Improve the API for initializing/setting mass-properties

CHANGELOG.md

@@ -1,3 +1,26 @@
+## Unreleased
+### Fixed
+
+### Modified
+- The `RigidBodyBuilder::additional_mass` method will now result in the additional angular inertia
+  being automatically computed based on the shapes of the colliders attached to the rigid-body.
+- Remove the deprecated methods `RigidBodyBuilder::mass`, `::principal_angular_inertia`, `::principal_inertia`.
+- Remove the methods `RigidBodyBuilder::additional_principal_angular_inertia`. Use
+  `RigidBodyBuilder::additional_mass_properties` instead.
+
+### Added
+- Add `RigidBody::recompute_mass_properties_from_colliders` to force the immediate computation
+  of a rigid-body’s mass properties (instead of waiting for them to be recomputed during the next
+  timestep). This is useful to be able to read immediately the result of a change of a rigid-body
+  additional mass-properties or a change of one of its collider’s mass-properties.
+- Add `RigidBody::set_additional_mass` to set the additional mass for the collider. The additional
+  angular inertia is automatically computed based on the attached colliders shapes.
+- Add `Collider::set_density`, `::set_mass`, `set_mass_properties`, to alter a collider’s mass
+  properties. Note that `::set_mass` will result in the collider’s angular inertia being automatically
+  computed based on this mass and on its shape.
+- Add `ColliderBuilder::mass` to set the mass of the collider instead of its density. Its angular
+  inertia tensor will be automatically computed based on this mass and its shape.
+
 ## v0.13.0 (31 May 2022)
 ### Fixed
 - Fix incorrect sensor events being generated after collider removal.

src/dynamics/rigid_body.rs

@@ -1,14 +1,14 @@
 use crate::dynamics::{
-    LockedAxes, MassProperties, RigidBodyActivation, RigidBodyCcd, RigidBodyChanges,
+    LockedAxes, MassProperties, RigidBodyActivation, RigidBodyAdditionalMassProps, RigidBodyCcd,
-    RigidBodyColliders, RigidBodyDamping, RigidBodyDominance, RigidBodyForces, RigidBodyIds,
+    RigidBodyChanges, RigidBodyColliders, RigidBodyDamping, RigidBodyDominance, RigidBodyForces,
-    RigidBodyMassProps, RigidBodyPosition, RigidBodyType, RigidBodyVelocity,
+    RigidBodyIds, RigidBodyMassProps, RigidBodyPosition, RigidBodyType, RigidBodyVelocity,
 };
 use crate::geometry::{
-    Collider, ColliderHandle, ColliderMassProps, ColliderParent, ColliderPosition, ColliderShape,
+    Collider, ColliderHandle, ColliderMassProps, ColliderParent, ColliderPosition, ColliderSet,
+    ColliderShape,
 };
 use crate::math::{AngVector, Isometry, Point, Real, Rotation, Vector};
-use crate::utils::{self, WCross};
+use crate::utils::WCross;
-use na::ComplexField;
 use num::Zero;
 
 #[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
@@ -123,7 +123,7 @@ impl RigidBody {
         }
     }
 
-    /// The mass properties of this rigid-body.
+    /// The mass-properties of this rigid-body.
     #[inline]
     pub fn mass_properties(&self) -> &MassProperties {
         &self.mprops.local_mprops
@@ -310,26 +310,82 @@ impl RigidBody {
         self.ccd.ccd_active
     }
 
