A processor is provided with first and second document images. The first image represents an instance of a reference document to which instance a mark has been added. The second image is selected from among a collection of document images and represents the reference document without the mark. The...http://www.google.ca/patents/US5692073?utm_source=gb-gplus-sharePatent US5692073 - Formless forms and paper web using a reference-based mark extraction technique 
