Hexen introduced the double sky effect, also known as parallax sky. If more skies than two are needed in a map, using skyboxes or Static_Init is necessary. The alternate sky is also used by lightning. Normal sectors with a sky flat will show sky1, however sectors of type 200 will show sky2. Since Hexen features two different skies per level, sky1 and sky2 as defined in MAPINFO, it provides a mechanism to use two different skies in the same map, provided that a double sky is not used. It can be overridden by using a skybox or by using the MBF sky transfer effect ( Static_Init property 255). The choice of the texture projected as the sky is set in a MAPINFO map definition with the sky1 and, optionally, sky2 properties. By default, it is F_SKY1 in Doom, Hacx, Heretic and most other games it is, however, F_SKY in Hexen and F_SKY001 in Strife. The flat which is replaced by a sky projection is determined in a MAPINFO GameInfo definition with the skyflatname property. (So, unless using a skybox, sky texture tiling will always happen in OpenGL whereas the effect can be replaced by sky stretching in software.)
The taller the texture, the further below the lower hemisphere will be "pushed away" on this projection, however it will not be removed entirely.
The sky texture is rendered twice, on each hemisphere. The OpenGL renderer used in GZDoom and Zandronum uses a spherical projection instead. ZDoom also allows to use geometrically constructed skyboxes with special cameras instead of an automatically generated cylinder. The sky in Doom engine games is a wall texture cylindrically projected and displayed in place of a flat. For more information on this article, visit the Sky page on the Doom Wiki.
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