Perceptual hash

For finding duplicates, it would be nice if Pillow would include some perceptual hashing algorithm:
https://en.wikipedia.org/wiki/Perceptual_hashing

The real use case for me is that my in my photo collection I often have the same image in different resolutions, e.g. if I sent it via WhatsApp: Once in full resolution from the camera, once in reduced resolution in WhatsApp.

Here is a blog post describing a simple algorithm that also uses Pillow:
https://www.safaribooksonline.com/blog/2013/11/26/image-hashing-with-python/
but it would be more convenient to have some function builtin,
most interesting something like image.signature or image.phash and then some function to calculate the similarity or distance between two or more images.

One library mentioned quite often in this context is phash:
http://www.phash.org/
Maybe their algorithms could be reused?

If this is out of scope for Pillow I would just use one the projects providing phash python bindings.

No Pillow file or test is named in the issue. Start by reviewing the requested perceptual-hashing API, the linked algorithm description, and the phash project, then clarify which algorithm and comparison behavior are in scope. Done means the project has an agreed implementation and API with corresponding tests.