Determining locations of electro-optical pens

1. An apparatus to determine a location of a pen, the apparatus comprising:

an image accessor to image a scribing surface from a first perspective to produce a first image and to image the scribing surface from a second perspective to produce a second image, the first image having a first field of view and the second image having a second field of view at least partially overlapping the first field of view;

a distance determiner to determine a distance from the pen to a scribing plane at a plurality of points to produce a plurality of distance measurements;

a tilt orientation determiner to determine a tilt orientation of the scribing plane relative to the pen based on the plurality of distance measurements;

a plane of view corrector to, based on the tilt orientation of the scribing plane, correct a plane of view associated with at least one of the first perspective and the second perspective;

a coordinate determiner to determine coordinates of the pen based on the corrected plane of view; and

a location determiner to determine the location of the pen on a display based on the coordinates.

2. The apparatus of claim 1 , wherein the distance determiner to determine the distance from the pen to the scribing plane by cross-correlating marks in the first image and the second image.

3. The apparatus of claim 1 , wherein the plane of view corrector is to correct the plane of view based on a comparison of a first depth of field associated with the first perspective and a second depth of field associated with the second perspective.

4. The apparatus of claim 1 , wherein the plane of view corrector is to correct the plane of view by correcting image distortion.

5. The apparatus of claim 1 , wherein the image accessor includes a stereoscopic camera set.

6. A method comprising:

acquiring, with a stereoscopic camera set, a first image of a scribing surface from a first perspective, the first image having a first field of view, acquiring, with the stereoscopic camera set, a second image of the scribing surface from a second perspective, the second image having a second field of view at least partially overlapping the first field of view;

determining, by executing an instruction with a processor, a distance from the pen to a scribing plane at a plurality of points to produce a plurality of distance measurements;

determining, by executing an instruction with a processor, a tilt orientation of the scribing plane relative to the pen based on the plurality of distance measurements;

based on the tilt orientation of the scribing plane, correcting, by executing an instruction with the processor, a plane of view associated with at least one of the first perspective and the second perspective;

determining, by executing an instruction with the processor, coordinates of the pen based on the corrected plane of view; and

determining, by executing an instruction with the processor, a position of the pen on a display based on the coordinates.

7. The method of claim 6 , wherein the determining of the distance from the pen to the scribing plane includes cross-correlating marks in the first image and the second image.

8. The method of claim 6 , wherein the correcting of the plane of view includes correcting image distortion.

9. The method of claim 6 , wherein the correcting of the plane of view is based on a comparison of a first depth of field associated with the first perspective and a second depth of field associated with the second perspective.

10. The method of claim 9 , wherein the comparison of the first depth of field associated with the first perspective and the second depth of field associated with the second perspective includes identifying a larger of the first and depth of field and the second depth of field.

11. A non-transitory computer readable storage medium comprising instructions that, when executed, cause a machine to at least:

image a scribing surface from a first perspective to produce a first image;

image the scribing surface from a second perspective to produce a second image, the first image having a first field of view and the second image having a second field first of view partially overlapping the first field of view;

identify distances between a pen and a scribing plane at a plurality of points;

determine a tilt orientation of the pen at the plurality of points;

based on the tilt orientation, adjust a plane of view associated with the first perspective based on a comparison of a first depth of field associated with the first perspective and a second depth of field associated with the second perspective;

determine coordinates of the pen based on the adjusted plane of view; and

locate the pen relative to a display based on coordinates.

12. The non-transitory computer readable storage medium of claim 11 , wherein the instructions cause the machine to identify the distances at the plurality of points cross-correlating marks in the first and the second images.

13. The non-transitory computer readable storage medium of claim 11 , wherein the instructions cause the machine to adjust the plane of view by correcting image distortion.

14. The non-transitory computer readable storage medium of claim 13 , wherein the instructions cause the machine to correct image distortion by producing an image with marks relative to a reference grid that enables positions of the marks to be decoded to determine a location of a tip of the pen on the scribing surface.

15. The non-transitory computer readable storage medium of claim 11 , wherein the instructions cause the machine to compare the depth of field associated with the first perspective and the depth of field associated with the second perspective by determining a larger of the depth of field associated with the first perspective and the depth of field associated with the second perspective.