Determines the overall lightness or darkness of an entire image.

I'm dead lol: a professor was traveling by boat. on his way he asked the sailor: “do you know biology, ecology, zoology, geography, physiology? the sailor said no to all his questions. professor: what the hell do you know on earth. you will die of illiteracy. after a while the boat started sinking. the sailor asked the professor, do you know swiminology & escapology from sharkology? the professor said no. sailor: “well, sharkology & crocodilogy will eat your assology, headology & you will dieology because of your mouthology.