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

Question

I have two points (a line segment) and a rectangle. I would like to know how to calculate if the line segment intersects the rectangle.

Answer 1

I hate browsing the MSDN docs (they're awfully slow and weird :-s) but I think they should have something similar to this Java method... and if they haven't, bad for them! XD (btw, it works for segments, not lines).

In any case, you can peek the open source Java SDK to see how is it implemented, maybe you'll learn some new trick (I'm always surprised when I look other people's code)

Answer 2

From my "Geometry" class:

public struct Line
{
    public static Line Empty;

    private PointF p1;
    private PointF p2;

    public Line(PointF p1, PointF p2)
    {
        this.p1 = p1;
        this.p2 = p2;
    }

    public PointF P1
    {
        get { return p1; }
        set { p1 = value; }
    }

    public PointF P2
    {
        get { return p2; }
        set { p2 = value; }
    }

    public float X1
    {
        get { return p1.X; }
        set { p1.X = value; }
    }

    public float X2
    {
        get { return p2.X; }
        set { p2.X = value; }
    }

    public float Y1
    {
        get { return p1.Y; }
        set { p1.Y = value; }
    }

    public float Y2
    {
        get { return p2.Y; }
        set { p2.Y = value; }
    }
}

public struct Polygon: IEnumerable<PointF>
{
    private PointF[] points;

    public Polygon(PointF[] points)
    {
        this.points = points;
    }

    public PointF[] Points
    {
        get { return points; }
        set { points = value; }
    }

    public int Length
    {
        get { return points.Length; }
    }

    public PointF this[int index]
    {
        get { return points[index]; }
        set { points[index] = value; }
    }

    public static implicit operator PointF[](Polygon polygon)
    {
        return polygon.points;
    }

    public static implicit operator Polygon(PointF[] points)
    {
        return new Polygon(points);
    }

    IEnumerator<PointF> IEnumerable<PointF>.GetEnumerator()
    {
        return (IEnumerator<PointF>)points.GetEnumerator();
    }

    public IEnumerator GetEnumerator()
    {
        return points.GetEnumerator();
    }
}

public enum Intersection
{
    None,
    Tangent,
    Intersection,
    Containment
}

public static class Geometry
{

    public static Intersection IntersectionOf(Line line, Polygon polygon)
    {
        if (polygon.Length == 0)
        {
            return Intersection.None;
        }
        if (polygon.Length == 1)
        {
            return IntersectionOf(polygon[0], line);
        }
        bool tangent = false;
        for (int index = 0; index < polygon.Length; index++)
        {
            int index2 = (index + 1)%polygon.Length;
            Intersection intersection = IntersectionOf(line, new Line(polygon[index], polygon[index2]));
            if (intersection == Intersection.Intersection)
            {
                return intersection;
            }
            if (intersection == Intersection.Tangent)
            {
                tangent = true;
            }
        }
        return tangent ? Intersection.Tangent : IntersectionOf(line.P1, polygon);
    }

    public static Intersection IntersectionOf(PointF point, Polygon polygon)
    {
        switch (polygon.Length)
        {
            case 0:
                return Intersection.None;
            case 1:
                if (polygon[0].X == point.X && polygon[0].Y == point.Y)
                {
                    return Intersection.Tangent;
                }
                else
                {
                    return Intersection.None;
                }
            case 2:
                return IntersectionOf(point, new Line(polygon[0], polygon[1]));
        }

        int counter = 0;
        int i;
        PointF p1;
        int n = polygon.Length;
        p1 = polygon[0];
        if (point == p1)
        {
            return Intersection.Tangent;
        }

        for (i = 1; i <= n; i++)
        {
            PointF p2 = polygon[i % n];
            if (point == p2)
            {
                return Intersection.Tangent;
            }
            if (point.Y > Math.Min(p1.Y, p2.Y))
            {
                if (point.Y <= Math.Max(p1.Y, p2.Y))
                {
                    if (point.X <= Math.Max(p1.X, p2.X))
                    {
                        if (p1.Y != p2.Y)
                        {
                            double xinters = (point.Y - p1.Y) * (p2.X - p1.X) / (p2.Y - p1.Y) + p1.X;
                            if (p1.X == p2.X || point.X <= xinters)
                                counter++;
                        }
                    }
                }
            }
            p1 = p2;
        }

