HULL # Last edited 1 year ago2023-12-16 20:28:20 UTC

The "outside" skin/face plane/facets boundary of a model or map. Half-Life maps have 4 hulls, one for visual rendering and three for collision detection. Hull is mentioned when you have a LEAK error when compiling a map.

Note: in the following table, boundaries and size are expressed as 3D vectors in the X, Y and Z coordinates respectively. Note that none of these are tied to a floor, ceiling, or wall especially.

Index	Minimum boundary	Maximum boundary	Size	Description*
	(0, 0, 0)	(0, 0, 0)	(0, 0, 0)	This is the physical hull, basically the planes and brush faces.
1	(-16, -16, -36)	(16, 16, 36)	(32, 32, 72)	This is the hull that limits where a standing player can go. Basically this one keeps players from getting "buried in the floor or the wall". The model has little to do with the hulls.
2	(-32, -32, -32)	(32, 32, 32)	(64, 64, 64)	This hull limits where large monsters can go such as the Gargantua, Gonarch and Bullsquid.
3	(-16, -16, -18)	(16, 16, 18)	(32, 32, 36)	This one is for the crouching player.
4	n/a	n/a	n/a	This is a bogus hull error and does not really exists.

* Sourced from VERC

Mechanism

John Carmack introduced clip hulls, along with the BSP format, with Quake in order to speed up collision detection.

A naive algorithm to calculate player collision would usually involve calculating the distance between a plane and a point (center of the player entity) then subtracting the thickness of the player entity from its center.

Carmack's solution is to pre-calculate the "thickness" part during compilation by expanding the collide-able planes outward the same distance to the center point of the player entity, so that inside the game loop the collision detection code only has to check if the distance is greater than zero. Hull 1 accounts for normal standing players, and hull 3 accounts for the crouched size of players. Hence for the purpose of world collision, the players are reduced to a singular point in space.

Hulls and clipnodes are very intertwined concepts, in fact clip nodes are the building blocks of the BSP hulls.

Controlling hull generation

Modern GoldSrc compilers like VHLT provide several ways to control aspects of hull generation.

The coarsest control knob is turning whole hulls off. This can be achieved by passing the following arguments to HLBSP:

• -noclip – skips clip generation entirely. the map will have no collision whatsoever
• -nohull# (#=1…3) – skips generation of specified hull number from world geometry

After that there are finer per-brush, per-area, or per-entity control on hull generation. This skips clip generation on all hulls:

• Using the NOCLIP texture on a brush
• Using the zhlt_noclip keyvalue on func_detail or brush entities

Third, comes the CLIP and co. family of tool textures which instructs the generation of clipnodes on some or all hulls. If using any of the earlier methods to skip building clipnodes off of world geometry, these clip brushes can be used to manually add back collision. The textures are:

• CLIP – all hulls
• CLIPHULL# (#=1…3) – hull #

Last but not least, a very powerful entity called info_hullshape can customize the sizes of the hulls for specific entities or for the whole map.

While the above can be safely used to customize clipnode generation using default hull sizes as the base, modifying hull sizes itself would have implications. For example, a lot of NPCs use the naive collision detection method since they don't express the same agility as players, so reducing hull sizes may not make NPCs magically fit narrower gaps than their original hull shape intended.