Collision shapes (3D)¶

This guide explains:

The types of collision shapes available in 3D in Redot.
Using a convex or a concave mesh as a collision shape.
Performance considerations regarding 3D collisions.

Redot provides many kinds of collision shapes, with different performance and accuracy tradeoffs.

You can define the shape of a PhysicsBody by adding one or more CollisionShapes as child nodes. Note that you must add a Shape resource to collision shape nodes in the Inspector dock.

Note

When you add multiple collision shapes to a single PhysicsBody, you don't have to worry about them overlapping. They won't "collide" with each other.

Primitive collision shapes¶

Redot provides the following primitive collision shape types:

You can represent the collision of most smaller objects using one or more primitive shapes. However, for more complex objects, such as a large ship or a whole level, you may need convex or concave shapes instead. More on that below.

We recommend favoring primitive shapes for dynamic objects such as RigidBodies and KinematicBodies as their behavior is the most reliable. They often provide better performance as well.

Convex collision shapes¶

Convex collision shapes are a compromise between primitive collision shapes and concave collision shapes. They can represent shapes of any complexity, but with an important caveat. As their name implies, an individual shape can only represent a convex shape. For instance, a pyramid is convex, but a hollow box is concave. To define a concave object with a single collision shape, you need to use a concave collision shape.

Depending on the object's complexity, you may get better performance by using multiple convex shapes instead of a concave collision shape. Redot lets you use convex decomposition to generate convex shapes that roughly match a hollow object. Note this performance advantage no longer applies after a certain amount of convex shapes. For large and complex objects such as a whole level, we recommend using concave shapes instead.

You can generate one or several convex collision shapes from the editor by selecting a MeshInstance and using the Mesh menu at the top of the 3D viewport. The editor exposes two generation modes:

Create Single Convex Collision Sibling uses the Quickhull algorithm. It creates one CollisionShape node with an automatically generated convex collision shape. Since it only generates a single shape, it provides good performance and is ideal for small objects.
Create Multiple Convex Collision Siblings uses the V-HACD algorithm. It creates several CollisionShape nodes, each with a convex shape. Since it generates multiple shapes, it is more accurate for concave objects at the cost of performance. For objects with medium complexity, it will likely be faster than using a single concave collision shape.

Concave or trimesh collision shapes¶

Concave collision shapes, also called trimesh collision shapes, can take any form, from a few triangles to thousands of triangles. Concave shapes are the slowest option but are also the most accurate in Redot. You can only use concave shapes within StaticBodies. They will not work with KinematicBodies or RigidBodies unless the RigidBody's mode is Static.

Note

Even though concave shapes offer the most accurate collision, contact reporting can be less precise than primitive shapes.

When not using GridMaps for level design, concave shapes are the best approach for a level's collision. That said, if your level has small details, you may want to exclude those from collision for performance and game feel. To do so, you can build a simplified collision mesh in a 3D modeler and have Redot generate a collision shape for it automatically. More on that below

Note that unlike primitive and convex shapes, a concave collision shape doesn't have an actual "volume". You can place objects both outside of the shape as well as inside.

You can generate a concave collision shape from the editor by selecting a MeshInstance and using the Mesh menu at the top of the 3D viewport. The editor exposes two options:

Create Trimesh Static Body is a convenient option. It creates a StaticBody containing a concave shape matching the mesh's geometry.
Create Trimesh Collision Sibling creates a CollisionShape node with a concave shape matching the mesh's geometry.

Note

Suppose you need to make a RigidBody slide on a concave collision shape. In that case, you may notice that sometimes, the RigidBody will bump upwards. To solve this, open Project > Project Settings and enable Physics > 3d > Smooth Trimesh Collision.

Once you've enabled smooth trimesh collision, make sure the concave shape is the only shape of your StaticBody and that it's located at its origin without any rotation. This way, the RigidBody should slide perfectly on the StaticBody.

Performance caveats¶

You aren't limited to a single collision shape per PhysicsBody. Still, we recommend keeping the number of shapes as low as possible to improve performance, especially for dynamic objects like RigidBodies and KinematicBodies. On top of that, avoid translating, rotating, or scaling CollisionShapes to benefit from the physics engine's internal optimizations.

When using a single non-transformed collision shape in a StaticBody, the engine's broad phase algorithm can discard inactive PhysicsBodies. The narrow phase will then only have to take into account the active bodies' shapes. If a StaticBody has many collision shapes, the broad phase will fail. The narrow phase, which is slower, must then perform a collision check against each shape.

If you run into performance issues, you may have to make tradeoffs in terms of accuracy. Most games out there don't have a 100% accurate collision. They find creative ways to hide it or otherwise make it unnoticeable during normal gameplay.