An Invisible Collision Hull (ICH) is a type of semisolid brush that is invisible. Its purpose is to block actors such as players and projectiles. ICHs were commonly used in pre-UT2003 versions of the engine, but in UnrealEd 3 they are superseded by the BlockingVolume.
To add one to a map:
- form the red builder brush into the required shape
- click the add special button in the toolbox
- a pop-up window appears: select "invisible collision hull" from the Presets drop-down list
- click Add
Technically, the ICH is a semisolid brush, with all the polys flagged to be invisible. Note that invisible polys are only visible in an editor 3D viewport when realtime preview is off, so if this is on, you won't see ICHs at all.
Invisible collision hulls can be used in combination with sheet brushes to make glass and masked textures such as grilles block players and projectiles.
ICHs should not touch walls or zone boundaries, otherwise errors may occur, e.g. becoming non-solid.
Some mappers prefer alternative techniques:
- making a transparent solid brush. This is prone to error and can lead to BSP holes.
- using several BlockAll actors. These simply provide an upright collision cylinder, and they don't interfere with the BSP. But for large areas they give awkward results: players can feel the row of cylinders is bumpy.
It's useful to use a particular texture for all of your ICH brushes; this way they are easier to see in the editor. Notably, ChrisToth's map DM-Axxon used the texture Botpack.Ammocount.AmmoCountBar for ICH polys. This is a plain green and makes them very easy to spot.