Using Transition Zones

From C4 Engine Wiki

A transition zone is a special type of zone intended to contain objects that must extend into more than one ordinary zone. It is often the case that the geometry making up a doorway between two zones needs to overlap both of the zones that it connects, but placing the geometry in one of the two zones could cause rendering artifacts if the portal leading from the zone containing the camera to the other isn't visible (or isn't enabled). The solution is to place a small transition zone between the two ordinary zones and put all of the doorway geometry in it.

A transition zone is created by drawing a zone normally in the World Editor and then checking the “Transition zone” box in the Node Info dialog, shown in the image to the right. A transition zone connecting two ordinary zones should be created at the same level in the scene graph as the highest level of the zones it's connecting. Normally, the two ordinary zones have the same parent zone (which may be the root infinite zone), and so the transition zone should also have the same parent.

Transition zones have the following special properties:

• For any portal leading out of the transition zone into an ordinary zone, the transition zone automatically becomes attached to the ordinary zone. Whenever part of the ordinary zone is visible, the contents of any attached transition zones are automatically qualified for rendering. It is not necessary (or desired) to create portals in the ordinary zone that lead to any transition zones.
• Geometries in a transition zone automatically get an ambient color channel that provides a smooth transition between the ambient light intensities of the ordinary zones connected by the transition zone. This can be disabled by checking the “Don't apply ambient transition” box in the geometry's Node Info dialog.

The general procedure for connecting two large rooms with a small transition zone containing doorway geometry is as follows.

1. Create both rooms in separate ordinary zones, but do not create portals that lead from either zone to the other.
2. Create a transition zone in between the ordinary zones where the doorway geometry will reside. A transition zone is allowed to overlap ordinary zones.
3. Place the doorway geometry in the transition zone. This is the only way that geometry is allowed to extend into multiple zones at once.
4. Create portals in the transition zone that lead to each of the ordinary zones. The ordinary zones will automatically detect which transition zones have portals connecting to them.