        return (counter % 2 == 1) ? Intersection.Containment : Intersection.None;
    }

    public static Intersection IntersectionOf(PointF point, Line line)
    {
        float bottomY = Math.Min(line.Y1, line.Y2);
        float topY = Math.Max(line.Y1, line.Y2);
        bool heightIsRight = point.Y >= bottomY &&
                             point.Y <= topY;
        //Vertical line, slope is divideByZero error!
        if (line.X1 == line.X2)
        {
            if (point.X == line.X1 && heightIsRight)
            {
                return Intersection.Tangent;
            }
            else
            {
                return Intersection.None;
            }
        }
        float slope = (line.X2 - line.X1)/(line.Y2 - line.Y1);
        bool onLine = (line.Y1 - point.Y) == (slope*(line.X1 - point.X));
        if (onLine && heightIsRight)
        {
            return Intersection.Tangent;
        }
        else
        {
            return Intersection.None;
        }
    }

}

Answer 3

If it is 2d, then all lines are on the only plane.

So, this is basic 3-D geometry. You should be able to do this with a straightforward equation.

Check out this page:

http://local.wasp.uwa.edu.au/~pbourke/geometry/planeline/.

The second solution should be easy to implement, as long as you translate the coordinates of your rectangle into the equation of a plane.

Furthermore, check that your denominator isn't zero (line doesn't intersect or is contained in the plane).

Answer 4

Do http://mathworld.wolfram.com/Line-LineIntersection.html for the line and each side of the rectangle.
Or: http://mathworld.wolfram.com/Line-PlaneIntersection.html

Answer 5

Isn't it possible to check the line against each side of the rectangle using simple line segment formula.

Answer 6

since it is missing i'll just add it for completeness

public static Intersection IntersectionOf(Line line1, Line line2)
    {
        //  Fail if either line segment is zero-length.
        if (line1.X1 == line1.X2 && line1.Y1 == line1.Y2 || line2.X1 == line2.X2 && line2.Y1 == line2.Y2)
            return Intersection.None;

        if (line1.X1 == line2.X1 && line1.Y1 == line2.Y1 || line1.X2 == line2.X1 && line1.Y2 == line2.Y1)
            return Intersection.Intersection;
        if (line1.X1 == line2.X2 && line1.Y1 == line2.Y2 || line1.X2 == line2.X2 && line1.Y2 == line2.Y2)
            return Intersection.Intersection;

        //  (1) Translate the system so that point A is on the origin.
        line1.X2 -= line1.X1; line1.Y2 -= line1.Y1;
        line2.X1 -= line1.X1; line2.Y1 -= line1.Y1;
        line2.X2 -= line1.X1; line2.Y2 -= line1.Y1;

        //  Discover the length of segment A-B.
        double distAB = Math.Sqrt(line1.X2 * line1.X2 + line1.Y2 * line1.Y2);

        //  (2) Rotate the system so that point B is on the positive X axis.
        double theCos = line1.X2 / distAB;
        double theSin = line1.Y2 / distAB;
        double newX = line2.X1 * theCos + line2.Y1 * theSin;
        line2.Y1 = line2.Y1 * theCos - line2.X1 * theSin; line2.X1 = newX;
        newX = line2.X2 * theCos + line2.Y2 * theSin;
        line2.Y2 = line2.Y2 * theCos - line2.X2 * theSin; line2.X2 = newX;

        //  Fail if segment C-D doesn't cross line A-B.
        if (line2.Y1 < 0 && line2.Y2 < 0 || line2.Y1 >= 0 && line2.Y2 >= 0)
            return Intersection.None;

        //  (3) Discover the position of the intersection point along line A-B.
        double posAB = line2.X2 + (line2.X1 - line2.X2) * line2.Y2 / (line2.Y2 - line2.Y1);

        //  Fail if segment C-D crosses line A-B outside of segment A-B.
        if (posAB < 0 || posAB > distAB)
            return Intersection.None;

        //  (4) Apply the discovered position to line A-B in the original coordinate system.
        return Intersection.Intersection;
    }

note that the method rotates the line segments so as to avoid direction-related problems