-    /// Sets the rigid-body's additional mass properties.
+    /// Recompute the mass-properties of this rigid-bodies based on its currently attached colliders.
+    pub fn recompute_mass_properties_from_colliders(&mut self, colliders: &ColliderSet) {
+        self.mprops.recompute_mass_properties_from_colliders(
+            colliders,
+            &self.colliders,
+            &self.pos.position,
+        );
+    }
+
+    /// Sets the rigid-body's additional mass.
+    ///
+    /// The total angular inertia of the rigid-body will be scaled automatically based on this
+    /// additional mass. If this scaling effect isn’t desired, use [`Self::additional_mass_properties`]
+    /// instead of this method.
+    ///
+    /// This is only the "additional" mass because the total mass of the  rigid-body is
+    /// equal to the sum of this additional mass and the mass computed from the colliders
+    /// (with non-zero densities) attached to this rigid-body.
+    ///
+    /// That total mass (which includes the attached colliders’ contributions)
+    /// will be updated at the name physics step, or can be updated manually with
+    /// [`Self::recompute_mass_properties_from_colliders`].
+    ///
+    /// This will override any previous mass-properties set by [`Self::set_additional_mass`],
+    /// [`Self::set_additional_mass_properties`], [`RigidBodyBuilder::additional_mass`], or
+    /// [`RigidBodyBuilder::additional_mass_properties`] for this rigid-body.
+    ///
+    /// If `wake_up` is `true` then the rigid-body will be woken up if it was
+    /// put to sleep because it did not move for a while.
+    #[inline]
+    pub fn set_additional_mass(&mut self, additional_mass: Real, wake_up: bool) {
+        self.do_set_additional_mass_properties(
+            RigidBodyAdditionalMassProps::Mass(additional_mass),
+            wake_up,
+        )
+    }
+
+    /// Sets the rigid-body's additional mass-properties.
+    ///
+    /// This is only the "additional" mass-properties because the total mass-properties of the
+    /// rigid-body is equal to the sum of this additional mass-properties and the mass computed from
+    /// the colliders (with non-zero densities) attached to this rigid-body.
+    ///
+    /// That total mass-properties (which include the attached colliders’ contributions)
+    /// will be updated at the name physics step, or can be updated manually with
+    /// [`Self::recompute_mass_properties_from_colliders`].
+    ///
+    /// This will override any previous mass-properties set by [`Self::set_additional_mass`],
+    /// [`Self::set_additional_mass_properties`], [`RigidBodyBuilder::additional_mass`], or
+    /// [`RigidBodyBuilder::additional_mass_properties`] for this rigid-body.
     ///
     /// If `wake_up` is `true` then the rigid-body will be woken up if it was
     /// put to sleep because it did not move for a while.
     #[inline]
     pub fn set_additional_mass_properties(&mut self, props: MassProperties, wake_up: bool) {
-        if let Some(add_mprops) = &mut self.mprops.additional_local_mprops {
+        self.do_set_additional_mass_properties(
-            self.mprops.local_mprops += props;
+            RigidBodyAdditionalMassProps::MassProps(props),
-            self.mprops.local_mprops -= **add_mprops;
+            wake_up,
-            **add_mprops = props;
+        )
-        } else {
+    }
-            self.mprops.additional_local_mprops = Some(Box::new(props));
-            self.mprops.local_mprops += props;
-        }
 
-        if self.is_dynamic() && wake_up {
+    fn do_set_additional_mass_properties(
-            self.wake_up(true);
+        &mut self,
-        }
+        props: RigidBodyAdditionalMassProps,
+        wake_up: bool,
+    ) {
+        let new_mprops = Some(Box::new(props));
 
-        self.update_world_mass_properties();
+        if self.mprops.additional_local_mprops != new_mprops {
+            self.changes.insert(RigidBodyChanges::LOCAL_MASS_PROPERTIES);
+            self.mprops.additional_local_mprops = new_mprops;
+
+            if self.is_dynamic() && wake_up {
+                self.wake_up(true);
+            }
+        }
     }
 
     /// The handles of colliders attached to this rigid body.
@@ -432,12 +488,6 @@ impl RigidBody {
         if let Some(i) = self.colliders.0.iter().position(|e| *e == handle) {
             self.changes.set(RigidBodyChanges::COLLIDERS, true);
             self.colliders.0.swap_remove(i);
-
-            let mass_properties = coll
-                .mass_properties()
-                .transform_by(coll.position_wrt_parent().unwrap());
-            self.mprops.local_mprops -= mass_properties;
-            self.update_world_mass_properties();
         }
     }
 
