EMR pens do NOT report the pens tilt to the tablet. For example it doesn't tell the tablet "I am tilted by 5 degrees"
Instead, an EMR digitizer can detect the tilt of the pen just by examining the strength and shape of the signal the digitizer coils receive from the pen.
When the pen is perpendicular, the digitizer detects a single perpendicular shape.
As the pen tilts, it produces two shapes. And the relationship between the two shapes indicates the tilt.
For example, in the diagram below, the pen is pointing to the lower left - meaning the top part of the pen is "falling" toward the lower left of the tablet.
The work for the EMR sensor and tablet firmware is to detect that there are two signal peaks and then disambiguate them.
Once two peaks are established and their relative strengths and positions are known, then the direction of tilt and amount of tilt can be established and then reported to the computer.
The specific algorithm to do this has, to my knowledge, never been exactly revealed. But those knowledgeable in how how signals like this are processed will probably have a clearer idea of how this tilt detection be done.
The tablet measures the tilt in both the x and y directions
Measuring as x tilt and y tilt is also equivalent to measuring as azimuth and altitude. Think of azimuth as an angle from the tablet measured from "north" and the altitude as how high the far end of the pen is from the tablet surface.
In the example below the orange lines indicator the azimuth. And the purple line indicates the altitude (also known as elevation).
