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Collision shapes (2D)

This guide explains:

  • The types of collision shapes available in 2D in Redot.

  • Using an image converted to a polygon as a collision shape.

  • Performance considerations regarding 2D collisions.

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

You can define the shape of a PhysicsBody2D by adding one or more CollisionShape2Ds or CollisionPolygon2Ds as child nodes. Note that you must add a Shape2D resource to collision shape nodes in the Inspector dock.

Note

When you add multiple collision shapes to a single PhysicsBody2D, 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 CharacterBodies as their behavior is the most reliable. They often provide better performance as well.

Convex collision shapes

Warning

Redot currently doesn't offer a built-in way to create 2D convex collision shapes. This section is mainly here for reference purposes.

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.

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 CharacterBodies 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 TileMaps for level design, concave shapes are the best approach for a level's collision.

You can configure the CollisionPolygon2D node's build mode in the inspector. If it is set to Solids (the default), collisions will include the polygon and its contained area. If it is set to Segments, collisions will only include the polygon edges.

You can generate a concave collision shape from the editor by selecting a Sprite2D and using the Sprite2D menu at the top of the 2D viewport. The Sprite2D menu dropdown exposes an option called Create CollisionPolygon2D Sibling. Once you click it, it displays a menu with 3 settings:

  • Simplification: Higher values will result in a less detailed shape, which improves performance at the cost of accuracy.

  • Shrink (Pixels): Higher values will shrink the generated collision polygon relative to the sprite's edges.

  • Grow (Pixels): Higher values will grow the generated collision polygon relative to the sprite's edges. Note that setting Grow and Shrink to equal values may yield different results than leaving both of them on 0.

Note

If you have an image with many small details, it's recommended to create a simplified version and use it to generate the collision polygon. This can result in better performance and game feel, since the player won't be blocked by small, decorative details.

To use a separate image for collision polygon generation, create another Sprite2D, generate a collision polygon sibling from it then remove the Sprite2D node. This way, you can exclude small details from the generated collision.

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 CharacterBodies. 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.