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I have a few questions regarding the technical details of the Apple Pencil:

  1. How does the iPad differentiate between the Pencil and your hand (palm rejection)? Is the Pencil creating some sort of a stronger electrostatic field at its tip compared to the intrinsic "field" of our skin, which the iPad's touch screen then measures?

    • I have a 3rd party active stylus that does not require a bluetooth connection with the iPad. For this stylus the palm rejection works just as fine as with the Apple Pencil. So clearly, this feature must work independently from the BT connection with the iPad. The electric field created at the tip is the only plausible explanation that comes to my mind.
    • The iPad also offers a setting called "Only draw with Apple Pencil" in which case the iPad won't allow any drawing with fingers; fingers will be used only for scrolling. This mode also works w/o any issues for the 3rd party stylus. Again, my assumption of a different electric field (compared to human skin) created by the pencil tip would explain how the iPad can tell Pencil and Finger apart.
  2. Why doesn't the Apple Pencil work with other devices, such as smartphones (I tried with my OnePlus 6 and it didn't work)?

    • Based on my assumptions from question 1, shouldn't the Apple Pencil work with any capacitive touch screen? However, this is not the case - it only works with supported iPads (also applies to my 3rd party stylus).
  3. How does the tilt detection (eg, for drawing shading) work?

    • Tilt detection also works with my 3rd party stylus, so again, this feature must work w/o the BT connection.
    • My assumption: The touch screen measures the capacitance between screen and tip. Depending on the tilt angle, this capacitance changes, so the iPad can calculate the tilt of the Pencil.

I watched this video, but it didn't answer any of my questions above (kind of expected, because it was the Apple Pencil ad).

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    \$\begingroup\$ It uses a proprietary technology that enables the integrated USB connector to snap off if you look at it incorrectly. \$\endgroup\$
    – Jason C
    Jul 11 at 9:27
  • \$\begingroup\$ If the pencil is not charged, does it work at all? \$\endgroup\$
    – Matt
    Jul 11 at 11:56
  • \$\begingroup\$ @Matt When I turn off my 3rd party stylus it doesn't work anymore. I guess the same would apply to when the Apple Pencil is discharged. \$\endgroup\$
    – Amplify
    Jul 11 at 12:04
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How does the iPad differentiate between the Pencil and your hand (palm rejection)?

A stylus is small, a palm is large. Software differentiates between small and large, ignores large. So that's why your 3rd-party stylus works just as well.

The pencil's own inertial sensing is, of course, not affected by the palm at all, and is probably used to increase accuracy of the drawing, or maybe replaces the capacitive sensing alltogether.

Why doesn't the Apple Pencil work with other devices, such as smartphones (I tried with my OnePlus 6 and it didn't work)?

Why should it? That's a business decision that would be very unlike Apple to make.

How does the tilt detection (eg, for drawing shading) work?

Inertial sensor in the stylus, inertial sensor in the iPad, I pad hence can calculate relative attitude.

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  • \$\begingroup\$ I think you are missing something here. If the large vs small touch area is how the iPad differentiates the two then this implies its just a normal touch input. If so, it would work with any other capacitive touch device. The fact is you need to charge the pencil. Its an active input, and doesnt need to be differentiated from the palm based on size. \$\endgroup\$
    – Matt
    Jul 11 at 1:29
  • \$\begingroup\$ Well, the sensors inside the stylus supplementing or replacing the capacitive touch sensing are explicitly mentioned, in the answer, @Matt. \$\endgroup\$
    – mmmm
    Jul 11 at 2:23
  • \$\begingroup\$ @mmmm At best the section you refer to is speculation when OP requested a definitive answer. The 3rd party stylus is active as well so without further evidence this line of reasoning is incomplete. An active stylus device doesnt rely on small vs large touch area to tell the stylus apart from the finger. The fact that none of these styluses work with other devices just about proves they arent regular touch input. If they arent touch input, its not small vs large touch area differentiation and this answer is wrong. \$\endgroup\$
    – Matt
    Jul 11 at 11:15
  • \$\begingroup\$ @Matt huh? How is looking at a teardown, seeing a microcontroller, an inertial sensor and then infering these are used to enhance or replace the touch sensing input pure speculation? They might not be great touch styli (high impedance between tip and hand of holder?), but as long as you can prove they are magically able to not change the capacity of the touch sensor when they approach the surface of the screen, I'd presume the effect is still there. It might not be strong – and thus other devices correct it away in software, again. The iPad can know the device is there, and thus adjust. \$\endgroup\$ Jul 11 at 11:26
  • \$\begingroup\$ @Marcus The words "might" and "maybe replaces" are clearly speculation. If you werent speculating you would have said "replaces" and wouldnt have said "might" at all. Adding this detail about adjusting touch thresholds when the pen is nearby to the answer would fill in most of the gaps and leave a self consistent answer. Still seems like a lot of speculation though. Your teardown doesnt seem to have told you a lot. Obviously the pencil contains electronics, its an active stylus. \$\endgroup\$
    – Matt
    Jul 11 at 11:31

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