bbiwarg-original/bbiwarg/Utility/Line2D.cs

using System;

namespace BBIWARG.Utility
{
    /// <summary>
    /// Values for describing if something is on, above or below a line.
    /// </summary>
    public enum LineSide
    {
        onLine = 0,
        above = 1,
        below = 2
    }

    /// <summary>
    /// Line lass which represents a infinity line in 2 dimensional space.
    /// </summary>
    public class Line2D
    {
        /// <summary>
        /// direction vector of the line
        /// </summary>
        public Vector2D Direction { get; private set; }

        /// <summary>
        /// one point on the line
        /// </summary>
        public Vector2D PointOnLine { get; private set; }

        /// <summary>
        /// Standard constructor which sets the essential attributes.
        /// </summary>
        /// <param name="pointOnLine">a point on the line</param>
        /// <param name="direction">the direction of the line</param>
        public Line2D(Vector2D pointOnLine, Vector2D direction)
        {
            PointOnLine = pointOnLine;
            Direction = direction.normalize();
        }

        /// <summary>
        /// Computes angle between current and an second line in degrees.
        /// </summary>
        /// <param name="line">second line</param>
        /// <returns>angle between this and line in degrees</returns>
        public float getAngleBetween(Line2D line)
        {
            float angle = Direction.getAngleBetween(line.Direction);
            return Math.Min(angle, 180 - angle);
        }

        /// <summary>
        /// Computes the intersection of two lines, iff the lines do not intersect it returns null.
        /// </summary>
        /// <param name="line">the second line</param>
        /// <returns>the intersection or null iff the lines do not intersect</returns>
        public Vector2D getIntersection(Line2D line)
        {
            Vector2D p = PointOnLine;
            Vector2D r = Direction;
            Vector2D q = line.PointOnLine;
            Vector2D s = line.Direction;

            float r_cross_s = r.crossProduct(s);
            float q_p_cross_s = (q - p).crossProduct(s);

            if (r_cross_s == 0.0)
                return null;

            float t = q_p_cross_s / r_cross_s;
            return p + t * r;
        }

        /// <summary>
        /// Calculates whether a point is above, below or on this line.
        /// </summary>
        /// <param name="point">point which is used to compute the side</param>
        /// <returns>LineSide.above iff point is above this, LineSide.below iff point is below this, LineSide.onLine iff point is on this line</returns>
        public LineSide onSide(Vector2D point)
        {
            float ratio = Direction.Y / Direction.X;
            float horizontalDifference = point.X - PointOnLine.X;
            float newY = PointOnLine.Y + ratio * horizontalDifference;
            if (newY < point.Y) return LineSide.above;
            else if (newY > point.Y) return LineSide.below;
            else return LineSide.onLine;
        }

        /// <summary>
        /// Projects a point on this line.
        /// </summary>
        /// <param name="point">point which is projected</param>
        /// <returns>the point on the line with minimal distance to input point</returns>
        public Vector2D projectToLine(Vector2D point)
        {
            float px = point.X, py = point.Y, dx = Direction.X, dy = Direction.Y, ox = PointOnLine.X, oy = PointOnLine.Y;
            float diffx = px - ox;
            float diffy = py - oy;

            float diff_d = diffx * dx + diffy * dy;
            float d_d = dx * dx + dy * dy;
            float q = diff_d / d_d;

            float newX = ox + q * dx;
            float newY = oy + q * dy;

            return new Vector2D(newX, newY);
        }
    }
}