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: model-based and portal-based.

A model-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 more advanced variant of model-based skybox is the sky model system used by Doomsday, where the sky geometry need not be a cube and can instead be any form, or even consist of multiple models.

A portal-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

 * 3DGE &mdash; uses DDFLEVL and DDFIMAGE to define skyboxes.
 * Doomsday &mdash;, a new method of defining sky models implemented in Doomsday 1.9 Beta6
 * Doomsday and derivatives &mdash;, pre Doomsday 1.9 Beta6 method of defining sky models, though it is currently still supported.
 * Eternity Engine &mdash; see
 * GLBoom+ &mdash; using the method from GZDoom
 * Vavoom &mdash; see
 * ZDoom and derivatives &mdash; see