Editing Doom engine (section)

===Floor and ceiling===
The system for drawing floors and ceilings ("flats") is less elegant {{Says who|date=February 2023}} than that used for the walls. Flats are drawn with a [[flood fill]]-like algorithm. Because of this, if a bad BSP builder is used, it is sometimes possible to get "holes" where the floor or ceiling bleeds down to the edges of the screen, a visual error commonly referred to as a "slime trail".<ref>[https://doomwiki.org/w/index.php?title=Slime_trail&oldid=193418 Slime trail - The Doom Wiki]</ref> This is also the reason why if the player travels outside of the level using the [[Noclip mode|noclip cheat]] the floors and ceilings will appear to stretch out from the level over the empty space.

The floor and ceiling are drawn as "visplanes". These represent horizontal runs of texture, from a floor or ceiling at a particular height, light level and texture (if two adjacent sectors have exactly the same floor, these can get merged into one visplane). Each x position in the visplane has a particular vertical line of texture which is to be drawn.

Because of this limit of drawing one vertical line at each x position, it is sometimes necessary to split visplanes into multiple visplanes. For example, consider viewing a floor with two [[concentric]] squares. The inner square will vertically divide the surrounding floor. In that horizontal range where the inner square is drawn, two visplanes are needed for the surrounding floor.

''Doom'' contained a static limit on the number of visplanes; if exceeded, a "visplane overflow" would occur, causing the game to exit to DOS with one of two errors, "No more visplanes!" or "visplane overflow (128 or higher)". The easiest way to invoke the visplane limit is a large checkerboard floor pattern; this creates a large number of visplanes.

As the segs are rendered, visplanes are also added, extending from the edges of the segs towards the vertical edges of the screen. These extend until they reach existing visplanes. Because of the way this works, the system is dependent on the fact that segs are rendered in order by the overall engine; it is necessary to draw nearer visplanes first, so that they can "cut off" by others further away. If unstopped, the floor or ceiling will "bleed out" to the edges of the screen, as previously described. Eventually, the visplanes form a "map" of particular areas of the screen in which to draw particular textures.

While visplanes are constructed essentially from vertical "strips", the actual low level rendering is performed in the form of horizontal "spans" of texture. After all the visplanes have been constructed, they are converted into spans which are then rendered to the screen. This appears to be a trade off: it is easier to construct visplanes as vertical strips, but because of the nature of how the floor and ceiling textures appear it is easier to draw them as horizontal strips.