Method for robust asymmetric modulation spatial marking with spatial sub-sampling
Example embodiments relate to a method for applying a special mark, that is invisible to the naked eye, on a medium. That is, the example embodiments provide a method for generating and applying on a medium a digital spatial mark of X×Y dots in accordance with a resolution of d 1 x by d 1 y dots per surface unit, provided that the ratio d 1 x /d 2 x and or d 1 y /d 2 y is more than 1. The method further includes over-sampling the digital spatial mark in X based on a factor nx=d 1 x /d 2 x and in Y based on a factor ny=d 1 y /d 2 y ; eroding the dots to be applied so as to leave one dot every nx dots in X and one dot every nx dots in Y; and applying the spatial mark on the support.
1. A method of generating and applying on a support of a digital spatial marking of X×Y points according to a resolution of d 1 x by d 1 y points per surface unit and intended to be read by a reading device with a resolution of d 2 x by d 2 y points per surface unit, taking into account that the ratio d 1 x /d 2 x and/or d 1 y /d 2 y is larger than 1, the method comprising:
defining a pseudo-random digital mark in a resolution d 2 x by d 2 y,
over-sampling the digital mark in X according to a factor nx=d 1 x /d 2 x and in Y according to a factor ny=d 1 y /d 2 y in order to obtain a digital mark according to a resolution d 1 x by d 1 y,
eroding the X×Y points intended to be applied so as to guarantee that a minimum distance between said X×Y points is respectively nx and ny in horizontal and vertical directions, and
applying the digital spatial marking on the support.
2. The method of generating and applying the digital spatial marking according to claim 1 , wherein the resolution of the reading device is identical in X and in Y (d 2 x =d 2 y ).
3. The method of generating and applying the digital spatial marking according to claim 1 , wherein the resolution of the initial digital spatial marking is identical in X and in Y (d 1 x =d 1 y ).
4. The method of generating and applying the digital spatial marking according to claim 1 , wherein the ratio of resolution in X (nx) and the ratio of resolution in Y (ny) is comprised between 2 and 5, 2 and 5 inclusive.
5. The method of generating and applying the digital spatial marking according to claim 1 , wherein the support is constituted by a printing process.
6. The method of generating and applying the digital spatial marking according to claim 1 , wherein the support is constituted by an engraving process.
7. A method of recognizing the digital spatial marking applied according to the generation method of claim 1 , comprising:
acquiring a digital image of the support,
filtering the acquired digital image to eliminate parts not including the digital spatial marking,
using autocorrelation properties to compensate every affine transformation introduced by the acquisition,
compensating in translation of the digital spatial marking using an intercorrelation between the obtained digital spatial marking and a group of possible positions of the digital spatial marking defined by a key, and
decoding the digital information by statistical correlation for each bit of information.
8. A method of detecting a digital spatial marking according to claim 7 , wherein the filtering of the acquired digital image is based on a compensation of a uniform initial color.
9. A method of detecting a digital spatial marking according to claim 7 , wherein the filtering stage is based on a prediction of the image of the initial support by a soundproofing filter.
10. A method of detecting a digital spatial marking according to claim 7 , wherein the digital acquisition of the image is carried out by a scanner.
11. A method of detecting a digital spatial marking according to claim 7 , wherein the digital acquisition of the image is carried out using a portable detector.
12. A method of detecting a spatial marking according to claim 7 , wherein the acquisition and processing of the spatial marking are carried out in two geographically remote locations.