Patent ID: 7440583

Claim:
A watermark information detecting method comprising: an image inputting step for reading a printed document with confidential information embedded as an input image, by preparing plural dot patterns with a direction of wave and/or wavelength changed according to an arrangement of dots, giving one symbol to one of the dot patterns and arranging the dot patterns combined with each other; a filtering step for obtaining, in each pixel of the input image, a filter type matrix related to a type of detection filter with a maximum output value among all detection filters and a filter output value matrix related to the output value of the detection filter, by performing filtering of the input image after preparing the detection filter having the same wave direction and wavelength as the dot patterns to be the same number of types in order to detect the dot patterns from the input image; a position searching step for determining the position of the dot patterns in order for the sum of the output values of the detection filter corresponding to a grid point of a position searching template to be maximum, while moving the position searching template in each area divided in a predetermined size with regard to the filter output value matrix; a symbol determining step for obtaining a symbol matrix by determining the symbol of the dot patterns embedded in a location determined in the position searching step from the type of the detection filter in the filter type matrix, corresponding to the location; a border determining step for determining a border of the area; and an information decoding step for decoding the confidential information embedded in the printed document based on the dot patterns embedded inside the border.