The Collisions module controls how particles 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.
When other objects surround a 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, 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 ColliderAn invisible shape that is used to handle physical collisions for an object. A collider doesn’t need to be exactly the same shape as the object’s mesh - a rough approximation is often more efficient and indistinguishable in gameplay. More info
See in Glossary in the scene by selecting 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 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 GizmosA graphic overlay associated with a GameObject in a Scene, and displayed in the Scene View. Built-in scene tools such as the move tool are Gizmos, and you can create custom Gizmos using textures or scripting. Some Gizmos are only drawn when the GameObject is selected, while other Gizmos are drawn by the Editor regardless of which GameObjects are selected. More info
See in Glossary 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.
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 voxels (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.