physics

Physics tags characterize the dynamic physics and propulsion of vehicles. They are essentially a collection of spherical mass points. Since vehicles can have both model_collision_geometry and physics, each tag is used in different situations:

Physics are used in collisions between vehicles and is responsible for Halo 1's charactistic "bouncy" vehicles.
Physics are used in collisions with the BSP. Because the mass points are somewhat "fuzzy" and are not continuous geometry, vehicles can become stuck in thin BSP.
Physics are used in collisions with device_machine and scenery.
Collision geometry is used in collisions with bipeds, items, and projectiles.
Note that vehicles cannot collide with device_light_fixture.

Physics tags are created by importing a JMS with specially-named markers parented to nodes. The markers become mass points.

Mass points

Mass points (also known as physics spheres) are spherical volumes with mass and density. They can have various types of friction and may provide powered impulse for flight and driving.

Even though mass points are parented to nodes, they do not move with base, overlay, or aiming animations unlike model_collision_geometry. However, when mass points are parented to nodes they can be used to engage a vehicle's suspension animation (e.g. the Warthog and Scorpion). The animation tag specifies the compression and extension depth. The mass points move with the node in this case.

	Function/global	Type
	`(debug_material_effects [boolean])` Displays cyan spheres wherever material_effects are being generated, like under the player's feet and where physics spheres intersect with the BSP or model_collision_geometry.	Global
	`(debug_objects_physics)` When `debug_objects` is enabled, displays physics mass points as white spheres.	Global
	`(debug_objects_position_velocity)` When `debug_objects` is enabled, displays red, green, and blue object-space reference axes and a yellow velocity vector on each object.	Global
	`(debug_objects_unit_vectors)` When `debug_objects` is enabled, displays white and red vectors on objects. Their meaning is unknown.	Global

Structure and fields

Field Type Comments

radius float

moment scale float

mass float

center of mass Point3D

Field	Type	Comments
x	`float`
y	`float`
z	`float`

density float

gravity scale float

ground friction

float

A coefficient which modifies the amount of friction applied to the vehicle when colliding with "ground" surfaces, which may just mean any BSP surface which is not a water plane. It is unknown if this also modifies friction on scenery collisions. A typical value from the Warthog is 0.23.

ground depth float

ground damp fraction float

ground normal k1

float

Used to determine how steep of a surface the vehicle can climb before its powered mass points no longer have an effect. A typical value is ~0.7, while a value near 1.0 allows vertical climbing assuming sufficient friction.

ground normal k0

float

This is also used to determine how steep of a surface the vehicle can climb and seems to be some sort of lower range bound for fading out powered mass points. A typical value is ~0.5, with -1.0 allowing vertical wall climbing.

water friction float

water depth float

water density float

air friction float

xx moment float

yy moment float

zz moment float

inertial matrix and inverse

Block

HEK max count: 2
Volatile
Min: 2
Max: 2

Field Type Comments

matrix Matrix

Field	Type	Comments
elements	`float[9]`

powered mass points

Block

HEK max count: 32

Field Type Comments

name TagString

flags bitfield

Flag	Mask	Comments
ground friction	`0x1`
water friction	`0x2`
air friction	`0x4`
water lift	`0x8`
air lift	`0x10`
thrust	`0x20`
antigrav	`0x40`

antigrav strength float

antigrav offset float

antigrav height float

antigrav damp fraction float

antigrav normal k1 float

antigrav normal k0 float

mass points

Block

HEK max count: 32

Field Type Comments

name TagString

powered mass point int16

model node int16

flags bitfield

Flag	Mask	Comments
metallic	`0x1`

relative mass float

mass float

relative density float

density float

position Point3D^?

forward Vector3D

Field	Type	Comments
i	`float`
j	`float`
k	`float`

up Vector3D^?

friction type enum

Option	Value	Comments
point	`0x0`
forward	`0x1`
left	`0x2`
up	`0x3`

friction parallel scale float

friction perpendicular scale float

radius float

Acknowledgements

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

Conscars (Mass point node testing)
Kavawuvi (Invader tag definitions)
MosesOfEgypt (Tag structure research)

physics

Mass points

Related HaloScript

Structure and fields

Acknowledgements