It's all about mystery, exoticism, masks and the art of undercover.
The deliberate omission of information is attributed to Leonardo, but there were many Renaissance artists who used the device of uncertainty to enhance the effect 'mirage' in his portraits. Evaporating key areas of recognition of expression (such as eye and mouth corners), it is relegated to the opinion of the viewer. That is why some works, according to our moods, behave differently making it a magic trick. Read Less