Error tolerant system for reading tags

A tag comprises cells represented by marks, such as white and black, arranged in a predetermined pattern. Reference cells are used to determine the edges of the tag and orientation, while others are used to encode data and error correction data. During processing, a tag is detected based on a variability metric within a given region of an image exceeding a threshold value. Once detected, the orientation of the tag is determined using the reference cells. Confidence values, indicative of a likelihood that the cell is a particular cell value, such as white or black, are determined for each cell in a first set of cell values. If the first set is invalid, the cell values for a set of lowest confidence cell values are inverted to produce a second set of cell values. The second set is tested, and if valid, is used to produce output data.