Skybox View Point
Thing type 780, Skybox View Point, is a type of Thing that is used to determine the viewing X, Y and Z coordinates of a skybox, which in turn determines what can be seen of the skybox in the level's sky when the map is first loaded. This Thing should be placed within the sectors making up the skybox itself, rather than the in-level sectors. The Z position of this Thing determines the Z position the skybox is viewed from, relative to the floor height of the sector the Thing is placed in.
If the Special flag is checked, this Thing instead becomes the skybox's center point (or reference point). It determines the in-level position at which the view created by the skybox view point will be displayed – when the player moves away from or towards this point in any direction, the skybox view displayed in the sky will appear to move with the player relative to this point. The speed of the movement in any direction is determined by the level header parameter
SkyboxScale; to modify movement speed along a particular axis rather than all,
SkyboxScaleZ should be used for the X, Y and Z axes respectively.
Note that a skybox center point will not work on its own; a skybox view point needs to exist as well in the map. If a skybox view point is used on its own without a center point, the skybox view will remain in the same position regardless of where the player is in the map.
The Parameter value of this Thing determines the ID of the skybox view point or center point. By default, a map will use the view point and center point with ID 0, but it is possible to switch to a skybox with a different ID via linedef type 448.