Encoding information within text printed on a page using differing gray or color levels

The present invention relates generally to image processing and information encoding and, more particularly, to encoding information within printed images or text using differences in gray or color levels which are imperceptible to the human eye. A document processing system for imperceptibly encoding information in displayed images or text. Printed text or other images are modified using gray levels to encode information within existing text or images. The text is modified using digital processing to provide text or images in which adjacent portions of individual text characters, for example, have slightly different gray levels in accordance with an encoding scheme to encode addition information within the existing text. The modifications to the text is so slight that it is imperceptible to the human eye, yet is easily detectable by a machine such as a scanner.