Intersection Tests

Simple 2D intersection tests can be used in many cases to add interactive components.

namespace Utilities
{
    using System;
    using Microsoft.Xna.Framework;
 
    class IntersectionTest
    {
        /// <summary>
        /// 2D Check if a circle is within another circle
        /// If the circles touch on a single point, they are 
        /// considered intersecting.
        /// </summary>
        /// <param name="centerA">Center of first circle.</param>
        /// <param name="radiusA">Radius of first circle.</param>
        /// <param name="centerB">Center of second circle.</param>
        /// <param name="radiusB">Radius of second circle.</param>
        /// <returns>
        /// A boolean flag indicating if the circles intersect.
        /// </returns>
        public static bool CircleInCircle(Vector2 centerA, double radiusA, Vector2 centerB, double radiusB)
        {
            double dX = (double)(centerB.X - centerA.X);
            double dY = (double)(centerB.Y - centerA.Y);
            double d = Math.Sqrt(dX * dX + dY * dY);
 
            // circles do not intersect if they are too far apart
            if (d > (radiusA + radiusB))
            {
                return false;
            }
 
            return true;
        }
 
        /// <summary>
        /// 2D Check if a point lies withing a polygon.
        /// </summary>
        /// <param name="polygonVertex">The points of the polygon.</param>
        /// <param name="testVertex">The point to check.</param>
        /// <returns>
        /// A boolean flag indicating if the test vertex
        /// is inside the polygon.
        /// </returns>
        public static bool PointInPolygon(Vector2[] polygonVertex, Vector2 testVertex)
        {
            bool c = false;
            int nvert = polygonVertex.Length;
            if (nvert > 2)
            {
                int i, j;
                for (i = 0, j = nvert - 1; i < nvert; j = i++)
                {
                    if (((polygonVertex[i].Y > testVertex.Y) != (polygonVertex[j].Y > testVertex.Y)) &&
                     (testVertex.X < (polygonVertex[j].X - polygonVertex[i].X) *
                     (testVertex.Y - polygonVertex[i].Y) /
                     (polygonVertex[j].Y - polygonVertex[i].Y) + polygonVertex[i].X))
                    {
                        c = !c;
                    }
                }
            }
 
            return c;
        }
    }
}