Ridigbody2d collision with Staticbody2d. How Bounce -property affects bouncing ?

[JuhaW]

The objects are Ridigbody2d and Staticbody2d. Both have Physic Material Bounce values set. Does Staticbody2d Bounce value affect how Rigidbody2d bounces ? Lets say Rigidbody bounce value is 0.5 and Staticbody2d bounce value is 0.3.

[Ughuuu]

Hi, good question. The colliders have set:

            collider.set_friction_combine_rule(CoefficientCombineRule::Min);
            collider.set_restitution_combine_rule(CoefficientCombineRule::Sum);

As per docs: https://rapier.rs/docs/user_guides/rust/collider_restitution/, and looking in code also of rapier:

impl CoefficientCombineRule {
    pub(crate) fn combine(
        coeff1: Real,
        coeff2: Real,
        rule_value1: CoefficientCombineRule,
        rule_value2: CoefficientCombineRule,
    ) -> Real {
        let effective_rule = rule_value1.max(rule_value2);

        match effective_rule {
            CoefficientCombineRule::Average => (coeff1 + coeff2) / 2.0,
            CoefficientCombineRule::Min => coeff1.min(coeff2).abs(),
            CoefficientCombineRule::Multiply => coeff1 * coeff2,
            CoefficientCombineRule::Max => coeff1.max(coeff2),
            CoefficientCombineRule::Sum => (coeff1 + coeff2).clamp(0.0, 1.0),
        }
    }
}

It will do the sum of them (eg. 0.3 + 0.5 = 0.8). Similarly, the Godot bounce property: https://docs.godotengine.org/en/stable/classes/class_physicsmaterial.html#class-physicsmaterial-property-bounce

In comparison, godot code does:

real_t combine_bounce(GodotBody2D *A, GodotBody2D *B) {
	return CLAMP(A->get_bounce() + B->get_bounce(), 0, 1);
}

So also does sum, and caps at 1 (same as rapier one).