Two types of skyboxes exist: model-based and portal-based.
A model-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 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
- EDGE — uses DDFLEVL and DDFIMAGE to define skyboxes.
- Doomsday — Sky definitions at the Doomsday wiki, a new method of defining sky models implemented in Doomsday 1.9 Beta6
- Doomsday and derivatives — Map_Info definitions at the Doomsday wiki, pre Doomsday 1.9 Beta6 method of defining sky models, though it is currently still supported.
- Eternity Engine — see skybox portals at the Eternity Engine wiki
- GLBoom+ — using the GLDEFS method from GZDoom
- Vavoom — see SKYBOXES lump at the Vavoom wiki (archived)
- ZDoom and derivatives — see skybox at the ZDoom wiki