Entities give life to your level. They are the monsters, the doors, the items, that certain something-something that gives a level its uniqueness.
There are two types of entities: point based and brush based.
Point based entities exist only at a certain exact point. These are entities like lights (non texture based, of course) and monsters/players. Monsters do have an area, but this is defined by the game code and is not modifiable from within the map. Some point type entities are just that: points. For example, the env_beam entity (controls Half-Life's beam effects) uses two info_target entities as targets.
Brush based entities are entities that depend on a brush for their physical presence (like doors, trains, and other moving entities) or "area" (like triggers, which require an activation field).
point based entity placement
The placing of entities is a simple procedure.
1. Select the entity tool (of course!)
2. From the New Objects dialog, select the entity you want.
Note: the entity list in the New Objects dialog is for point entities only. For information on brush based entities, please refer to the following section.
3. Click in a 2D window, then position the entity cursor with the mouse.
4. Pressing Enter will create the entity.
Tip: You will most likely need to edit the entity properties after you place a point entity. This can be done by selecting the entity and pressing Alt-Enter. The properties dialog pops up automatically for brush based entities.
brush based entity placement
Brush based entities are a bit more complicated to place, but they also follow a simple procedure.
1. Build the object where you would like it. You can use as many brushes as you'd like while building a brush based entity. This example uses 6 brushes to build a door (pictured at top right).
2. Select the entire object.
3. On the New Objects dialog, press the toEntity key, which will turn the selected objects into a brush based entity and...
4. ...bring up the entity properties dialog. You can select the appropriate entity from the Class list box, in this case func_door. Once the entity type is selected, you will be able to modify the entity properties as needed.
Tip: You can bring up the entity properties for a brush based entity in the same way you do for a point based entity: select, and press Alt-Enter.
5. You will notice that brush based entities will appear as a different color in the 2D windows, typically the purplish color shown here.
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