Enum CoefficientCombineRule 

pub enum CoefficientCombineRule {
    Average = 0,
    Min = 1,
    Multiply = 2,
    Max = 3,
    ClampedSum = 4,
    GeometricMean = 5,
}

How to combine friction/restitution values when two colliders touch.

When two colliders with different friction (or restitution) values collide, Rapier needs to decide what the effective friction/restitution should be. Each collider has a combine rule, and the “stronger” rule wins (GeometricMean > ClampedSum > Max > Multiply > Min > Average).

Combine Rules

Most games use Average (the default) and never change this.

• Average (default): (friction1 + friction2) / 2 - Balanced, intuitive
• Min: min(friction1, friction2).abs() - “Slippery wins” (ice on any surface = ice)
• Multiply: friction1 × friction2 - Both must be high for high friction
• Max: max(friction1, friction2) - “Sticky wins” (rubber on any surface = rubber)
• ClampedSum: sum(friction1, friction2).clamp(0, 1) - Sum of both frictions, clamped to range 0, 1.
• GeometricMean: sqrt(friction1 × friction2) - Between Multiply and Average; zero if either is zero.

Example

// Ice collider that makes everything slippery
let ice = ColliderBuilder::cuboid(10.0, 0.1, 10.0)
    .friction(0.0)
    .friction_combine_rule(CoefficientCombineRule::Min)  // Ice wins!
    .build();

Priority System

If colliders disagree on rules, the “higher” one wins: GeometricMean > ClampedSum > Max > Multiply > Min > Average

Variants

Average = 0

Average the two values (default, most common).

Min = 1

Use the smaller value (“slippery/soft wins”).

Multiply = 2

Multiply the two values (both must be high).

Max = 3

Use the larger value (“sticky/bouncy wins”).

ClampedSum = 4

The clamped sum of the two coefficients.

GeometricMean = 5

The square root of the product of the two values.

A common convention in other engines (e.g. Bullet, PhysX): stricter than Average (either value being zero results in zero) but less aggressive than Multiply for values below 1.

Trait Implementations

impl Clone for CoefficientCombineRule

fn clone(&self) -> CoefficientCombineRule

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for CoefficientCombineRule

impl Debug for CoefficientCombineRule

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for CoefficientCombineRule

fn default() -> CoefficientCombineRule

Returns the “default value” for a type.

impl Eq for CoefficientCombineRule

impl Ord for CoefficientCombineRule

fn cmp(&self, other: &CoefficientCombineRule) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl PartialEq for CoefficientCombineRule

fn eq(&self, other: &CoefficientCombineRule) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl PartialOrd for CoefficientCombineRule

fn partial_cmp(&self, other: &CoefficientCombineRule) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl StructuralPartialEq for CoefficientCombineRule