HOWTO: Crop Image Using Elliptical Marquee

// Crop rectangle
System.Drawing.Rectangle cropRect = new System.Drawing.Rectangle(300, 0, 500, 300);

// Source bitmap
Aurigma.GraphicsMill.Bitmap bmp = new Aurigma.GraphicsMill.Bitmap(@"D:\temp\source.jpg");

// Apply rectangular crop
bmp.Transforms.Crop(cropRect);

// Create ellipse mask and add it to the cropped bitmap
using (Aurigma.GraphicsMill.Bitmap alpha = new Aurigma.GraphicsMill.Bitmap(cropRect.Width, 
  cropRect.Height, Aurigma.GraphicsMill.PixelFormat.Format24bppRgb))
{
    // Draw ellipse on the mask
    using (System.Drawing.Graphics gr = alpha.GetGdiplusGraphics())
    {
        gr.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
        gr.FillEllipse(new System.Drawing.SolidBrush(System.Drawing.Color.White), 
          new System.Drawing.Rectangle(0, 0, cropRect.Width, cropRect.Height));
    }
    alpha.ColorManagement.Convert(Aurigma.GraphicsMill.PixelFormat.Format8bppGrayScale);

    // Add the mask to cropeed bitmap
    bmp.Channels.AddAlpha(1.0f);
    bmp.Channels[bmp.Channels.AlphaChannelIndex] = alpha;
}

// Save the result
bmp.Save(@"d:\temp\EllipseCrop.png");