Method and system for printing an original image and for determining if a printed image is an original or has been altered
View Patent ↗A method for printing an original image which is protected against copying or alteration, such as a postal indicium, and for determining if that image has been altered. The image includes a copy detection feature and coded information linked to the copy detection feature. Altered images are detected by testing to determine if the link between the copy detection feature and the coded information in fact exists. The copy detection feature and the coded information can be linked by: 1) scanning the image to recover the coded information and the copy detection feature; 2) test the coded information and copy detection feature, and 3) accept the printed image as unaltered if the test indicates that the nominal link exists in fact.
1. A method for determining if a printed image is unaltered by determining if said image includes coded information and a copy detection feature linked to said coded information, said method comprising the steps of:
a) scanning said image to recover said coded information and said copy detection feature;
b) testing said coded information and said copy detection feature; where said copy detection feature includes robust elements and is determined to be unaltered if copy detection feature is determined to have been linked to said coded information by the steps of: generating said copy detection feature as a pseudorandom function of said coded information; creating a robust descriptor of said copy detection feature; and incorporating said descriptor into said coded information, and said testing step comprises the substeps of:
a) regenerating said robust descriptor from said recovered copy detection feature;
b) recovering said robust descriptor from said coded information;
c) comparing said recovered and said regenerated robust descriptors; and
d) if said descriptors are at least substantially similar; then,
e) regenerating said copy detection feature as a pseudorandom function of said coded information;
f) computing a distance between said recovered copy detection feature and said regenerated copy detection feature; and
g) indicating that said link exists if said distance is less than a predetermined threshold; and
h) accepting said printed image as unaltered if said testing step indicates that said coded information and said copy detection feature are linked.
2. A method as described in claim 1 where said descriptors are compared by computing a distance between said descriptors and determining that said descriptors are substantially similar if said distance is less than a predetermined threshold.
3. A method as described in claim 1 , where said image includes information identifying a particular printer used to print said image, comprising the additional steps of:
a) if said distance is greater than said threshold, adding a difference between said distance and said threshold to a copy likelihood index associated with said particular printer; and
b) indicating a possible problem with said particular printer if said copy likelihood index is greater than a second predetermined threshold.
4. A method as described in claim 1 , where said image includes information identifying a printer type used to print said image, comprising the additional steps of:
a) recovering said printer type information; and
b) applying a print-scan model corresponding to said printer type information to said regenerated copy detection feature and said regenerated robust descriptors to transform said regenerated feature and descriptors so as to more closely approximate a scanned image.