@@ -859,8 +909,8 @@ pub struct RigidBodyBuilder {
     pub angular_damping: Real,
     body_type: RigidBodyType,
     mprops_flags: LockedAxes,
-    /// The additional mass properties of the rigid-body being built. See [`RigidBodyBuilder::additional_mass_properties`] for more information.
+    /// The additional mass-properties of the rigid-body being built. See [`RigidBodyBuilder::additional_mass_properties`] for more information.
-    pub additional_mass_properties: MassProperties,
+    additional_mass_properties: RigidBodyAdditionalMassProps,
     /// Whether or not the rigid-body to be created can sleep if it reaches a dynamic equilibrium.
     pub can_sleep: bool,
     /// Whether or not the rigid-body is to be created asleep.
@@ -887,7 +937,7 @@ impl RigidBodyBuilder {
             angular_damping: 0.0,
             body_type,
             mprops_flags: LockedAxes::empty(),
-            additional_mass_properties: MassProperties::zero(),
+            additional_mass_properties: RigidBodyAdditionalMassProps::default(),
             can_sleep: true,
             sleeping: false,
             ccd_enabled: false,
@@ -968,18 +1018,41 @@ impl RigidBodyBuilder {
         self
     }
 
-    /// Sets the additional mass properties of the rigid-body being built.
+    /// Sets the additional mass-properties of the rigid-body being built.
     ///
-    /// Note that "additional" means that the final mass properties of the rigid-bodies depends
+    /// This will be overridden by a call to [`Self::additional_mass`] so it only makes sense to call
+    /// either [`Self::additional_mass`] or [`Self::additional_mass_properties`].    
+    ///
+    /// Note that "additional" means that the final mass-properties of the rigid-bodies depends
     /// on the initial mass-properties of the rigid-body (set by this method)
     /// to which is added the contributions of all the colliders with non-zero density
     /// attached to this rigid-body.
     ///
-    /// Therefore, if you want your provided mass properties to be the final
+    /// Therefore, if you want your provided mass-properties to be the final
-    /// mass properties of your rigid-body, don't attach colliders to it, or
+    /// mass-properties of your rigid-body, don't attach colliders to it, or
     /// only attach colliders with densities equal to zero.
-    pub fn additional_mass_properties(mut self, props: MassProperties) -> Self {
+    pub fn additional_mass_properties(mut self, mprops: MassProperties) -> Self {
-        self.additional_mass_properties = props;
+        self.additional_mass_properties = RigidBodyAdditionalMassProps::MassProps(mprops);
+        self
+    }
+
+    /// Sets the additional mass of the rigid-body being built.
+    ///
+    /// This will be overridden by a call to [`Self::additional_mass_properties`] so it only makes
+    /// sense to call either [`Self::additional_mass`] or [`Self::additional_mass_properties`].    
+    ///
+    /// This is only the "additional" mass because the total mass of the  rigid-body is
+    /// equal to the sum of this additional mass and the mass computed from the colliders
+    /// (with non-zero densities) attached to this rigid-body.
+    ///
+    /// The total angular inertia of the rigid-body will be scaled automatically based on this
+    /// additional mass. If this scaling effect isn’t desired, use [`Self::additional_mass_properties`]
+    /// instead of this method.
+    ///
+    /// # Parameters
+    /// * `mass`- The mass that will be added to the created rigid-body.
+    pub fn additional_mass(mut self, mass: Real) -> Self {
+        self.additional_mass_properties = RigidBodyAdditionalMassProps::Mass(mass);
         self
     }
 
