7. Two-Stream CNNs for Gesture-Based Verification and Identification: Learning User Style

A gesture is a short body motion that contains both static (nonrenewable) anatomical information and dynamic (renewable) behavioral information. Unlike traditional biometrics such as face, fingerprint, and iris, which cannot be easily changed, gestures can be modified if compromised. We consider two types of gestures: full-body gestures, such as a wave of the arms, and hand gestures, such as a subtle curl of the fingers and palm, as captured by a depth sensor (Kinect v1 and v2 in our case). Most prior work in this area evaluates gestures in the context of a “password,” where each user has a single, chosen gesture motion. Contrary to this, we aim to learn a user’s gesture “style” from a set of training gestures. This allows for user convenience since an exact user motion is not required for user recognition. To achieve the goal of learning gesture style, we use two-stream convolutional neural networks, a deep learning framework that leverages both the spatial (depth) and temporal (optical flow) information of a video sequence. First, we evaluate the generalization performance during testing of our approach against gestures of users that have not been seen during training. Then, we study the importance of dynamics by suppressing the use of dynamic information in training and testing. Finally, we assess the capacity of the aforementioned techniques to learn representations of gestures that are invariant across users (gesture recognition) or to learn representations of users that are invariant across gestures (user style in verification and identification) by visualizing the two-dimensional t-Distributed Stochastic Neighbor Embedding (t-SNE) of neural network features. We find that our approach outperforms state-of-the-art methods in identification and verification on two biometrics-oriented gesture datasets for full-body and in-air hand gestures.