  Multi-Task Learning has emerged as a methodology in which multiple tasks are
jointly learned by a shared learning algorithm, such as a DNN. MTL is based on
the assumption that the tasks under consideration are related; therefore it
exploits shared knowledge for improving performance on each individual task.
Tasks are generally considered to be homogeneous, i.e., to refer to the same
type of problem. Moreover, MTL is usually based on ground truth annotations
with full, or partial overlap across tasks. In this work, we deal with
heterogeneous MTL, simultaneously addressing detection, classification &
regression problems. We explore task-relatedness as a means for co-training, in
a weakly-supervised way, tasks that contain little, or even non-overlapping
annotations. Task-relatedness is introduced in MTL, either explicitly through
prior expert knowledge, or through data-driven studies. We propose a novel
distribution matching approach, in which knowledge exchange is enabled between
tasks, via matching of their predictions' distributions. Based on this
approach, we build FaceBehaviorNet, the first framework for large-scale face
analysis, by jointly learning all facial behavior tasks. We develop case
studies for: i) continuous affect estimation, action unit detection, basic
emotion recognition; ii) attribute detection, face identification.
  We illustrate that co-training via task relatedness alleviates negative
transfer. Since FaceBehaviorNet learns features that encapsulate all aspects of
facial behavior, we conduct zero-/few-shot learning to perform tasks beyond the
ones that it has been trained for, such as compound emotion recognition. By
conducting a very large experimental study, utilizing 10 databases, we
illustrate that our approach outperforms, by large margins, the
state-of-the-art in all tasks and in all databases, even in these which have
not been used in its training.
