public override void OnMouseDown(object sender, MouseEventArgs e)
{
if (e.Button != MouseButtons.Left || !this.IsSelected || isDrawn)
{
return;
}
pt = Element.ToImageCoordinate(e.Location).ToPt();
var imageSize = Element.Image.Size.ToSize();
this.Annotation.Polygon = new PointF[] { pt.Clamp(imageSize) };
isDrawn = true;
}