 'funny' thing I am doing is COM Interop with Excel. This is happening after the transaction has been committed for the part with this algorithm, and I am sure I am cleaning up all my COM objects. I have not been able to reproduce the failure without the COM Interop part but I don't think I've tested it enough yet to have enough absence of evidence.

Any ideas what could be wrong?

bool IsInsidePolygon(Polyline polygon, Point3d pt)
    {
        int n = polygon.NumberOfVertices;
        double angle = 0;
        Point pt1, pt2;

        for (int i = 0; i < n; i++)
        {
            pt1.X = polygon.GetPoint2dAt(i).X - pt.X;
            pt1.Y = polygon.GetPoint2dAt(i).Y - pt.Y;
            pt2.X = polygon.GetPoint2dAt((i + 1) % n).X - pt.X;
            pt2.Y = polygon.GetPoint2dAt((i + 1) % n).Y - pt.Y;
            angle += Angle2D(pt1.X, pt1.Y, pt2.X, pt2.Y);
        }

        if (Math.Abs(angle) < Math.PI)
            return false;
        else
            return true;
    }

    public struct Point
    {
        public double X, Y;
    };

    public static double Angle2D(double x1, double y1, double x2, double y2)
    {
        double dtheta, theta1, theta2;

        theta1 = Math.Atan2(y1, x1);
        theta2 = Math.Atan2(y2, x2);
        dtheta = theta2 - theta1;
        while (dtheta > Math.PI)
            dtheta -= (Math.PI * 2);
        while (dtheta < -Math.PI)
            dtheta += (Math.PI * 2);
        return (dtheta);
    }

Upvotes: 0

Views: 2855

Answers (3)

Woestebanaan
Woestebanaan

Reputation: 1

I used some code from Kean Walmsley to convert 3d lines into 2d lines. But be aware that the following is not (always) true:

Point2d pt = lwp.GetPoint2dAt(i);
Point2d npt = new Point2d(lwp.GetPoint3dAt(i).X, lwp.GetPoint3dAt(i).Y);

pt == npt;

I encountered it using it on a polylines, with 3d vertices. I ended up using the npt.

http://through-the-interface.typepad.com/through_the_interface/2007/04/iterating_throu.html

Upvotes: 0

Dobriyan Benov
Dobriyan Benov

Reputation: 11

another method. Make one "temporary" point outside the polygon (find the min X and Y and make a point with X-1 and Y-1). Then make a line between your point and the new "temporary" point. Check if this line crosses the polygon - use polyline.IntersectWith. If number of cross points is odd - then your point is inside, if the number of crosses is even - the your point is not inside. This works for me, hope it will helps you also. If you find trouble with implementing this, i can send you an example code. Regards, Dobriyan Benov

Upvotes: 1

loic
loic

Reputation: 185

Some ideas:

  • floating point comparison have to be done using a tolerence, this might cause kind of arbitrary results especially in case where the point lies on the polyline (same remark for point3d, they must be compared using a tolerence)

  • maybe the last point of your polyline is at the same location as the first one, in that case, the angle cannot be computed (perhaps this is why you get a double pi value for the last point). You should then test is first and last points are equals.

  • I'm not sure your algorithm works regardless if the polyline is clockwise or counterclockwise (I think yes)

  • you may convert your polyline into a region and rely on region point containment method

Upvotes: 1

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