1. A method of searching a digital image of a document for a predetermined keyword comprising one more keyword shapes, the method comprising the steps of: using a computer, identifying a word in the digital image, the word comprising one or more shapes; using a computer, generating a test matrix comprising a difference vector for each shape of the word, a difference vector representing the differences between the visual features of a respective shape and the visual features of a collection of reference shapes; using a computer, generating a template matrix comprising a difference vector for each keyword shape of the keyword; and using a computer, determining a measure of similarity between the word and the keyword by comparing the test matrix and the template matrix.