Doom Wiki

Skyboxes are different methods by which a fully 3D sky can be constructed (instead of the cylinder projection of the normal sky textures, or the dome projection commonly used in OpenGL ports).

Two types of skyboxes exist: texture-based and geometry-based.

A texture-based skybox uses cube mapping to project the sky texture on all six directions (east, west, north, south, up, down). The source images must be distorted in a way that compensates for the cube projection, or the skybox's boxy nature becomes too apparent. Programs such as Terragen are specialized in creating such source pictures.

A more advanced variant of texture-based skyboxes is the sky model system used by Doomsday. Very similar to the above except that the model doesn't have to be a cube and can instead be any shape or even multiple models.

A geometry-based skybox uses a special "camera" object placed in a part of the level (normally inaccessible to the player) which is used as the point of reference for projecting the sky. This method, basically a type of portal, is usually combined with normal sky or a texture-based skybox.

Source ports which have implemented skyboxes may provide one or more of the above types of skyboxes.

Source ports with skyboxes