/// <summary>
/// Draws rectangle with the specified text on top.
/// </summary>
/// <param name="image">Image.</param>
/// <param name="rect">User specified area to annotate.</param>
/// <param name="text">Label.</param>
/// <param name="font">Font to use.</param>
public static void DrawAnnotation(this Bgr <byte>[,] image, Rectangle rect, string text, Font font)
{
const int VERTICAL_OFFSET = 5;
image.Draw(rect, Bgr <byte> .Red, 1);
var textSize = font.GetTextSize(text, 0);
var bottomLeftPt = new Point(rect.X + rect.Width / 2 - textSize.Width / 2, rect.Top - VERTICAL_OFFSET);
image.Draw(text, font, bottomLeftPt, Bgr <byte> .Black);
}