battleMETAL - Why that was a bad idea, a terrible idea - Part 3 - Collision Systems

About those multi-part models

Collision systems a core part of any video game engine. From the first version of Pong up to the latest games on any platform, these systems are responsible for processing impacts between game objects in the game space. In general, like most computer code, collision systems have to be built at some point because a computer does not intrinsically know when two ‘objects’ have hit each other. The complexity of these systems can vary wildly depending on how detailed the game wants to get for collisions and how complicated the game itself is. Quake’s collision system is fairly simple for a 3D FPS.

The collision system for Quake starts with a series of boxes, or specifically, axis-aligned bounding boxes (AABB). Woah there’s some jargon, AABB simply means that the collision model for a game object is a box (technically a rectangle because box proportions can be altered). The axis-aligned part means that this bounding box does not rotate. The model can change angles all it wants, but in the eyes of the collision system, it never rotates - this is an important distinction.

Here we see Quake Guy and his AABB. Whenever Quake Guy hits another entity, or is hit by entity, the game doesn’t check the Quake Guy model for this collision. Instead, the game sees if Quake Guy’s AABB was contacted; which is mathematically simpler and therefore a faster calculation. The downsides to simple AABB is the lack of detail in collision detection, this can be best seen when Quake Guy is standing on a staircase or slope. Notice how only a few corners of the AABB are touching the ground?

Let’s make one quick jump back to the previous post about models, specifically the models that make up a mech in battleMETAL. These entities use a move type called movetype_follow which tells the game to always keep the entity’s center as an offset from a code-defined center in game space. There’s one important detail here, entities with movetype_follow cannot have a collision AABB. As a restriction, this exists because the use-case for movetype_follow is for cosmetic models put on the parent entity - weapons, clothes, etc. For battleMETAL, this means that the various mech parts that are seen on-screen don’t actually have a link to the collision system. Hm, but battleMETAL requires a feature where mechs various parts can be blown off the mech.

The solution implemented is a bit obtuse to manages to satisfy the requirement and still work. The player seen on screen is a heavy composite of several code pieces and models. To start, the player doesn’t actually have a model bound to its entity structure. Rather, the visual components - the mech piece models - are all separate entities using the movetype_follow functionality to appear in the correct location during gameplay. From the perspective of the collisions system however, these pieces are ‘wrapped’ inside the player’s main AABB. When a player is struck by a weapon, the following is executed in the code:

1. Determine the gamespace location of the hit, this is a 3D vector of X Y Z
2. Query the gamespace location of every mech piece that the player-being-hit owns.
3. Find the closest mech piece to the hit origin.
4. Closest mech piece becomes the area that is ‘hit’.

More modern game engines can execute what is called Per-Poly collision where the gamepsace location of a collision between objects can be focused on the smallest parts of the models themselves. Quake is much more primitive as we have seen. The solution that battleMETAL uses is not the most precise for players, but works surprisingly well in-action. The majority of hits against a target are applied to the correct piece of the mech (if the target is a mech) and battleMETAL uses this multi-part collision detection only in very limited cases. Every weapon strike is a short loop through all the mech parts of the mech target, which can get computationally expensive if a lot of mechs are on screen.

There are some quirks to the written algorithm as well.

1. Ignores the volume of each model piece on the mech
2. An imprecision thats hard to detect
3. Effects on mech design
   

In the end, this was deemed acceptable if it allowed players to destroy components of mech models in the game. There are several other problems with the collision system being so primitive - such as interactions between moving objects and the terrain models. The 4-point ground detection code can get a bit cagey when trying to detect collisions with a terrain piece that changes its slope too quickly- something like a really bumpy road or rocky area. This necessitated keeping terrain features more simple in their mesh complexity.