#1,149 – Drawing an Arc in a Custom Shape

We can use a StreamGeometryContext to render some geometry in a custom Shape element that we can then use in XAML.  Below is an example that draws a simple arc from 0 degress to 90 degrees.  It uses a PolarPoint class to allow describing the arc start and finish as polar coordinates.  (A future post will allow a user to specify arc start and end).

    public class Arc : Shape
    {
        protected override Geometry DefiningGeometry
        {
            get
            {
                double maxWidth = RenderSize.Width;
                double maxHeight = RenderSize.Height;
                double maxRadius = Math.Min(maxWidth, maxHeight) / 2.0;

                PolarPoint arcStart = new PolarPoint(maxRadius, 0.0);
                PolarPoint arcFinish = new PolarPoint(maxRadius, 90.0);

                StreamGeometry geom = new StreamGeometry();
                using (StreamGeometryContext ctx = geom.Open())
                {
                    ctx.BeginFigure(
                        new Point((maxWidth / 2.0) + arcStart.X,
                                   (maxHeight / 2.0) - arcStart.Y),
                        false,
                        false);
                    ctx.ArcTo(
                        new Point((maxWidth / 2.0) + arcFinish.X,
                                  (maxHeight / 2.0) - arcFinish.Y),
                        new Size(maxRadius, maxRadius),
                        0.0,     // rotationAngle
                        false,   // greater than 180 deg?
                        SweepDirection.Counterclockwise,
                        true,    // isStroked
                        true);
                }

                return geom;
            }
        }
    }

Using the arc:

        <loc:Arc Stroke="Black" StrokeThickness="1"
                 Height="100" Width="100" Margin="5"
                 HorizontalAlignment="Center"/>

1149-001

#1,148 – Sample Code to Convert from Polar to Cartesian Coordinates

Below is some simple code (not productized) that can convert from two-dimensional polar to cartesian coordinates.

    public class PolarPoint
    {
        // Angle expressed in degrees
        public PolarPoint(double radius, double angleDeg)
        {
            if (radius < 0.0)
                throw new ArgumentException("Radius must be non-negative");
            if ((angleDeg < 0) || (angleDeg >= 360.0))
                throw new ArgumentException("Angle must be in range [0,360)");

            Radius = radius;
            AngleDeg = angleDeg;
        }

        // Polar coordinates
        public double Radius { get; set; }
        public double AngleDeg { get; set; }

        // Cartesian coordinates
        public double X
        {
            get { return Radius * Math.Cos(AngleDeg * Math.PI / 180.0); }
        }

        public double Y
        {
            get { return Radius * Math.Sin(AngleDeg * Math.PI / 180.0); }
        }

        public override string ToString()
        {
            return string.Format("({0},{1})", X, Y);
        }
    }

We can then use this class as follows.

            Console.WriteLine(new PolarPoint(0.0, 0.0));
            Console.WriteLine(new PolarPoint(1.0, 0.0));
            Console.WriteLine(new PolarPoint(1.0, 45.0));
            Console.WriteLine(new PolarPoint(1.0, 90.0));
            Console.WriteLine(new PolarPoint(1.0, 135.0));
            Console.WriteLine(new PolarPoint(1.0, 180.0));

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