That's how i've always thought of it.
If you view it as half-empty, you're looking at what's missing from your life as it stands to that point, and perhaps subconsciously want that missing material.
At least that's what I feel this perception test signifies....
Half-full for me.
If the glass has snot and puke in it, then it wouldn't necessarily be seen as being a pessimist if its half empty or an optimist if its half full, because that's just gross and nobody wants a half a glass of snot and puke.
If that is the case, then all of you must be henceforth terminated on the spot without the beastliness of forethought.