14. A system for retrieving an electronic document using invisible junction features, the system comprising: a processor; a feature extraction unit stored on a memory and executable by the processor, the feature extraction unit having an input and an output for producing an invisible junction feature descriptor from an image, the input of the feature extraction unit coupled to receive an input image; and a feature retrieval unit stored on the memory and executable by the processor, the feature retrieval unit having an input and an output for retrieving information about an output electronic document using the invisible junction feature descriptor and a geometric constraint, the input of the feature retrieval unit coupled to the output of the feature extraction unit, the output of the feature retrieval unit providing the information about the output electronic document.
14. A system for retrieving an electronic document using invisible junction features, the system comprising: a processor; a feature extraction unit stored on a memory and executable by the processor, the feature extraction unit having an input and an output for producing an invisible junction feature descriptor from an image, the input of the feature extraction unit coupled to receive an input image; and a feature retrieval unit stored on the memory and executable by the processor, the feature retrieval unit having an input and an output for retrieving information about an output electronic document using the invisible junction feature descriptor and a geometric constraint, the input of the feature retrieval unit coupled to the output of the feature extraction unit, the output of the feature retrieval unit providing the information about the output electronic document. 19. The system of claim 14 , wherein the information is one from the group of a document identification, a point in the document, a page in the document, a viewing region in the document, and the output document itself.