Editing Doom engine (section)

==Level structure==
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A simple setup demonstrating how ''Doom'' represents levels internally
[[Image:Doom-map-format-map.svg|240px|thumb|Map view in editor]]

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Viewed from the top down, all ''Doom'' levels are actually two-dimensional, demonstrating one of the key limitations of the ''Doom'' engine: [[room-over-room]] is not possible. This limitation, however, has a silver lining: a "map mode" can be easily displayed, which represents the walls and the player's position, much like the first image to the right.

===Basic objects===
The base unit is the [[vertex (geometry)|vertex]], which represents a single 2D point. Vertices (or "vertexes" as they are referred to internally) are then joined to form [[line (mathematics)|lines]], known as "linedefs". Each linedef can have either one or two sides, which are known as "sidedefs". Sidedefs are then grouped together to form [[polygon]]s; these are called "sectors". Sectors represent particular areas of the level.

===Sectors===
Each sector contains a number of properties: a floor height, ceiling height, light level, a floor [[Texture mapping|texture]] and a ceiling texture. To have a different light level in a particular area, for example, a new sector must be created for that area with a different light level. One-sided linedefs therefore represent solid walls, while two-sided linedefs represent bridge lines between sectors.

===Sidedefs===
Sidedefs are used to store wall [[Texture mapping|textures]]; these are completely separate from the floor and ceiling textures. Each sidedef can have three textures; these are called the middle, upper and lower textures. In one-sided linedefs, only the middle texture is used for the texture on the wall. In two-sided linedefs, the situation is more complex. The lower and upper textures are used to fill the gaps where adjacent sectors have different floor and ceiling heights: lower textures are used for steps, for example. The sidedefs can have a middle texture as well, although most do not; this is used to make textures hang in mid air. For example, when a transparent bar texture is seen forming a cage, this is an example of a middle texture on a two-sided linedef.