Bepu: Bounce and Friction

Every Bepu collidable, a BodyComponent or a StaticComponent, has four properties that decide what happens where it touches another one. Nothing bounces with the default values, and the way two collidables combine their values decides which of them has any effect.

The Physics Materials example shows every case on this page and prints what it measures.

Properties

Property	Default	Meaning
`SpringFrequency`	30	Stiffness of the contact, in hertz. Lower is softer.
`SpringDampingRatio`	3	Share of a hit the contact absorbs. 0 returns all of it, 1 or more returns none.
`FrictionCoefficient`	1	Resistance to sliding. 0 is frictionless.
`MaximumRecoveryVelocity`	1000	Highest speed at which two overlapping collidables are pushed apart. Also selects whose spring is used, see below.

var ball = new BodyComponent
{
    Collider = new CompoundCollider(),
    SpringFrequency = 10f,
    SpringDampingRatio = 0f,
};

How two collidables combine

A contact has one set of values, made from the two collidables that touch.

Value of the pair	Rule
Friction	The two coefficients multiplied.
Maximum recovery velocity	The larger of the two.
Spring frequency and damping ratio	Taken together from the collidable with the larger `MaximumRecoveryVelocity`. When the two are equal and one side is a static, the body's are used.

The springs are not blended. One collidable's spring is used and the other's is ignored.

Bounce

A contact bounces when its damping ratio is below 1. The default ratio of 3 absorbs the whole hit, so the default spring does not bounce.

How much comes back then depends on the frequency. The simulation takes 60 steps a second, and a stiff contact is over in a step or two, which loses most of the bounce. The lower the frequency, the more of it survives.

Rebound of a ball dropped 3 m onto the default ground, as a share of the drop, at 60 steps a second:

`SpringFrequency`, with a damping ratio of 0	Rebound
30	9%
20	18%
15	27%
10	40%
5	59%

`SpringDampingRatio`, at 10 Hz	Rebound
0	40%
0.1	22%
0.2	13%
0.5	3%
1	0%

A lower frequency is also softer: at 5 Hz the ball sinks about 0.2 m into the ground before it comes back.

A bouncy surface

A static with a bouncy spring does nothing under a body with default values, because the two MaximumRecoveryVelocity values are equal and the body's spring is used. Raise the static's value above the body's:

var trampoline = new StaticComponent
{
    Collider = new CompoundCollider(),
    SpringFrequency = 10f,
    SpringDampingRatio = 0f,
    MaximumRecoveryVelocity = 2000f,
};

Pad under a ball with default values	Rebound
10 Hz, damping 0, `MaximumRecoveryVelocity` 1000	0%
10 Hz, damping 0, `MaximumRecoveryVelocity` 2000	40%

The reverse also holds: lowering MaximumRecoveryVelocity on a bouncy body below the ground's hands the contact to the ground's spring, and the body stops bouncing.

Friction

Friction is the product of the two coefficients, so a coefficient of 0 on either side removes it whatever the other side has. A body of 4 on a surface of 0.5 slides like a body of 1 on a surface of 2.

Note

Stride 4.4 uses BepuPhysics 2.5.0-beta.28, which divides the friction of a contact between two convex shapes by its number of contact points. A box resting on a face has four, so it slides as if the friction were a quarter of the product: a box with the default coefficient of 1 slides down a 30 degree slope. Later Bepu versions apply the full product. Until Stride moves to one, multiply the coefficient to compensate. A sphere has one contact point and is not affected.

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