It is a serious mathematical and logical question, called "unit rule" in mathematics. The same you can say for a half closed window or half open window. For me, both are correct, a glass half full, or half empty.

It can be any way.

Hi... It depends on the situation and my mood...

The glass is something which I fill, then empty, then refill...

Whether it is half full or half empty, I always look at the half-full part of the glass. :) I'm an optimist.