How to tell if a line intersects a polygon in C#?

Question

I have a question very similar to this:

How to know if a line intersects a plane in C#?

I am searching for a method (in C#) that tells if a line is intersecting an arbitrary polygon.

I think the algorithm by Chris Marasti-Georg was very helpful, but missing the most important method, i.e. line to line intersection.

Does anyone know of a line intersection method to complete Chris Marasti-Georg's code or have anything similar?

Is there a built-in code for this in C#?

This method is for use with the Bing Maps algorithm enhanced with a forbidden area feature. The resulting path must not pass through the forbidden area (the arbitrary polygon).

Answer 1

There is no builtin code for edge detection built into the .NET framework.

Here's code (ported to C#) that does what you need (the actual algorithm is found at comp.graphics.algorithms on Google groups) :

public static PointF FindLineIntersection(PointF start1, PointF end1, PointF start2, PointF end2)
{
    float denom = ((end1.X - start1.X) * (end2.Y - start2.Y)) - ((end1.Y - start1.Y) * (end2.X - start2.X));

    //  AB & CD are parallel 
    if (denom == 0)
        return PointF.Empty;

    float numer = ((start1.Y - start2.Y) * (end2.X - start2.X)) - ((start1.X - start2.X) * (end2.Y - start2.Y));

    float r = numer / denom;

    float numer2 = ((start1.Y - start2.Y) * (end1.X - start1.X)) - ((start1.X - start2.X) * (end1.Y - start1.Y));

    float s = numer2 / denom;

    if ((r < 0 || r > 1) || (s < 0 || s > 1))
        return PointF.Empty;

    // Find intersection point
    PointF result = new PointF();
    result.X = start1.X + (r * (end1.X - start1.X));
    result.Y = start1.Y + (r * (end1.Y - start1.Y));

    return result;
 }