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 pill

Biped bodies are approximated by vertically-oriented capsule/pill shapes for their physical interactions with the level BSP, the model_collision_geometry of objects (like scenery, devices, vehicles), and with other biped physics pills. Note that the physics pill is not used as the "hitbox" for projectiles; a biped's model_collision_geometry is tested for projectile ray casts.

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).

You can enable debug_objects and debug_objects_biped_physics_pills to visualize physics pills.

Autoaim pill

Autoaim pills are part of the autoaim system which causes projectiles to fire towards a biped when the pill is with in a weapon's autoaim angle and range. The width of this pill is controlled within this tag, but its height and placement depends on the model:

• 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.

You can enable debug_objects and debug_objects_biped_autoaim_pills to visualize autoaim pills.

NTSC vs PAL physics

Halo CE for the Xbox was originally released under two different analog TV standards: NTSC in the Americas and PAL in Europe. The NTSC version ran at 30 FPS, while PAL ran at 25 FPS. Since the simulation tick rate was tied to the frame rate, this meant time in the PAL edition would run slower than the NTSC version. To compensate, the developers increased speeds and firing rates across a variety of tags, including player speed in globals.

This tag's unit uses old ntsc player physics flag was a hack added for the PAL edition and remained in the engine through the PC port, H1A, and MCC. It causes the engine to override the player globals values with the following hard-coded values:

• walking speed: 0.512
• run forward speed: 2.25
• run backward speed: 2.0
• run sideways speed: 2.0
• run acceleration: 0.32

These values are identical to the globals in all non-PAL versions of the game. The flag only has a visible effect on the Xbox PAL version due to the difference in globals there, and is only used on the cyborg_cinematic.biped. This is presumably to fix recorded animations which assumed NTSC physics.

Structure and fields

Acknowledgements

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

• Aerocatia (NTSC player physics research)
• Kavawuvi (Invader tag definitions)
• MosesOfEgypt (Tag structure research)