@@ -1037,78 +1110,6 @@ impl RigidBodyBuilder {
         self
     }
 
-    /// Sets the additional mass of the rigid-body being built.
-    ///
-    /// This is only the "additional" mass because the total mass of the  rigid-body is
-    /// equal to the sum of this additional mass and the mass computed from the colliders
-    /// (with non-zero densities) attached to this rigid-body.
-    pub fn additional_mass(mut self, mass: Real) -> Self {
-        self.additional_mass_properties.set_mass(mass, false);
-        self
-    }
-
-    /// Sets the additional mass of the rigid-body being built.
-    ///
-    /// This is only the "additional" mass because the total mass of the  rigid-body is
-    /// equal to the sum of this additional mass and the mass computed from the colliders
-    /// (with non-zero densities) attached to this rigid-body.
-    #[deprecated(note = "renamed to `additional_mass`.")]
-    pub fn mass(self, mass: Real) -> Self {
-        self.additional_mass(mass)
-    }
-
-    /// Sets the additional angular inertia of this rigid-body.
-    ///
-    /// This is only the "additional" angular inertia because the total angular inertia of
-    /// the rigid-body is equal to the sum of this additional value and the angular inertia
-    /// computed from the colliders (with non-zero densities) attached to this rigid-body.
-    #[cfg(feature = "dim2")]
-    pub fn additional_principal_angular_inertia(mut self, inertia: Real) -> Self {
-        self.additional_mass_properties.inv_principal_inertia_sqrt =
-            utils::inv(ComplexField::sqrt(inertia.max(0.0)));
-        self
-    }
-
-    /// Sets the angular inertia of this rigid-body.
-    #[cfg(feature = "dim2")]
-    #[deprecated(note = "renamed to `additional_principal_angular_inertia`.")]
-    pub fn principal_angular_inertia(self, inertia: Real) -> Self {
-        self.additional_principal_angular_inertia(inertia)
-    }
-
-    /// Use `self.principal_angular_inertia` instead.
-    #[cfg(feature = "dim2")]
-    #[deprecated(note = "renamed to `additional_principal_angular_inertia`.")]
-    pub fn principal_inertia(self, inertia: Real) -> Self {
-        self.additional_principal_angular_inertia(inertia)
-    }
-
-    /// Sets the additional principal angular inertia of this rigid-body.
-    ///
-    /// This is only the "additional" angular inertia because the total angular inertia of
-    /// the rigid-body is equal to the sum of this additional value and the angular inertia
-    /// computed from the colliders (with non-zero densities) attached to this rigid-body.
-    #[cfg(feature = "dim3")]
-    pub fn additional_principal_angular_inertia(mut self, inertia: AngVector<Real>) -> Self {
-        self.additional_mass_properties.inv_principal_inertia_sqrt =
-            inertia.map(|e| utils::inv(ComplexField::sqrt(e.max(0.0))));
-        self
-    }
-
-    /// Sets the principal angular inertia of this rigid-body.
-    #[cfg(feature = "dim3")]
-    #[deprecated(note = "renamed to `additional_principal_angular_inertia`.")]
-    pub fn principal_angular_inertia(self, inertia: AngVector<Real>) -> Self {
-        self.additional_principal_angular_inertia(inertia)
-    }
-
-    /// Use `self.principal_angular_inertia` instead.
-    #[cfg(feature = "dim3")]
-    #[deprecated(note = "renamed to `additional_principal_angular_inertia`.")]
-    pub fn principal_inertia(self, inertia: AngVector<Real>) -> Self {
-        self.additional_principal_angular_inertia(inertia)
-    }
-
     /// Sets the damping factor for the linear part of the rigid-body motion.
     ///
     /// The higher the linear damping factor is, the more quickly the rigid-body
@@ -1169,9 +1170,11 @@ impl RigidBodyBuilder {
         rb.body_type = self.body_type;
         rb.user_data = self.user_data;
 
-        if self.additional_mass_properties != MassProperties::default() {
+        if self.additional_mass_properties
+            != RigidBodyAdditionalMassProps::MassProps(MassProperties::zero())
+            && self.additional_mass_properties != RigidBodyAdditionalMassProps::Mass(0.0)
+        {
             rb.mprops.additional_local_mprops = Some(Box::new(self.additional_mass_properties));
-            rb.mprops.local_mprops = self.additional_mass_properties;
         }
 
         rb.mprops.flags = self.mprops_flags;

src/dynamics/rigid_body_components.rs

@@ -110,6 +110,8 @@ bitflags::bitflags! {
         const TYPE        = 1 << 4;
         /// Flag indicating that the `RigidBodyDominance` component of this rigid-body has been modified.
         const DOMINANCE   = 1 << 5;
+        /// Flag indicating that the local mass-properties of this rigid-body must be recomputed.
+        const LOCAL_MASS_PROPERTIES = 1 << 6;
     }
 }
 
