Huffman’s original paper on line labeling and drawing interpretation is: David A. Huffman, 1971. Impossible objects as nonsense sentences. In Machine intelligence 6: Proceedings of the 6th annual machine intelligence workshop (1970), ed. Bernard Meltzer and Donald Michie, 295–323. Edinburgh: Edinburgh University Press.

You’ll find a well-written summary in Section 4.1 of Nalwa’s A guided tour of computer vision.