base, aux and snd

The .base, .aux, and .snd files of a sector contain its items and nodes. The structure of all three files is identical; the only difference is which item types they contain.

Item types

.base contains the following item types:
Bus Stop, City, Company, Cut Plane, Cutscene, Ferry, Fuel Pump, Garage, Map Area, Map Overlay, Prefab, Road, Service, Sign (if it has a traffic rule), Traffic Area, Trajectory, Trigger, Visibility Area.

.aux contains the following item types:
Animated Model, Bezier Patch, Buildings, Camera Path, Camera Point, Compound, Curve, Environment Area, Far Model, Gate, Hinge, Hookup, Model, Mover, Sign (if it doesn't have a traffic rule), Terrain, Walker.

.snd contains the following item types:
Sound

The .snd file is optional; .base and .aux must be present even if only one of them contains items.

Far Models

There is one exception to this: if a Bezier Patch, Buildings, Curve, Model, or Terrain is parented by a Far Model, the item is written to .base instead of .aux.

Structure

Field(s)	Description
Header	See Header.
u32 item_count array_struct items	Contains the items of the sector. Each entry starts with a k-DOP item followed by item-specific data.
u32 node_count array_struct nodes	Contains the nodes of the items in this file. The order in which nodes are stored is unimportant, but they are typically sorted by UID in ascending order.
u32 vis_area_child_count array_u64 uids	If there are any items in the file which are parented by a visibility area, and that area is set to "Show objects" (items are only rendered if the camera is inside the area), those items' UIDs are cached here. The UID reference is written to the sector of the items, not the area. Each item's UID will only be written once, even if it is parented by multiple areas.

Determining the sector of an item

Which sector a map item should be written to is determined by the center point of its nodes. For example, a Road item going from (-15, 0, 35) to (35, 0, -15) will have a center point of (10, 0, 10) and therefore belongs to sector (0, 0), despite the fact that neither of its nodes fall in that sector.