Electronic pen with retraction mechanism and force sensor

1. An electronic pen for interacting with a substrate having coded data disposed thereon, the pen comprising:

an image sensor for sensing at least some of the coded data when the pen is used to interact with the surface;

a cartridge having a nib for contacting said surface;

a retraction mechanism for slidably moving said cartridge between a refracted position wherein said nib is retracted within a body of said pen and an extended position wherein said nib protrudes from said body;

a force sensor cooperating with said retraction mechanism, said force sensor being configured to sense a force applied to said surface by said nib when said nib is in said extended position; and

a processor configured to generate, using at least some of the sensed coded data, indicating data indicative of the interaction with the substrate;

wherein said image sensor is configured to sense said coded data only when said force sensor detects that said nib is in contact with said substrate.

2. A pen as claimed in claim 1 , wherein said image sensor is configured to sense said coded data only when a threshold force is sensed by said force sensor.

3. A pen as claimed in claim 1 , wherein said processor is configured to generate force data indicative of the force applied to the surface by the nib.

4. A pen as claimed in claim 3 , wherein said force data is captured as a continuous stream of sensed forces during a stroke.

5. A pen as claimed in claim 1 , further comprising communication means configured to communicate force data indicative of the sensed force to a computer system.

6. A pen as claimed in claim 1 , wherein said cartridge is an ink cartridge having a marking nib.

7. A pen as claimed in claim 1 , wherein said cartridge is a stylus cartridge having a non-marking nib.

8. A pen as claimed in claim 1 , further comprising a user-operable actuator for actuating said retraction mechanism.

9. A pen as claimed in claim 1 , which is configured to enter a quiescent state after a predetermined period of inactivity.

10. A pen as claimed in claim 1 , wherein said processor is configured to initiate wake-up of said pen when said force sensor detects contact between said nib and said substrate.

11. A pen as claimed in claim 1 , wherein said coded data identifies a plurality of positions on the substrate, and said indicating data is indicative of a position of the nib on the substrate.

12. A pen as claimed in claim 1 , wherein said coded data is printed as a plurality of tags, each tag identifying a position, and wherein said indicating data is indicative of a position of the nib on the substrate.

13. A pen as claimed in claim 12 , wherein each tag further identifies a region identity, and said indicating data is further indicative of said region identity.

14. A pen as claimed in claim 12 , wherein said nib is offset from an optical axis of the pen, and wherein said processor is configured to compute the position of the nib from an observed perspective distortion on an imaged tag and a known spatial relationship between the pen's nib and the pen's optical axis.

15. A pen as claimed in claim 14 , wherein said processor is configured to compute the position of the nib using a multiple degree-of-freedom perspective transform inferred from a perspective-distorted imaged tag.

16. A pen according to claim 1 , further comprising memory storing a pen identifier, said identifier distinguishing the pen from other pens of the same type.