Traditionally, facial expression recognition (FER) issues have been studied mostly based on modalities of 2D images, 2D videos, and 3D static models. In this paper, we propose a spatio-temporal expression analysis approach based on a new modality, 3D dynamic geometric facial model sequences, to tackle the FER problems. Our approach integrates a 3D facial surface descriptor and Hidden Markov Models (HMM) to recognize facial expressions. To study the dynamics of 3D dynamic models for FER, we investigated three types of HMMs: temporal 1D-HMM, pseudo 2D-HMM (a combination of a spatial HMM and a temporal HMM), and real 2D-HMM. We also created a new dynamic 3D facial expression database for the research community. The results show that our approach achieves a 90.44% person-independent recognition rate for distinguishing six prototypic facial expressions. The advantage of our method is demonstrated as compared to methods based on 2D texture images, 2D/3D Motion Units, and 3D static range models. Further experimental evaluations also verify the benefits of our approach with respect to partial facial surface occlusion, expression intensity changes, and 3D model resolution variations.
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