Data-driven Dimensional Expression Generation via Encapsulated Variational Auto-Encoders

Concerning facial expression generation, relying on the sheer volume of training data, recent advances on generative models allow high-quality generation of facial expressions free of the laborious facial expression annotating procedure. However, these generative processes have limited relevance to the psychological conceptualised dimensional plane, i.e., the Arousal-Valence two-dimensional plane, resulting in the generation of psychological uninterpretable facial expressions. For this, in this research, we seek to present a novel generative model, targeting learning the psychological compatible (low-dimensional) representations of facial expressions to permit the generation of facial expressions along the psychological conceptualised Arousal-Valence dimensions. To generate Arousal-Valence compatible facial expressions, we resort to a novel form of the data-driven generative model, i.e., the encapsulated variational auto-encoders (EVAE), which is consisted of two connected variational auto-encoders. Two harnessed variational auto-encoders in our EVAE model are concatenated with a tuneable continuous hyper-parameter, which bounds the learning of EVAE. Since this tuneable hyper-parameter, along with the linearly sampled inputs, largely determine the process of generating facial expressions, we hypothesise the correspondence between continuous scales on the hyper-parameter and sampled inputs, and the psychological conceptualised Arousal-Valence dimensions. For empirical validations, two public released facial expression datasets, e.g., the Frey faces and FERG-DB datasets, were employed here to evaluate the dimensional generative performance of our proposed EVAE. Across two datasets, the generated facial expressions along our two hypothesised continuous scales were observed in consistent with the psychological conceptualised Arousal-Valence dimensions. Applied our proposed EVAE model to the Frey faces and FERG-DB facial expression datasets, we demonstrate the feasibility of generating facial expressions along with the conceptualised Arousal-Valence dimensions. In conclusion, to generate facial expressions along the psychological conceptualised Arousal-Valance dimensions, we propose a novel type of generative model, i.e., encapsulated variational auto-encoders (EVAE), allowing the generation process to be disentangled into two tuneable continuous factors. Validated in two publicly available facial expression datasets, we demonstrate the association between these factors and Arousal-Valence dimensions in facial expression generation, deriving the data-driven Arousal-Valence plane in affective computing. Despite its embryonic stage, our research may shed light on the prospect of continuous, dimensional affective computing.