5. A method, comprising: obtaining an image of at least a portion of a document; processing the image to determine a plurality of image characteristics of the image wherein the plurality of image characteristics comprise at least one of a first image characteristic representing ascending letters in the image, a second image characteristic representing descending letters in the image, a third image characteristic representing a number of letters in a word in the image and a fourth image characteristic representing a number of words in a line of text in the image; and identifying a previously stored source document based at least in part on the plurality of image characteristics.
5. A method, comprising: obtaining an image of at least a portion of a document; processing the image to determine a plurality of image characteristics of the image wherein the plurality of image characteristics comprise at least one of a first image characteristic representing ascending letters in the image, a second image characteristic representing descending letters in the image, a third image characteristic representing a number of letters in a word in the image and a fourth image characteristic representing a number of words in a line of text in the image; and identifying a previously stored source document based at least in part on the plurality of image characteristics. 10. The method of claim 5 , in which the plurality of image characteristics comprises: a fifth image characteristic representing word length of words in the image in standard units of length; a sixth image characteristic representing word length proportional to word heights of words in the image; a seventh image characteristic representing letter height in standard units of length of letters in the image; and an eighth image characteristic representing letter height proportional to an average letter height of letters in the image.