Foreshortening is just a property of linear perspective. When artists draw in linear perspective, they intentionally distort the distances between points/lines (such as the sides of a cube) to become shorter as the points move farther away from the observer. That distortion of points moving ‘closer’ together as they get farther from the observer is foreshortening. This mimics how we see things in the real world and makes drawings look like they actually have real depth (despite, ultimately, just being 2D images).
Latest Answers