1. A method of generating fine-grain fingerprints for identifying content in a rendered document, the method comprising: applying image-based techniques to identify patterns in a document rendered by an electronic document rendering system, irrespective of a file format in which the rendered document was electronically created, the applying of the image-based techniques including: i. scanning, by at least one electronic processor, the rendered document one scanline at a time to identify image pixels, wherein at each current pixel a determination is made by the at least one electronic processor whether the local neighborhood of pixels corresponding to a local image neighborhood is a member of a new or previously existing local image neighborhood, ii. identifying candidate keypoints at locations in a local image neighborhood of the document, by the at least one electronic processor, and iii. combining the locations of the candidate keypoints to form fine-grain fingerprints indentifying patterns representing content in the document, by the at least one electronic processor.