从极坐标转换为直角坐标

/// <summary>
/// Convert from polar coordinates to rectangular coordinates.
/// Only works for points to the left of the origin.
/// </summary>
/// <param name="radius">The radius of the point in pixels.</param>
/// <param name="theta">The angle of the point in radians.</param>
/// <returns>The point in rectangular coordinates.</returns>
internal Point PolarToRectangular(
        double radius,
        double theta)
{
    try
    {
        double sin = Math.Sin(theta);

        // This is faster then:
        // double cos = Math.Cos(theta);
        double cos = -Math.Sqrt(1 - (sin * sin));

        Int32 x = _offset.X + (Int32)Math.Round(radius * cos);
        Int32 y = _offset.Y + (Int32)Math.Round(radius * sin);

        return new Point(x, y);
    }
    catch (OverflowException ex)
    {
        ex.Data.Add("Screen polar Radius", radius);
        ex.Data.Add("Screen polar Theta", theta);
        throw;
    }
}