The touch pressure for paper is done with a new touch Radius property. The radius property on touches was available as a private API, but only made available publicly in iOS 8.

The Ink and Adonit touch pixel point, which are basically the same (We manufacture the Ink and Slide for adobe) use a pressure sensitive tip in the pen itself, and communicate that pressure over Bluetooth. This gives you a much higher resolution on the pressure, with the trade off of the BT stack latency.

Is there any way to buy Ink & Slide in the UK?

It's really interesting to me that most of these are just Bluetooth-connected buttons. There's no pressure or angle sensitivity in the FiftyThree Pencil, it's just a button.

I tried to find the "pressure" API in iOS 8, but I just found a new majorRadius field (with an error tolerance). Android supports a bunch of information (including "pressure" and radius) -- I bet you get pretty rich data on devices with dedicated stylus support.

I was able to connect to the Pencil from Android via BTLE and observe the tip being pressed and released (though not the eraser for some reason; maybe I have to write some value out for that characteristic to come alive?) -- maybe later this summer I'll try to make a basic Pencil-compatible Android drawing app :).

In a capacitive touchscreen there really is no way to measure how hard your finger is pressing on the screen. What you can measure is how large of a footprint your meat-finger creates when it smashes down on the glass. The center of the point hasn't changed, but the radius of the circle has.

FWIW, the Ink is a lot more than that, including accelerometer data, and pressure sensitivity.

That's cool; thanks for sharing.

So you could use the accelerometer independently of the touchscreen for gestures like flicking ink blots or swishing the "brush" in "water" -- or maybe you can do some fancy things on the screen like tapping on the same spot for stippling (using the accelerometer to approximate height and velocity, independent of the final pressure on the tip). There's so much fun stuff to do here!

Definitely, that is what we hope developers will start using it for. We expose the accelerometer data in the SDK, so developers can get access to the data. We also use the relative pen position to the iPad to sort out some offset issues to make sure the line appears to be coming from the tip itself.

Does it also include a gyro sensor?