[Frankos]

If you are on an Android device or an iPad with a TrueDepth camera, you will find the face gesture functionality in the Settings

What does TrueDepth camera mean?
I've recently ordered an android tablet online especially to use this facial recognition feature.
I have a Boox e-ink tablet but that does not have a camera at all so I decided to get a new tablet that has one.
I've purchased a Doogee T30 Pro, I hope this is going to work.

[Zubersoft]

I can't speak to that device, but in theory it should work fine. 

As far as the TrueDepth camera, this is what is written in the first article that came up with a search:

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What is a TrueDepth camera?

TrueDepth is what Apple calls the technology in the iPhone’s front camera that powers the phone’s Face ID and Memoji features.

The camera system can also automatically adjust settings like brightness and volume based on whether or not you’re paying attention to your phone, and is used to create depth with Portrait mode.

How does it work?

The TrueDepth camera works by projecting thousands of invisible dots onto your face and analysing them to create a depth map of your face. The camera will also capture an infrared image of your face to help in providing accurate face data.

The neural engine in the Bionic chip then works to transform the depth map and infrared image into a mathematical representation of your face and – in the case of Face ID – will compare that data with your enrolled facial data to confirm they match and unlock your phone.

Because the TrueDepth camera captures depth information that isn’t found in 2D images, it makes it significantly harder for others to break into your phone using a photo or mask of your face. The camera can even detect if your eyes are closed or directed away from the device to prevent anyone from unlocking your phone when you’re asleep or paying less attention.

The technology is smart enough to adapt to changes in your appearance, like make-up, glasses, facial hair and hats, but, if you’ve ever owned an iPhone, you’ll know Face ID can stumble when it comes to bigger adjustments, like face masks (though Apple has updated the software to deal with this in response to the pandemic).

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A TrueDepth camera is required to use ARKit which is what I'm utilizing for the face gestures. Once I get fixes for the PDF library, then I'll be able to use standard cameras with Google's library to do the facial landmark detection on older devices (although it's less accurate than Apple's solution due to the hardware difference).

Mike