The Greek painter Apollodorus (active ca. 408 B.C.) was recognized in antiquity as the inventor of a systematic technique for shading to simulate the appearance of mass and space. He achieved this through the modulation of light and shade, a technique which in Greek was called "skiagraphia."
A metaphor is a figure of speech that is used to make a comparison between two things that aren't alike but do have something in common. Unlike a simile, where two things are compared directly using like or as, a metaphor's comparison is more indirect, usually made by stating something is something else. A metaphor is very expressive; it is not meant to be taken literally. You may have to work a little to find the meaning in a metaphor.