// load input image
var img = Dlib.LoadImage<RgbPixel>(inputFilePath);

// find all faces in the image
var faces = fd.Operator(img);
foreach (var face in faces)
{
    // find the landmark points for this face
    var shape = sp.Detect(img, face);

    // draw the landmark points on the image
    for (var i = 0; i < shape.Parts; i++)
    {
        var point = shape.GetPart((uint)i);
        var rect = new Rectangle(point);
        Dlib.DrawRectangle(img, rect, color: new RgbPixel(255, 255, 0), thickness: 4);
    }
}

// the rest of the code goes here....