Free look (or in a single word freelook) is the ability ZDoom offers of looking up and down. Although Doom and other games directly based on Doom such as Chex Quest or Hacx did not feature free look, this was implemented in Heretic, Hexen and Strife through a method known as Y-shearing: the render is still performed horizontally by the engine, but the area rendered is moved away from the camera. The use of this method in vintage 2.5D engines can be identified by the simple fact that vertical lines (that is to say, the walls) remain vertical on the screen, as in two-point perspective. This effect is often called a distortion as it does not correspond to natural perceptions. Y-shearing is also limited, in that greater the angle of the pitch from the horizontal, the larger the area to render becomes. Looking up 90° up or down with this method would be an infinite task, and is therefore not possible.
Polygonal engines, such as the OpenGL renderer offered by some source ports, allow a true free look, including straight up and down, without distortion. However, looking at extreme angles will still cause problems for the engine, notably with sprite rendering, since sprites remain strictly 2D. This issue can be addressed through billboarding, but it is merely a choice between two imperfect solutions.
Freelook can be enabled or disabled from the gameplay options menu or in a MAPINFO map definition. If enabled, it is generally used with mouse look (the always mouselook option corresponds to the freelook console variable), but it is also possible to use keyboard look instead.