What if the glass isn't exactly half full?

[Igo kyu]

Suppose the glass is 1/3 full.

You can legitimately call that half empty, because over half of it is empty, but if you called it half full, you'd be exaggerating.

Equally, what if the glass is 2/3 full?

Then, if you called it half empty, you'd be telling porkies, but if you said it was half full you'd be telling a half truth at worst.