Collision not detected correctly with multi-geometry shapes - ios

Question

I have a spaceship object which has a complex geometry, and since SceneKit's physics doesn't work with complex bodies, I have adopted a workaround: I'm using some basic shapes like cylinders and cubes so simulate the whole spaceship's body. In Blender I created a set of objects that approximate the shape of the spaceship:
Then when I load the scene I remove these objects, but use their geometry to construct a SCNPhysicsShape to be used as the physics body of the spaceship:
// First I retrieve all of these bodies, which I named "Body1" up to 9:
let bodies = _scene.rootNode.childNodes(passingTest: { (node:SCNNode, stop:UnsafeMutablePointer<ObjCBool>) -> Bool in
if let name = node.name
{
return name.contains("Body")
}
return false
})
// Then I create an array of SCNPhysicsShape objects, and an array
// containing the transformation associated to each shape
var shapes = [SCNPhysicsShape]()
var transforms = [NSValue]()
for body in bodies
{
shapes.append(SCNPhysicsShape(geometry: body.geometry!, options: nil))
transforms.append(NSValue(scnMatrix4:body.transform))
// I remove it from the scene because it shouldn't be visible, as it has
// the sole goal is simulating the spaceship's physics
body.removeFromParentNode()
}
// Finally I create a SCNPhysicsShape that contains all of the shapes
let shape = SCNPhysicsShape(shapes: shapes, transforms: transforms)
let body = SCNPhysicsBody(type: .dynamic, shape: shape)
body.isAffectedByGravity = false
body.categoryBitMask = SpaceshipCategory
body.collisionBitMask = 0
body.contactTestBitMask = RockCategory
self.spaceship.physicsBody = body
The SCNPhysicsShape object should contain all the shapes that I created in the Blender file. But when I test the program, the spaceship just behaves like an empty body, and collisions are not detected.
PS: my goal is only to detect collisions. I don't want the physics engine to simulate physics.

Answer 1

You want to use a concave shape for your ship. Example below. In order to use concave your body must be static or kinematic. Kinematic is ideal for animated objects.
No need for:
body.collisionBitMask = 0
A contact bit mask is all you will need for contact only.
The code below should work for what you are looking for. This is swift 3 code fyi.
let body = SCNPhysicsBodyType.kinematic
let shape = SCNPhysicsShape(node: spaceship, options: [SCNPhysicsShape.Option.type: SCNPhysicsShape.ShapeType.concavePolyhedron])
spaceship.physicsBody = SCNPhysicsBody(type: body, shape: shape)
Without seeing your bit mask, I am unable to tell if you are handling those properly. You should have something like:
spaceship.physicsBody.categoryBitMask = 1 << 1
Anything you want to contact your spaceship should have:
otherObject.physicsBody.contactTestBitMask = 1 << 1
This will only register a contact of the "otherObject" to the "spaceship" and not the "spaceship" to the "otherObject". So make sure your physics world contact delegate is handled that way or vise versa. No need for both objects to look for each other. That would be less efficient.
Example of delegate below:
func physicsWorld(_ world: SCNPhysicsWorld, didBegin contact: SCNPhysicsContact) {
var tempOtherObject: SCNNode!
var tempSpaceship: SCNNode!
// This will assign the tempOtherObject and tempSpaceship to the proper contact nodes
if contact.nodeA.node == otherObject {
tempOtherObject = contact.nodeA
tempSpaceship = contact.nodeB
}else{
tempOtherObject = contact.nodeB
tempSpaceship = contact.nodeA
}
print("tempOtherObject = ", tempOtherObject)
print("tempSpaceship = ", tempSpaceship)
}