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Common mistakes

My local build of Rapier is slower than the online demos​

Make sure you are building your project (or just the Rapier dependency) in release mode, e.g., cargo build --release. Rapier can be 100 times slower without optimizations enabled. Keep in mind that it is possible to compile your project without optimizations while keeping optimizations enabled for Rapier itself:

# Add this to your Cargo.toml
[profile.dev.package.rapier3d]
opt-level = 3

See the cargo book about profile overrides for details about this technique.

info

Also note that setting the number codegen units to 1 will further improve performances in a noticeable way, even for a release build (though the build itself will take longer to complete):

# Add this to your Cargo.toml
[profile.release]
codegen-units = 1

Rigid-body isn't affected by gravity​

If you expect your rigid-body to fall because of gravity but it doesn't, please make sure to double-check the following:

  • Your gravity vector is non-zero.
  • Your rigid-body is a dynamic rigid-body.
  • You didn't lock the translations of the rigid-body.
  • The rigid-body has a non-zero mass.

Note that a collider not attached to a dynamic rigid-body will never fall because it won't be affected by forces.

note

If the rigid-body has no collider attached to it, its mass will be zero unless you gave it a mass (or mass properties) explicitly. If the rigid-body has colliders attached to it and you didn't give the rigid-body a mass explicitly, make sure that at least one of the colliders has a non-zero density (or non-zero mass if you set it explicitly on the collider).

warning

Currently, colliders with triangle-meshes won't have their mass properties computed automatically. So if a rigid-body only has triangle-mesh colliders attached to it, you need to set its mass/angular inertia manually.

Applying a force to a rigid-body doesn't work​

If applying a force or an impulse to a rigid-body doesn't work, please make sure to double-check the following:

  • The rigid-body is a dynamic rigid-body.
  • The rigid-body has a non-zero mass (or non-zero angular inertia for torques).
  • The force or impulse must be strong enough to actually push the rigid-body. You may for example try with a very high force/impulse value (say, with a magnitude of 100_000.0) and see if this stronger force works.
  • The rigid-body is awake, either by calling RigidBody::wake_up(true) explicitly, or by using true as the last argument of the force/impulse application method.
note

If the rigid-body has no collider attached to it, its mass will be zero unless you gave it a mass (or mass properties) explicitly. If the rigid-body has colliders attached to it and you didn't give the rigid-body a mass explicitly, make sure that at least one of the colliders has a non-zero density (or non-zero mass if you set it explicitly on the collider).

The simulation panics!​

It may happen that Rapier panics with the following panic message:

"assertion failed: proxy.aabb.maxs[dim] >= self.min_bound"

This panics generally means that something went wrong in the simulation, resulting in NaN values appearing in the collider positions. The most common configuration that makes this panic happen is if two dynamic rigid-bodies with a zero mass start being in contact. To fix this, make sure that your dynamic rigid-bodies have a non-zero mass.

note

If the rigid-body has no collider attached to it, its mass will be zero unless you gave it a mass (or mass properties) explicitly. If the rigid-body has colliders attached to it and you didn't give the rigid-body a mass explicitly, make sure that at least one of the colliders has a non-zero density (or non-zero mass if you set it explicitly on the collider).

Why is everything moving in slow-motion?​

A common mistake, especially in 2D, is to use pixels as the length unit in the physics world. Let's say that in 2D you have a 100x100 pixels sprite for your player. It may be tempting to use a 100x100 cuboid collider for this sprite: ColliderBuilder::cuboid(50.0, 50.0) (we set 50.0 because this is the half-width of the cuboid). Doing this will make it look like the simulation runs in slow-motion because the cuboid will be huge compared to the magnitude of the default gravity (-9.81).

The recommended way of using Rapier is to use SI units (meters, seconds, kilograms, etc.) If the player sprite is a 100x100 cuboid, then it is as if your player is 100 meters tall and 100 meters wide, which is huge. Therefore it is recommended to have a scaling factor between the graphics and the physics. For example we can say that 1 physics meter is equal to 50 pixels. This means that we can initialize our player collider as a 2x2 cuboid while still using a 100x100 pixels sprite.

All we need to do to keep measures in sync is to multiply by our scaling factor 50 all the positions given by the physics engine before rendering:

// Scale the translation to convert from meters to pixels
sprite.set_translation(rigid_body.translation() * 50.0);
// Rotation angles don't need to be scaled
sprite.set_rotation(rigid_body.rotation().angle());