@@ -222,7 +224,23 @@ bitflags::bitflags! {
     }
 }
 
-// TODO: split this into "LocalMassProps" and `WorldMassProps"?
+/// Mass and angular inertia added to a rigid-body on top of its attached colliders’ contributions.
+#[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum RigidBodyAdditionalMassProps {
+    /// Mass properties to be added as-is.
+    MassProps(MassProperties),
+    /// Mass to be added to the rigid-body. This will also automatically scale
+    /// the attached colliders total angular inertia to account for the added mass.
+    Mass(Real),
+}
+
+impl Default for RigidBodyAdditionalMassProps {
+    fn default() -> Self {
+        RigidBodyAdditionalMassProps::MassProps(MassProperties::default())
+    }
+}
+
 #[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
 #[derive(Clone, Debug, PartialEq)]
 /// The mass properties of this rigid-bodies.
@@ -232,7 +250,7 @@ pub struct RigidBodyMassProps {
     /// The local mass properties of the rigid-body.
     pub local_mprops: MassProperties,
     /// Mass-properties of this rigid-bodies, added to the contributions of its attached colliders.
-    pub additional_local_mprops: Option<Box<MassProperties>>,
+    pub additional_local_mprops: Option<Box<RigidBodyAdditionalMassProps>>,
     /// The world-space center of mass of the rigid-body.
     pub world_com: Point<Real>,
     /// The inverse mass taking into account translation locking.
@@ -294,18 +312,20 @@ impl RigidBodyMassProps {
         self.effective_world_inv_inertia_sqrt.squared().inverse()
     }
 
-    /// Recompute the mass-properties of this rigid-bodies based on its currentely attached colliders.
+    /// Recompute the mass-properties of this rigid-bodies based on its currently attached colliders.
     pub fn recompute_mass_properties_from_colliders(
         &mut self,
         colliders: &ColliderSet,
         attached_colliders: &RigidBodyColliders,
         position: &Isometry<Real>,
     ) {
-        self.local_mprops = self
+        let added_mprops = self
             .additional_local_mprops
             .as_ref()
             .map(|mprops| **mprops)
-            .unwrap_or_else(MassProperties::default);
+            .unwrap_or_else(|| RigidBodyAdditionalMassProps::MassProps(MassProperties::default()));
+
+        self.local_mprops = MassProperties::default();
 
         for handle in &attached_colliders.0 {
             if let Some(co) = colliders.get(*handle) {
@@ -319,6 +339,16 @@ impl RigidBodyMassProps {
             }
         }
 
+        match added_mprops {
+            RigidBodyAdditionalMassProps::MassProps(mprops) => {
+                self.local_mprops += mprops;
+            }
+            RigidBodyAdditionalMassProps::Mass(mass) => {
+                let new_mass = self.local_mprops.mass() + mass;
+                self.local_mprops.set_mass(new_mass, true);
+            }
+        }
+
         self.update_world_mass_properties(position);
     }
 

src/geometry/collider.rs

@@ -241,10 +241,54 @@ impl Collider {
     pub fn density(&self) -> Option<Real> {
         match &self.mprops {
             ColliderMassProps::Density(density) => Some(*density),
+            ColliderMassProps::Mass(_) => None,
             ColliderMassProps::MassProperties(_) => None,
         }
     }
 
+    /// Sets the uniform density of this collider.
+    ///
+    /// This will override any previous mass-properties set by [`Self::set_density`],
+    /// [`Self::set_mass`], [`Self::set_mass_properties`], [`ColliderBuilder::density`],
+    /// [`ColliderBuilder::mass`], or [`ColliderBuilder::mass_properties`]
+    /// for this collider.
+    ///
+    /// The mass and angular inertia of this collider will be computed automatically based on its
+    /// shape.
+    pub fn set_density(&mut self, density: Real) {
+        self.do_set_mass_properties(ColliderMassProps::Density(density));
+    }
+
+    /// Sets the mass of this collider.
+    ///
+    /// This will override any previous mass-properties set by [`Self::set_density`],
+    /// [`Self::set_mass`], [`Self::set_mass_properties`], [`ColliderBuilder::density`],
+    /// [`ColliderBuilder::mass`], or [`ColliderBuilder::mass_properties`]
+    /// for this collider.
+    ///
+    /// The angular inertia of this collider will be computed automatically based on its shape
+    /// and this mass value.
+    pub fn set_mass(&mut self, mass: Real) {
+        self.do_set_mass_properties(ColliderMassProps::Mass(mass));
+    }
+
+    /// Sets the mass properties of this collider.
+    ///
+    /// This will override any previous mass-properties set by [`Self::set_density`],
+    /// [`Self::set_mass`], [`Self::set_mass_properties`], [`ColliderBuilder::density`],
+    /// [`ColliderBuilder::mass`], or [`ColliderBuilder::mass_properties`]
+    /// for this collider.
+    pub fn set_mass_properties(&mut self, mass_properties: MassProperties) {
+        self.do_set_mass_properties(ColliderMassProps::MassProperties(Box::new(mass_properties)))
+    }
+
+    fn do_set_mass_properties(&mut self, mprops: ColliderMassProps) {
+        if mprops != self.mprops {
+            self.changes |= ColliderChanges::LOCAL_MASS_PROPERTIES;
+            self.mprops = mprops;
+        }
+    }
+
     /// The geometric shape of this collider.
     pub fn shape(&self) -> &dyn Shape {
         self.shape.as_ref()
@@ -284,10 +328,7 @@ impl Collider {
 
