Robust Sign Language Recognition with Hierarchical Conditional Random Fields

Sign language spotting is the task of detection and recognition of signs (words in the predefined vocabulary) and fingerspellings (a combination of continuous alphabets that are not found in signs) in a signed utterance. The internal structures of signs and fingerspellings differ significantly. Therefore, it is difficult to spot signs and fingerspellings simultaneously. In this paper, a novel method for spotting signs and fingerspellings is proposed, which can distinguish signs, fingerspellings, and nonsign patterns. This is achieved through a hierarchical framework consisting of three steps; (1) Candidate segments of signs and fingerspellings are discriminated with a two-layer conditional random field (CRF). (2) Hand shapes of detected signs and fingerspellings are verified by BoostMap embeddings. (3) The motions of fingerspellings are verified in order to distinguish those which have similar hand shapes and differ only in hand trajectories. Experiments demonstrate that the proposed method can spot signs and fingerspellings from utterance data at rates of 83% and 78%, respectively.