bitmap.ToLuminanceImage() is not a method

Question

all

I wont use Shipwreck.Phash(https://github.com/pgrho/phash) for image,but I have trouble with the example:

var bitmap = (Bitmap)Image.FromFile(fullPathToImage);
var hash = ImagePhash.ComputeDigest(bitmap.ToLuminanceImage());

vs2015 cannot find ToLuminanceImage();

I used net4.5.2. System.Drawing is v4.0.0

After import all packages,but do nothing.

So , how can i do?

Best wish.