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/// <summary>
/// Rotates the image by angle.
/// </summary>
/// <param name="oldBitmap">The old bitmap.</param>
/// <param name="angle">The angle.</param>
/// <returns></returns>
private static Bitmap RotateImageByAngle(System.Drawing.Image oldBitmap, float angle)
{
    var newBitmap = new Bitmap(oldBitmap.Width, oldBitmap.Height);
    var graphics = Graphics.FromImage(newBitmap);
    graphics.TranslateTransform((float)oldBitmap.Width / 2, (float)oldBitmap.Height / 2);
    graphics.RotateTransform(angle);
    graphics.TranslateTransform(-(float)oldBitmap.Width / 2, -(float)oldBitmap.Height / 2);
    graphics.DrawImage(oldBitmap, new Point(0, 0));
    return newBitmap;
}