Collision module

This module controls how particlesA small, simple image or mesh that is emitted by a particle system. A particle system can display and move particles in great numbers to represent a fluid or amorphous entity. The effect of all the particles together creates the impression of the complete entity, such as smoke. More info
See in Glossary collide with GameObjectsThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary in the SceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
See in Glossary. Use the first drop-down to define whether your collision settings apply to Planes or to the WorldThe area in your scene in which all objects reside. Often used to specify that coordinates are world-relative, as opposed to object-relative.
See in Glossary. If you choose World, use the Collision Mode drop-down to define whether your collision settings apply for a 2D or 3D world.

Using the Collision module

This module is part of the Particle SystemA component that simulates fluid entities such as liquids, clouds and flames by generating and animating large numbers of small 2D images in the scene. More info
See in Glossary component. When you create a new Particle System GameObject, or add a Particle System component to an exiting GameObject, Unity adds the Collision module to the Particle System. By default, Unity disables this module. To create a new Particle System and enable this module:

1. Click GameObject > Effects > Particle System.
2. In the InspectorA Unity window that displays information about the currently selected GameObject, asset or project settings, allowing you to inspect and edit the values. More info
   See in Glossary, find the Particle System component.
3. In the Particle System component, find the Collision module fold-out.
4. To the left of the fold-out header, enable the checkbox.

API

Since this module is part of the Particle System component, you access it through the ParticleSystem class. For information on how to access it and change values at runtime, see the Collision module API documentation.

Planes module properties

For some properties in this section, you can use different modes to set their value. For information on the modes you can use, see Varying properties over time.

World module properties

For some properties in this section, you can use different modes to set their value. For information on the modes you can use, see Varying properties over time.

Details

When other objects surround a Particle System, the effect is often more convincing when the particles interact with those objects. For example, water or debris should be obstructed by a solid wall rather than simply passing through it. With the CollisionA collision occurs when the physics engine detects that the colliders of two GameObjects make contact or overlap, when at least one has a Rigidbody component and is in motion. More info
See in Glossary module enabled, particles can collide with objects in the Scene.

A Particle System can be set so its particles collide with any Collider in the scene by selecting World mode from the pop-up. Colliders can also be disabled according to the layer they are on by using the Collides With property. The pop-up also has a Planes mode option which allows you to add a set of planes to the Scene that don’t need to have Colliders. This option is useful for simple floors, walls and similar objects, and has a lower processor overhead than World mode.

When Planes mode is enabled, a list of transforms (typically empty GameObjects) can be added via the Planes property. The planes extend infinitely in the objects’ local XZ planes, with the positive Y axis indicating the planes’ normal vectors. To assist with development, the planes will be shown as Gizmos in the Scene, regardless of whether or not the objects have any visible MeshThe main graphics primitive of Unity. Meshes make up a large part of your 3D worlds. Unity supports triangulated or Quadrangulated polygon meshes. Nurbs, Nurms, Subdiv surfaces must be converted to polygons. More info
See in Glossary themselves. The Gizmos can be shown as a wireframe grid or a solid plane, and can also be scaled. However, the scaling only applies to the visualization - the collision planes themselves extend infinitely through the Scene.

When collisions are enabled, the size of a particle is sometimes a problem because its graphic can be clipped as it makes contact with a surface. This can result in a particle appearing to “sink” partway into a surface before stopping or bouncing. The Radius Scale property addresses this issue by defining an approximate circular radius for the particles, as a percentage of its actual size. This size information is used to prevent clipping and avoid the sinking-in effect.

The Dampen and Bounce properties are useful for when the particles represent solid objects. For example, gravel will tend to bounce off a hard surface when thrown but a snowball’s particles might lose speed during a collision. Lifetime Loss and Min Kill Speed can help to reduce the effects of residual particles following a collision. For example, a fireball might last for a few seconds while flying through the air but after colliding, the separate fire particles should dissipate quickly.

You can also detect particle collisions from a script if Send Collision Messages is enabled. The script can be attached to the object with the particle system, or the one with the Collider, or both. By detecting collisions, you can use particles as active objects in gameplay, for example as projectiles, magic spells and power-ups. See the script reference page for MonoBehaviour.OnParticleCollision for further details and examples.

World Collision Quality

The World Collision module has a Collision Quality property, which you can set to High, Medium or Low. When Collision Quality is set to Medium (Static Colliders) or Low (Static Colliders), collisions use a grid of voxelsA 3D pixel. More info
See in Glossary (values on a 3D grid) to cache previous collisions, for fast re-use in later frames.

This cache consists of a plane in each voxel, where the plane represents the collision surface at that location. On each frame, Unity checks the cache for a plane at the position of the particle, and if there is one, Unity uses it for collision detectionAn automatic process performed by Unity which determines whether a moving GameObject with a Rigidbody and collider component has come into contact with any other colliders. More info
See in Glossary. Otherwise, it asks the physics system. If a collision is returned, it is added to the cache for fast querying on subsequent frames.

This is an approximation, so some missed collisions might occur. You can reduce the Voxel Size value to help with this; however, doing so uses extra memory, and is less efficient.

The only difference between Medium and Low is how many times per frame the system is allowed to query the physics system. Low makes fewer queries per frame than Medium. Once the per-frame budget has been exceeded, only the cache is used for any remaining particles. This can lead to an increase in missed collisions, until the cache has been more comprehensively populated.

• 2017–05–30 Page amended

• Functionality of Collision module changed in Unity 2017.1 NewIn20171