Here’s where there’s a difference between Touch ID and Face ID: Touch ID throws away the original enrollment images of your fingerprints almost immediately. Face ID keeps the original enrollment images of your face (but crops them as tightly as possible so as not to store background information). That’s for convenience. Apple wants to be able to update the neural networks for Face ID without you having to re-register your face each time.
The True Depth camera reads the data and captures a randomized sequence of 2D images and depth maps which are then digitally signed and send to the Secure Enclave for comparison. (Randomization also protects against spoofing attacks.)
The portion of the Neural Engine inside the Secure Enclave converts the captured data into math and the secure Face ID neural networks compare it with the math from the registered face. If the math matches, a “yes” token is released and you’re on your way. If it doesn’t, you need to try again, fall back to passcode, or stay locked out of the device.
None of the neural networks have yet been trained to distinguish multiple registered faces. They can tell you or not you, but not you, someone else, and not either of you. That’s a level of complexity beyond the first iteration of the system. Right now, very few people reportedly register multiple fingers for Touch ID, but Apple could add that functionality to a future implementation of Face ID, if there’s significant demand.
Once the app asks for authentication, it hands off to the system, and all it ever gets back is that authentication or rejection. Apple has a separate system, built into ARKit, the company’s augmented reality framework, that provides basic face tracking for Animoji or any apps that want to provide similar functionality, but it only gets rudimentary mesh and depth data, and never gets anywhere near Face ID data or the Face ID process.
As with his previous iPhone reviews, Rene doesn’t leave out any detail, his reviews are always worth a read.