5. Discussion

Emotional communication is a key piece for enhancing HRI, after all it will be very useful if our smart phones, personal computers, cars or busses, and other devices could exploit our emotional information to improve our experience and communication. While nowadays, several proposals for robotic emotional communication are undergoing, emoji as a framework for the latter present a novel approach with high applicability and big usage opportunities. Some of the works presented here discussed the linguistic aspects of emoji, as well as the technical aspects in terms of ML and NLP to R&E emotions, utterances, gestures in texts, which contain emoji. Furthermore, we also presented some related works in the area of HRI, which can easily adopt emoji for imbuing an embodied artificial intelligent entity with the capacity for expressing and recognizing emotional aspects of the communication. On the whole, ML models support these issues, but we do not exclude the important task that involves the processing and transformation of data to reach a suitable input representation for training an appropriate model.

On the other hand, there are several open questions regarding the usage of emoji for emotional communication. For instance, are emoji suitable for the communication of every robotic entity? Emoji are mostly employed in a friendly manner and for maintaining a positive communication. If the objective is to model a virtual human, emoji usage will clearly restrain the spectrum of emotions, which may be detected and expressed due to its knowledge base. An important example to consider is the humanoid robot designed by Hiroshi Ishiguro, the man who made a copy of himself [45]. Ishiguro's proposal is that in order to understand and model emotions, we must first understand ourselves. Hence, this humanoid robot, namely Geminoid HI-1, is capable of displaying ultrarealistic human-like behaviors. However, do we really want to interact with service robots, which may have bad personality traits such as been unsociable and fickle, or whose mood can be affected by heat and noise like a human does? Do we really want to interact with service robots, which can be rude as a real elderly caretaker could? In this sense, emoji usage for the emotional communication may be best suited when the task at hand (e.g., robotic retail store cashier or an educational agent) requires keeping a friendly tone with the human interlocutor. Another question is, should the entire emoji lexicon be used or be restricted only to the core lexicon, which refers to facial expressions? In an ultrarealistic anthropomorphic robot such as Geminoid HI-1, all hand gestures might be carried out by robot's hands itself, thus it should be unnecessary to even fit a screen for displaying a waving emoji ( ) while greeting. On the contrary, more constrained entities such as a Roomba®2 or Pepper® may clearly be benefited from both core and peripheral emoji lexicons for improving its emotional communication with humans. Also, since most of the emoji knowledge is based on short text messages, multimodal data first need to be converted into their corresponding discourse text message, which is, by itself, an open research question.
