---
title: SCNParticleModifierStage.preCollision
framework: scenekit
role: symbol
role_heading: Case
path: scenekit/scnparticlemodifierstage/precollision
---

# SCNParticleModifierStage.preCollision

The stage before SceneKit simulates the results of collisions between particles and scene geometry.

## Declaration

```swift
case preCollision
```

## Discussion

Discussion Insert a modifier block at this stage to alter the inputs to collision resolution. For example, if you modify the bounce factors of particles during this stage, SceneKit uses the modified factors to compute the bounce velocity of each particle.

## See Also

### Constants

- [SCNParticleModifierStage.preDynamics](scenekit/scnparticlemodifierstage/predynamics.md)
- [SCNParticleModifierStage.postDynamics](scenekit/scnparticlemodifierstage/postdynamics.md)
- [SCNParticleModifierStage.postCollision](scenekit/scnparticlemodifierstage/postcollision.md)