[aniviacat]

AFAIK Pixel phones, including the Pixel 9, only use 2D images for face unlock. So it's definitely possible to reach mainstream quality with conventional cameras.

(Unless you'd argue that the face unlock found on Pixels is not passable either)

[MengerSponge]

I don't know how Google does it, but it's possible to extract 3d information from a 2d sensor. You either need a variable focus or phase detection in the sensor.

[westurner]

It is possible to infer phase from second order intensity via the Huygens-Steiner theorem for rigid body rotation, FWIU: https://news.ycombinator.com/item?id=42663342 .. https://news.ycombinator.com/item?id=37226121#37226160

Doesn't that mean that any camera can be used to infer phase (and thus depth for face ID, which is a high risk application)?

> variable focus

A light field camera (with "infinite" focus) would also work.

[MengerSponge]

Very cool. Yes, probably? I'll have to think about the relationship between image quality and the fidelity of the derived phase measurement, because it's not obvious how good a camera needs to be to be "good enough" for a secure system.

Light field? I remember Lytro! Such cool technology that never found its niche. https://en.wikipedia.org/wiki/Lytro

Is anybody making a successor product?

[westurner]

I guess the task is to design an experiment to test the error between phase inferred from intensity in a digital camera by Huygens-Steiner and a barycentric coordinate map And far more expensive photonic phase sensors.

Is (framerate-1 Hz) a limit, due to the discrete derivative being null for the first n points?

Fortunately this article explained the implications of said breakthrough; "Physicists use a 350-year-old theorem [Huygens-Steiner] to reveal new properties of light waves" https://phys.org/news/2023-08-physicists-year-old-theorem-re... :

> This means that hard-to-measure optical properties such as amplitudes, phases and correlations—perhaps even these of quantum wave systems—can be deduced from something a lot easier to measure: light intensity.

IDK what happened with wave field cameras like the Lytro. They're possibly useful for face ID, too?

"SAR wavefield". There's a thing.

From https://news.ycombinator.com/item?id=32819838 :

> Wave Field recordings are probably [would probably be] the most complete known descriptions of the brain and its nonlinear fields?