     /// Compute the local-space mass properties of this collider.
     pub fn mass_properties(&self) -> MassProperties {
-        match &self.mprops {
+        self.mprops.mass_properties(&*self.shape)
-            ColliderMassProps::Density(density) => self.shape.mass_properties(*density),
-            ColliderMassProps::MassProperties(mass_properties) => **mass_properties,
-        }
     }
 
     /// The total force magnitude beyond which a contact force event can be emitted.
@@ -303,11 +344,8 @@ impl Collider {
 pub struct ColliderBuilder {
     /// The shape of the collider to be built.
     pub shape: SharedShape,
-    /// The uniform density of the collider to be built.
+    /// Controls the way the collider’s mass-properties are computed.
-    pub density: Option<Real>,
+    pub mass_properties: ColliderMassProps,
-    /// Overrides automatic computation of `MassProperties`.
-    /// If None, it will be computed based on shape and density.
-    pub mass_properties: Option<MassProperties>,
     /// The friction coefficient of the collider to be built.
     pub friction: Real,
     /// The rule used to combine two friction coefficients.
@@ -341,8 +379,7 @@ impl ColliderBuilder {
     pub fn new(shape: SharedShape) -> Self {
         Self {
             shape,
-            density: None,
+            mass_properties: ColliderMassProps::default(),
-            mass_properties: None,
             friction: Self::default_friction(),
             restitution: 0.0,
             position: Isometry::identity(),
@@ -627,9 +664,6 @@ impl ColliderBuilder {
     }
 
     /// Sets whether or not the collider built by this builder is a sensor.
-    ///
-    /// Sensors will have a default density of zero,
-    /// but if you call [`Self::mass_properties`] you can assign a mass to a sensor.
     pub fn sensor(mut self, is_sensor: bool) -> Self {
         self.is_sensor = is_sensor;
         self
@@ -679,19 +713,34 @@ impl ColliderBuilder {
 
     /// Sets the uniform density of the collider this builder will build.
     ///
-    /// This will be overridden by a call to [`Self::mass_properties`] so it only makes sense to call
+    /// This will be overridden by a call to [`Self::mass`] or [`Self::mass_properties`] so it only
-    /// either [`Self::density`] or [`Self::mass_properties`].
+    /// makes sense to call either [`Self::density`] or [`Self::mass`] or [`Self::mass_properties`].
+    ///
+    /// The mass and angular inertia of this collider will be computed automatically based on its
+    /// shape.
     pub fn density(mut self, density: Real) -> Self {
-        self.density = Some(density);
+        self.mass_properties = ColliderMassProps::Density(density);
+        self
+    }
+
+    /// Sets the mass of the collider this builder will build.
+    ///
+    /// This will be overridden by a call to [`Self::density`] or [`Self::mass_properties`] so it only
+    /// makes sense to call either [`Self::density`] or [`Self::mass`] or [`Self::mass_properties`].
+    ///
+    /// The angular inertia of this collider will be computed automatically based on its shape
+    /// and this mass value.
+    pub fn mass(mut self, mass: Real) -> Self {
+        self.mass_properties = ColliderMassProps::Mass(mass);
         self
     }
 
     /// Sets the mass properties of the collider this builder will build.
     ///
-    /// If this is set, [`Self::density`] will be ignored, so it only makes sense to call
+    /// This will be overridden by a call to [`Self::density`] or [`Self::mass`] so it only
-    /// either [`Self::density`] or [`Self::mass_properties`].
+    /// makes sense to call either [`Self::density`] or [`Self::mass`] or [`Self::mass_properties`].
     pub fn mass_properties(mut self, mass_properties: MassProperties) -> Self {
-        self.mass_properties = Some(mass_properties);
+        self.mass_properties = ColliderMassProps::MassProperties(Box::new(mass_properties));
         self
     }
 
