Patent ID: 6941026

Claim:
A method of decoding a two-dimensional symbol matrix comprising the steps of: acquiring an image of an object; pre-processing said image to obtain a first filtered image and a second filtered image, wherein said first and second filtered images are obtained using different filters; and evaluating each of said first and second filtered images for a valid symbol, the evaluating step comprising the steps of: defining one of said first and second filtered image as a current image; locating at least one coarse location within said current image that appear to contain a symbol shape to determine a set of symbol candidate locations, the locating step comprising the steps of: measuring a first and second derivative of image intensity values across said image; and identifying areas of said image in which said first and second derivatives correspond to areas approximating predetermined shape and size parameters of said two-dimensional symbol; whereby information for each such identified area comprises a location and an orientation; refining at least one of said symbol candidate locations to obtain a refined symbol image; evaluating said refined symbol image by decoding to determine if it corresponds to a valid symbol; and if said symbol image fails to correspond to a valid symbol, then repeating said steps of locating, refining and evaluating after having redefined said current image to contain said second filtered image.