# contactTestBitMask A mask that defines which categories of bodies cause intersection notifications with this physics body. ``` var contactTestBitMask: Int { get set } ``` ## Discussion When two physics bodies overlap, a contact may occur. SceneKit compares the body’s contact mask to the other body’s category mask by performing a bitwise AND operation. If the result is a nonzero value, SceneKit creates an [`SCNPhysicsContact`](/documentation/SceneKit/SCNPhysicsContact) object describing the contact and sends messages to the [`contactDelegate`](/documentation/SceneKit/SCNPhysicsWorld/contactDelegate) object of the scene’s physics world. For best performance, only set bits in the contact mask for interactions you are interested in. For applications running in OS X v10.10 or iOS 8, this property’s value matches that of the [`collisionBitMask`](/documentation/SceneKit/SCNPhysicsBody/collisionBitMask) property—that is, SceneKit sends contact messages if and only if a collision occurs. For applications running in OS X v10.11 or iOS 9 or later, this property’s value defaults to zero and need not match the collision mask—that is, a pair of bodies generates contact messages whenever the bodies intersect, regardless of whether they collide or pass through one another. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)