Skybox

 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 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 are specialized in creating such source pictures.

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 either type of skyboxes, or both.

Source ports with portals

 * Eternity Engine &mdash; see portals
 * Vavoom &mdash; see SKYBOXES lump
 * ZDoom and derivative &mdash; see skybox