Biped tags represent the "bodies" of AI- and player-driven characters. They can take on a wide range of forms, like flood infection forms, sentinels, marines, grunts, and scripted characters like Cortana. Invisible bipeds are even used to pilot dropships and detecting explosive damage to the ship's reactor in The Maw. Bipeds are a type of unit.

On its own, a biped will not do anything unless being controlled by a player or actor_variant. This tag defines key physical characteristics like height, speed, collision, and appearance.

Physics and autoaim pills

Biped bodies are approximated with "pills" for physics and autoaim, rather than using their collision model. These simple capsule shapes are sufficient for these purposes and efficient to test against. Pills are parameterized by radius/width and height.

Physics pills are how the biped collides with model_collision_geometry and the BSP. It is not used as the "hitbox" for projectiles. Its width depend on this tag's collision radius. The height depends on standing and crouching collision heights only if the biped uses player physics, and otherwise has 0 height (making the pill a sphere).

Autoaim pills are part of the autoaim system which causes projectiles to fire towards a physics pill when the pill is with in a weapon's autoaim angle and range. The width of this pill is controlled solely by this tag, however its height depends:

• If the biped has head and pelvis nodes...
  • If the spherical flag is set, then the pill is a sphere at the midpoint between head and pelvis.
  • Otherwise, the capsule spans between the head and pelvis nodes.
• If the biped doesn't have both of these nodes, the autoaim pill is vertically-aligned and its height is a fraction of the physics pill's. It will be slightly elevated from the physics pill's base.

Structure and fields

Acknowledgements

Thanks to the following individuals for their research or contributions to this topic:

• Kavawuvi (Invader tag definitions)
• MosesOfEgypt (Tag structure research)