SKPhysicsContact detect the contact not expecting

Question

Here is my code:

func didBegin(_ contact: SKPhysicsContact) {

  print("contact.bodyA.categoryBitMask:::\(contact.bodyA.categoryBitMask)")
  print("contact.bodyA.contactTestBitMask:::\(contact.bodyA.contactTestBitMask)")
  print("contact.bodyA.isDynamic:::\(contact.bodyA.isDynamic)")
  print("contact.bodyB.categoryBitMask:::\(contact.bodyB.categoryBitMask)")
  print("contact.bodyB.contactTestBitMask:::\(contact.bodyB.contactTestBitMask)")
  print("contact.bodyB.isDynamic:::\(contact.bodyB.isDynamic)")

and I got the result:

contact.bodyA.categoryBitMask:::1 contact.bodyA.contactTestBitMask:::9 contact.bodyA.isDynamic:::true contact.bodyB.categoryBitMask:::5 contact.bodyB.contactTestBitMask:::8 contact.bodyB.isDynamic:::false

I found the contact nodes are not collected by its categoryBitMask and contactTestBitMask. I mean A's categoryBitMask not match B's contactTestBitMask, but their contact can be detect.

Somebody tell me why, and how can I optimize the SKPhysicsContact.

SpriteKit

548

Posted by

pigmore

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Answer 1

body A category = 0001, contact = 1001

body B category = 0101, contact = 1000

(Body A category) bitwise-& (body B contact) = 0000, would not flag collision

(Body B category) bitwise-& (body A contact) = 0001, flags collision

According to the documentation, if either comparison gives a nonzero result, you'll get a collision notification.

https://developer.apple.com/documentation/spritekit/skphysicsbody/1519781-contacttestbitmask

If you don't want that notification, you have to make sure that body B's category mask does not overlap with body A's contact mask.

Posted by

bg2b

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Answer 2

but my confuse is

contact.bodyA.categoryBitMask = Uint32(1) contactTestBitMask = Uint32(9)

contact.bodyB.categoryBitMask = Uint32(5) contactTestBitMask = Uint32(8)

Their collision can be detect, and I am sure the unnecessary collision detect drops my game fps down.

Posted by

pigmore

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Answer 3

Uint32(5) bitwise-& Uint32(9) = Uint32(1). Because that result isn't 0, it's notifying you of the collision.

If you don't want that notification, you have to either change B's category bitmask or A's contact bitmask.

The fact that the overlap between A's category and B's contact is 0 doesn't matter, since it notifies you if either result isn't 0.

Posted by

bg2b

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SKPhysicsContact detect the contact not expecting

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