dotUpdate()

//Calculates the color of a point in a rectangle that is filled

//with a LinearGradientBrush.

private Color GetColorAtPoint(Rectangle theRec, Point thePoint)

{

    //Get properties

    LinearGradientBrush br = (LinearGradientBrush)theRec.Fill;

    double y3 = thePoint.Y;

    double x3 = thePoint.X;

    double x1 = br.StartPoint.X * theRec.ActualWidth;

    double y1 = br.StartPoint.Y * theRec.ActualHeight;

    Point p1 = new Point(x1, y1); //Starting point

    double x2 = br.EndPoint.X * theRec.ActualWidth;

    double y2 = br.EndPoint.Y * theRec.ActualHeight;

    Point p2 = new Point(x2, y2);  //End point

    //Calculate intersecting points 

    Point p4 = new Point(); //with tangent

    if (y1 == y2) //Horizontal case

    {

        p4 = new Point(x3, y1);

    }

    else if (x1 == x2) //Vertical case

    {

        p4 = new Point(x1, y3);

    }

    else //Diagnonal case

    {

        double m = (y2 - y1) / (x2 - x1);

        double m2 = -1 / m;

        double b = y1 - m * x1;

        double c = y3 - m2 * x3;

        double x4 = (c - b) / (m - m2);

        double y4 = m * x4 + b;

        p4 = new Point(x4, y4);

    }

    //Calculate distances relative to the vector start

    double d4 = dist(p4, p1, p2);

    double d2 = dist(p2, p1, p2);

    double x = d4 / d2;

    //Clip the input if before or after the max/min offset values

    double max = br.GradientStops.Max(n => n.Offset);

    if (x > max)

    {

        x = max;

    }

    double min = br.GradientStops.Min(n => n.Offset);

    if (x < min)

    {

        x = min;

    }

    //Find gradient stops that surround the input value

    GradientStop gs0 = br.GradientStops.Where(n => n.Offset <= x).OrderBy(n => n.Offset).Last();

    GradientStop gs1 = br.GradientStops.Where(n => n.Offset >= x).OrderBy(n => n.Offset).First();

    float y = 0f;

    if (gs0.Offset != gs1.Offset)

    {

        y = (float)((x - gs0.Offset) / (gs1.Offset - gs0.Offset));

    }

    //Interpolate color channels

    Color cx = new Color();

    if (br.ColorInterpolationMode == ColorInterpolationMode.ScRgbLinearInterpolation)

    {

        float aVal = (gs1.Color.ScA - gs0.Color.ScA) * y + gs0.Color.ScA;

        float rVal = (gs1.Color.ScR - gs0.Color.ScR) * y + gs0.Color.ScR;

        float gVal = (gs1.Color.ScG - gs0.Color.ScG) * y + gs0.Color.ScG;

        float bVal = (gs1.Color.ScB - gs0.Color.ScB) * y + gs0.Color.ScB;

        cx = Color.FromScRgb(aVal, rVal, gVal, bVal);

    }

    else

    {

        byte aVal = (byte)((gs1.Color.A - gs0.Color.A) * y + gs0.Color.A);

        byte rVal = (byte)((gs1.Color.R - gs0.Color.R) * y + gs0.Color.R);

        byte gVal = (byte)((gs1.Color.G - gs0.Color.G) * y + gs0.Color.G);

        byte bVal = (byte)((gs1.Color.B - gs0.Color.B) * y + gs0.Color.B);

        cx = Color.FromArgb(aVal, rVal, gVal, bVal);

    }

    return cx;

}

//Helper method for GetColorAtPoint

//Returns the signed magnitude of a point on a vector with origin po and pointing to pf

private double dist(Point px, Point po, Point pf)

{

    double d = Math.Sqrt((px.Y - po.Y) * (px.Y - po.Y) + (px.X - po.X) * (px.X - po.X));

    if (((px.Y < po.Y) && (pf.Y > po.Y)) ||

        ((px.Y > po.Y) && (pf.Y < po.Y)) ||

        ((px.Y == po.Y) && (px.X < po.X) && (pf.X > po.X)) ||

        ((px.Y == po.Y) && (px.X > po.X) && (pf.X < po.X)))

    {

        d = -d;

    }

    return d;

}