Machine-readable symbols

1. A multimode machine-readable symbol comprising:

a first machine-readable symbol instantiated on an object at a first location, the first machine-readable symbol readable by a first methodology; and

a second machine-readable symbol instantiated on the object at a second location that at least partially overlaps the first location, the second machine-readable symbol readable by a second methodology different from the first methodology, wherein at least one of said first and second methodologies is one of a topographic methodology and a polarization methodology.

2. The multimode machine-readable symbol recited in claim 1 further comprising a third machine-readable symbol instantiated on the object at a third location that overlaps the first and second locations, the third machine-readable symbol readable by a third methodology different from the first and second methodologies.

3. The multimode machine-readable symbol recited in claim 1 wherein the second machine-readable symbol comprises a pattern of marks formed in a surface of the object.

4. The multimode machine-readable symbol recited in claim 3 wherein the first machine-readable symbol comprises a barcode printed on the surface of the object.

5. The multimode machine-readable symbol recited in claim 3 wherein the first machine-readable symbol comprises a barcode printed on a conformal layer applied over the surface of the object.

6. The machine-readable symbol recited in claim 1 , wherein said second machine-readable symbol comprises topographic features formed in a substrate beneath said machine-readable symbol.

7. The machine-readable symbol recited in claim 1 , wherein said second machine-readable symbol comprises topographic features combined together with said first machine-readable symbol.

8. The machine-readable symbol recited in claim 1 , wherein said first machine-readable symbol and said second machine-readable symbol are combined to define a code of higher order than binary.

9. A method of decoding a multimode machine-readable symbol having a first machine-readable symbol instantiated on an object at a first location and a second machine-readable symbol instantiated on the object at a second location that at least partially overlaps the first location, the method comprising:

acquiring an image of the multimode machine-readable symbol;

processing the image to separately evaluate first and second characteristics of the image, wherein the first characteristics arise from the first machine-readable symbol and the second characteristics arise from the second machine-readable symbol;

applying a first methodology to decode the first machine-readable symbol from the evaluated first characteristics; and

applying a second methodology to decode the second machine-readable symbol from the second evaluated characteristics, wherein at least one of said first and second methodologies is one of a topographic methodology and a polarization methodology.

10. The method recited in claim 9 wherein the first and second methodologies are different.

11. The method recited in claim 9 wherein:

the first characteristics comprise reflectance characteristics; and

the second characteristics comprise topographic characteristics.

12. The method recited in claim 9 wherein processing the image comprises applying a photometric stereo technique to the image.

13. The method recited in claim 9 wherein:

the multimode machine-readable symbol further has a third machine-readable symbol instantiated on the object at a third location that overlaps the first and second locations; and

processing the image comprises separately evaluating third characteristics of the image, wherein the third characteristics arise from the third machine-readable symbol,

the method further comprising applying a third methodology to decode the third machine-readable symbol from the third evaluated characteristics.

14. The method in claim 9 , wherein said second machine-readable symbol comprises topographic features formed in a substrate beneath said machine-readable symbol.

15. The method recited in claim 9 , wherein said second machine-readable symbol comprises topographic features combined together with said first machine-readable symbol.

16. The method recited in claim 9 , wherein said first machine-readable symbol and said second machine-readable symbol are combined to define a code of higher order than binary.