You don't need two eyes to
There are two key criteria that we use to judge the quality of a particular facial expression: clarity and intensity. If a face has high clarity, more than 80-90% of viewers will agree on the emotion it is portraying. Intensity, on the other hand, is a measure of how strong that emotion appears to be. As you might imagine, faces with high intensity generally also have high clarity, and less intense faces are somewhat harder to interpret.
But not necessarily! With certain expressions, like anger, it is possible to have faces with both low intensity and high clarity, allowing an artist or animator to successfully portray a full range of emotional states, from mere irritation to full-on rage like our one-eyed, mythological friend, the Cyclops.
Below, I have created a serial lineup of a progressively angrier face all with high clarity, from least intense to most intense, based on my test results:
One of our research goals has been to determine the optimal patterns for the cardinal expressions, the “secret” formula that will make joy, or surprise, or sadness, the most recognizable. And there is no doubt that such patterns exist, because faces which contain the right configuration of features will be recognized by random viewers in exactly the same way, time and time again.