Stylus tilt tracking with a digitizer

1. A method comprising:

detecting a signal emitted by a stylus at a plurality of coordinates on a digitizer sensing surface;

determining coordinates of a writing tip of the stylus;

defining at least one feature characterizing an asymmetrical spread of the stylus signal on the digitizer sensing surface with respect to the writing tip coordinates;

defining asymmetry in a peak formed by amplitudes of the signal detected at the plurality of coordinates; and

detecting an orientation of the stylus with respect to a digitizer sensing surface based on the characterization of the asymmetry in the peak.

2. The method according to claim 1 , wherein the at least one feature is a gradient of a decaying function defined to characterize decay of amplitude from the coordinates of a writing tip as a function of distance from the coordinates of the writing tip.

3. The method according to claim 1 , wherein the at least one feature is asymmetry in rate of decay from opposite sides of the coordinates of a writing tip.

4. The method according to claim 1 , wherein the at least one feature includes a first rate of a decay of amplitude on one side of the writing tip coordinates along one axis of the digitizer sensor compared to a second rate of a decay of amplitude on an opposite side of the writing tip coordinates along the one axis of the digitizer sensor.

5. The method according to claim 4 , wherein the at least one feature includes a third rate of a decay of amplitude on one side of the writing tip coordinates along a second axis of the digitizer sensor compared to a third rate of a decay of amplitude on an opposite side of the writing tip coordinates along that second axis of the digitizer sensor.

6. The method according claim 1 , comprising correcting the writing tip coordinates determined, responsive to the orientation of the stylus as characterized.

7. The method according to claim 1 , wherein characterizing an orientation of the stylus includes determining tilt angle of the stylus toward the digitizer sensing surface.

8. The method according to claim 1 , wherein characterizing an orientation of the stylus includes determining a direction of a tilt of the stylus.

9. The method according to claim 1 , determining the writing tip location is determined from an amplitude pattern obtained from the spread of the stylus signal on the digitizer sensing surface.

10. The method according to claim 9 , wherein the writing tip coordinates is defined as coordinates at which amplitude of the detected stylus signal is estimated to have a peak.

11. The method according to claim 9 comprising defining an envelope over the amplitude pattern obtained from the spread of the stylus signal on the digitizer sensing surface; and defining the writing tip coordinates at a location of a peak of the envelope.

12. The method according to claim 9 comprising defining a pair of decay functions that characterize decays of amplitude on either side of an estimated area of a peak and defining the writing tip coordinates at a location at which the pair of decay functions intersect.

13. The method according to claim 1 , wherein the orientation of the stylus is reported to a host associated with the digitizer sensor.

14. A method comprising:

tracking first orientation of a digitizer sensing surface in three dimensions with respect to gravity;

tracking second orientation of a stylus interacting with the digitizer sensing surface in three dimensions with respect to gravity, wherein the first orientation of the digitizer sensing surface is tracked concurrently with tracking of the second orientation of the stylus;

comparing the first orientation of the digitizer sensing surface with the second orientation of the stylus; and

detecting a third orientation of the stylus with respect to the digitizer sensing surface responsive to the comparing.

15. The method according to claim 14 , wherein the first orientation is tracked with an accelerometer rigidly connected to the digitizer sensing surface.

16. The method according to claim 14 , wherein the second orientation is tracked with an accelerometer positioned on the stylus.

17. The method according to claim 14 , wherein the third orientation is tilt angle of the stylus toward the digitizer sensing surface.

18. The method according to claim 14 , wherein the third orientation includes direction of a tilt of the stylus with respect to the digitizer sensing surface.