Subsector

From Doom Wiki

Subsectors in E1M1: Hangar with both segs and implicit edges shown. The implicit edges were determined using node partition lines.

SSECTORS (an abbreviation for subsectors) is the name of a WAD lump which is a component of a level. It is normally generated automatically from other data for the level using a node builder tool.

A subsector is a range of seg (linedef segment) numbers. These segs form part (or all) of a single sector. Each subsector is constructed so that when a player is anywhere within a particular subsector, no part of any one seg will block the view of any other in that subsector.

A subsector can be considered to be a convex polygon, but some of its edges may be implicit. This is because segs are generated only where linedefs already exist. (It is not uncommon for a subsector to consist of a single seg.) The Doom engine uses the partition lines in the nodes structure to determine which subsector any particular point lies in.

Subsectors are referenced from the nodes lump.

[edit] Subsector structure

Doom level format

Extensions:

Offset	Size (bytes)	Description
0	2	Seg count
2	2	First seg number

The subsectors and segs are both stored in sequential order. That is, the seg numbers in subsector 0 are 0 through N₀ - 1 (where N₀ is the subsector 0 seg count), the seg numbers in subsector 1 are N₀ through N₀ + N₁ - 1 (where N₁ is the subsector 1 seg count), and so on.

At level load time, the sector that each subsector is a part of is determined by looking at the first seg, and a link to this sector is saved in the internal subsector data structure.