  Most research on facial expression recognition (FER) is conducted in highly
controlled environments, but its performance is often unacceptable when applied
to real-world situations. This is because when unexpected objects occlude the
face, the FER network faces difficulties extracting facial features and
accurately predicting facial expressions. Therefore, occluded FER (OFER) is a
challenging problem. Previous studies on occlusion-aware FER have typically
required fully annotated facial images for training. However, collecting facial
images with various occlusions and expression annotations is time-consuming and
expensive. Latent-OFER, the proposed method, can detect occlusions, restore
occluded parts of the face as if they were unoccluded, and recognize them,
improving FER accuracy. This approach involves three steps: First, the vision
transformer (ViT)-based occlusion patch detector masks the occluded position by
training only latent vectors from the unoccluded patches using the support
vector data description algorithm. Second, the hybrid reconstruction network
generates the masking position as a complete image using the ViT and
convolutional neural network (CNN). Last, the expression-relevant latent vector
extractor retrieves and uses expression-related information from all latent
vectors by applying a CNN-based class activation map. This mechanism has a
significant advantage in preventing performance degradation from occlusion by
unseen objects. The experimental results on several databases demonstrate the
superiority of the proposed method over state-of-the-art methods.