@@ -745,16 +794,7 @@ impl ColliderBuilder {
 
     /// Builds a new collider attached to the given rigid-body.
     pub fn build(&self) -> Collider {
-        let mass_info = if let Some(mp) = self.mass_properties {
-            ColliderMassProps::MassProperties(Box::new(mp))
-        } else {
-            let default_density = Self::default_density();
-            let density = self.density.unwrap_or(default_density);
-            ColliderMassProps::Density(density)
-        };
-
         let shape = self.shape.clone();
-        let mprops = mass_info;
         let material = ColliderMaterial {
             friction: self.friction,
             restitution: self.restitution,
@@ -779,7 +819,7 @@ impl ColliderBuilder {
 
         Collider {
             shape,
-            mprops,
+            mprops: self.mass_properties.clone(),
             material,
             parent: None,
             changes,

src/geometry/collider_components.rs

@@ -47,21 +47,23 @@ bitflags::bitflags! {
     pub struct ColliderChanges: u32 {
         /// Flag indicating that any component of the collider has been modified.
         const MODIFIED = 1 << 0;
+        /// Flag indicating that the density or mass-properties of this collider was changed.
+        const LOCAL_MASS_PROPERTIES = 1 << 1; // => RigidBody local mass-properties update.
         /// Flag indicating that the `ColliderParent` component of the collider has been modified.
-        const PARENT   = 1 << 1; // => BF & NF updates.
+        const PARENT   = 1 << 2; // => BF & NF updates.
         /// Flag indicating that the `ColliderPosition` component of the collider has been modified.
-        const POSITION = 1 << 2; // => BF & NF updates.
+        const POSITION = 1 << 3; // => BF & NF updates.
         /// Flag indicating that the collision groups of the collider have been modified.
-        const GROUPS   = 1 << 3; // => NF update.
+        const GROUPS   = 1 << 4; // => NF update.
         /// Flag indicating that the `ColliderShape` component of the collider has been modified.
-        const SHAPE    = 1 << 4; // => BF & NF update. NF pair workspace invalidation.
+        const SHAPE    = 1 << 5; // => BF & NF update. NF pair workspace invalidation.
         /// Flag indicating that the `ColliderType` component of the collider has been modified.
-        const TYPE     = 1 << 5; // => NF update. NF pair invalidation.
+        const TYPE     = 1 << 6; // => NF update. NF pair invalidation.
         /// Flag indicating that the dominance groups of the parent of this collider have been modified.
         ///
         /// This flags is automatically set by the `PhysicsPipeline` when the `RigidBodyChanges::DOMINANCE`
         /// or `RigidBodyChanges::TYPE` of the parent rigid-body of this collider is detected.
-        const PARENT_EFFECTIVE_DOMINANCE = 1 << 6; // NF update.
+        const PARENT_EFFECTIVE_DOMINANCE = 1 << 7; // NF update.
     }
 }
 
@@ -86,7 +88,7 @@ impl ColliderChanges {
         //       bottleneck at some point in the future (which is very unlikely)
         //       we could do a special-case for dominance-only change (so that
         //       we only update the relative_dominance of the pre-existing contact.
-        self.bits() > 1
+        self.bits() > 2
     }
 }
 
@@ -134,6 +136,10 @@ pub enum ColliderMassProps {
     /// Its actual `MassProperties` are computed automatically with
     /// the help of [`SharedShape::mass_properties`].
     Density(Real),
+    /// The collider is given a mass.
+    ///
+    /// Its angular inertia will be computed automatically based on this mass.
+    Mass(Real),
     /// The collider is given explicit mass-properties.
     MassProperties(Box<MassProperties>),
 }
@@ -159,8 +165,23 @@ impl ColliderMassProps {
     /// If `self` is the `MassProperties` variant, then this returns the stored mass-properties.
     pub fn mass_properties(&self, shape: &dyn Shape) -> MassProperties {
         match self {
-            Self::Density(density) => shape.mass_properties(*density),
+            ColliderMassProps::Density(density) => {
-            Self::MassProperties(mprops) => **mprops,
+                if *density != 0.0 {
+                    shape.mass_properties(*density)
+                } else {
+                    MassProperties::default()
+                }
+            }
+            ColliderMassProps::Mass(mass) => {
+                if *mass != 0.0 {
+                    let mut mprops = shape.mass_properties(1.0);
+                    mprops.set_mass(*mass, true);
+                    mprops
+                } else {
+                    MassProperties::default()
+                }
+            }
+            ColliderMassProps::MassProperties(mass_properties) => **mass_properties,
         }
     }
 }

