The result of emotion are like this: happiness, sadness, surprise, anger, fear, contempt, disgust or neutral.
You can tap hte shutter button when the app recognizes "face" with square frame.
As mental stress goes less when you are happy because parasympathetic nerve system is dominant over sympathetic one which cause active oxygen and aging. As a means of anti-aging tool, this might be usable app, I wish.
I wish you a happy day!