Sky hack

From DoomWiki.org

A demonstration of the sky hack in E1M1: Hangar.
The sky hack is a feature of the Doom rendering engine that prevents a linedef's upper textures from being displayed between two sectors using both a sky ceiling. When missiles or hitscan traces collide with a sky hack wall, they simply disappear as they do when they collide with the ceiling itself.

On the example picture, taken in a map editor, the ceilings of the front and back sectors are highlighted in blue, the upper texture is highlighted in yellow. This upper texture should "normally" display STARTAN3, but with the sky hack it is not actually drawn, and the sky is seen instead. This allows to have outdoors area with outer walls at differing heights.

The sky hack does not apply to lower textures between two sky floors.