Lance Williams

Dynamic hand gesture recognition: An exemplar-based approach from motion divergence fields

Exemplar-based approaches for dynamic hand gesture recognition usually require a large collection of gestures to achieve high-quality performance. Efficient visual representation of the motion patterns hence is very important to offer a scalable solution for gesture recognition when the databases are large. In this paper, we propose a new visual representation for hand motions based on the motion divergence fields, which can be normalized to gray-scale images. Salient regions such as Maximum Stable Extremal Regions (MSER) are then detected on the motion divergence maps. From each detected region, a local descriptor is extracted to capture local motion patterns. We further leverage indexing techniques from image search into gesture recognition. The extracted descriptors are indexed using a pre-trained vocabulary. A new gesture sample accordingly can be efficiently matched with database gestures through a term frequency-inverse document frequency (TF-IDF) weighting scheme. We have collected a hand gesture database with 10 categories and 1050 video samples for performance evaluation and further applications. The proposed method achieves higher recognition accuracy than other state-of-the-art motion and spatio-temporal features on this database. Besides, the average recognition time of our method for each gesture sequence is only 34.53ms.

What characterizes a shadow boundary under the sun and sky

Despite decades of study, robust shadow detection remains difficult, especially within a single color image. We describe a new approach to detect shadow boundaries in images of outdoor scenes lit only by the sun and sky. The method first extracts visual features of candidate edges that are motivated by physical models of illumination and occluders. We feed these features into a Support Vector Machine (SVM) that was trained to discriminate between most-likely shadow-edge candidates and less-likely ones. Finally, we connect edges to help reject non-shadow edge candidates, and to encourage closed, connected shadow boundaries. On benchmark shadow-edge data sets from Lalonde et al. and Zhu et al., our method showed substantial improvements when compared to other recent shadow-detection methods based on statistical learning.

Motion divergence fields for dynamic hand gesture recognition

Although it is in general difficult to track articulated hand motion, exemplar-based approaches provide a robust solution for hand gesture recognition. Presumably, a rich set of dynamic hand gestures are needed for a meaningful recognition system. How to build the visual representation for the motion patterns is the key for scalable recognition. We propose a novel representation based on the divergence map of the gestural motion field, which transforms motion patterns into spatial patterns. Given the motion divergence maps, we leverage modern image feature detectors to extract salient spatial patterns, such as Maximum Stable Extremal Regions (MSER). A local descriptor is extracted from each region to capture the local motion pattern. The descriptors from gesture exemplars are subsequently indexed using a pre-trained vocabulary tree. New gestures are then matched efficiently with the database gestures with a TF-IDF scheme. Our extensive experiments on a large hand gesture database with 10 categories and 1050 video samples validate the efficacy of the extracted motion patterns for gesture recognition. The proposed approach achieves an overall recognition rate of 97.62%, while the average recognition time is only 34.53 ms.

A template-based automatic bird phrase classification in noisy environments using limited training data

Bird Songs typically comprise a sequence of smaller units, termed phrases, separated from one another by longer pauses; songs are thought to assist in mate attraction and territory defense. Studies of bird song would often be helped by automated phrase classification. Past classification studies usually employed techniques from speech recognition, such as MFCC feature extraction and HMMs. Problems with these methods include degradation from background noise, and often require a large amount of training data. We present a novel approach to robust bird phrase classification using template-based techniques. One (or more) template is assigned to each phrase with its specific information, such as prominent time-frequency components. In our trials with 1022 phrases from Cassin’s Vireo (Vireo cassinii) that had been hand-identified into 32 distinct classes, far fewer few examples per class were required for training in some cases only 1 to 4 examples for 84.95%-90.27% accuracy. The choice of distance metrics was...