src/pipeline/physics_pipeline.rs

@@ -407,6 +407,9 @@ impl PhysicsPipeline {
         hooks: &dyn PhysicsHooks,
         events: &dyn EventHandler,
     ) {
+        self.counters.reset();
+        self.counters.step_started();
+
         // Apply some of delayed wake-ups.
         for handle in impulse_joints
             .to_wake_up
@@ -417,18 +420,15 @@ impl PhysicsPipeline {
         }
 
         // Apply modifications.
-        let modified_bodies = bodies.take_modified();
         let mut modified_colliders = colliders.take_modified();
         let mut removed_colliders = colliders.take_removed();
-
-        self.counters.reset();
-        self.counters.step_started();
-
         super::user_changes::handle_user_changes_to_colliders(
             bodies,
             colliders,
             &modified_colliders[..],
         );
+
+        let modified_bodies = bodies.take_modified();
         super::user_changes::handle_user_changes_to_rigid_bodies(
             Some(islands),
             bodies,

src/pipeline/user_changes.rs

@@ -2,17 +2,12 @@ use crate::dynamics::{
     IslandManager, RigidBodyChanges, RigidBodyHandle, RigidBodySet, RigidBodyType,
 };
 use crate::geometry::{ColliderChanges, ColliderHandle, ColliderPosition, ColliderSet};
-use parry::utils::hashmap::HashMap;
 
 pub(crate) fn handle_user_changes_to_colliders(
     bodies: &mut RigidBodySet,
     colliders: &mut ColliderSet,
     modified_colliders: &[ColliderHandle],
 ) {
-    // TODO: avoid this hashmap? We could perhaps add a new flag to RigidBodyChanges to
-    //       indicated that the mass properties need to be recomputed?
-    let mut mprops_to_update = HashMap::default();
-
     for handle in modified_colliders {
         // NOTE: we use `get` because the collider may no longer
         //       exist if it has been removed.
@@ -26,22 +21,19 @@ pub(crate) fn handle_user_changes_to_colliders(
                 }
             }
 
-            if co.changes.contains(ColliderChanges::SHAPE) {
+            if co
-                if let Some(co_parent) = co.parent {
+                .changes
-                    mprops_to_update.insert(co_parent.handle, ());
+                .intersects(ColliderChanges::SHAPE | ColliderChanges::LOCAL_MASS_PROPERTIES)
+            {
+                if let Some(rb) = co
+                    .parent
+                    .and_then(|p| bodies.get_mut_internal_with_modification_tracking(p.handle))
+                {
+                    rb.changes |= RigidBodyChanges::LOCAL_MASS_PROPERTIES;
                 }
             }
         }
     }
-
-    for (to_update, _) in mprops_to_update {
-        let rb = bodies.index_mut_internal(to_update);
-        rb.mprops.recompute_mass_properties_from_colliders(
-            colliders,
-            &rb.colliders,
-            &rb.pos.position,
-        );
-    }
 }
 
 pub(crate) fn handle_user_changes_to_rigid_bodies(
@@ -163,6 +155,16 @@ pub(crate) fn handle_user_changes_to_rigid_bodies(
                 }
             }
 
+            if changes
+                .intersects(RigidBodyChanges::LOCAL_MASS_PROPERTIES | RigidBodyChanges::COLLIDERS)
+            {
+                rb.mprops.recompute_mass_properties_from_colliders(
+                    colliders,
+                    &rb.colliders,
+                    &rb.pos.position,
+                );
+            }
+
             rb.changes = RigidBodyChanges::empty();
             rb.ids = ids;
             rb.activation = activation;