Question Relation between SetLayerCollisionMask and IgnoreLayerCollision not documented

If two specific colliders are set to ignore each other then nothing can override that.

The order of evaluation is:

1. Kinematic/Kinematic and not using full-kinematic contacts? Yes so no Contact
2. Ignore Collider/Collider? Yes so no Contact
3. Either Collider using an Effector Collider Mask? Yes, so contact is defined by that.
   
4. Combine the Layer Collision Matrix with +Include Layer(s) and -Exclude Layer(s) + Layer Priority and use that.
5. If Layer masks are conflicting and they have the same layer priority then no Contact (sort of a soft user error)

4 is what you'd use most of the time with 2 being the per-collider-pair override.

 

Oh sorry, think I had too much of your other question on my mind!

So yes, it's a simple bit toggle which updates the Layer Collision Matrix. It's actually the call we use in the Editor itself so setting it outside of play-mode will persist the change. Obviously in play-mode it'll won't be persisted.

Here's it being used in the Layer Collision Matrix Editor itself:

https://github.com/Unity-Technologi...aged/Settings/Physics2DSettingsEditor.cs#L196

Internally the Layer Collision Matrix is an array of 32 x UInt32 where each is a bit-mask of what that layer (array index) can interact with. This means that when you use "SetLayerCollisionMask" to set a whole mask, the whole set of masks are updated appropriately. You can find this stored in the "ProjectSettings/Physics2DSettings.asset" as "m_LayerCollisionMatrix".

Yes! The "ignore" part inverses it.

Or layer 0 can interact only with layers 3 and 6

Code (CSharp):

1. Physics2D.SetLayerCollisionMask(0, (1 << 3) | (1 << 6));

 

Some of the source for ignoring a layer collision:

Code (CSharp):

1. void Physics2DSettings::IgnoreLayerCollision(int layer1, int layer2, bool ignore)
2. {
3.     <snip>
4.  
5.     // m_LayerCollisionMatrix is an array of UInt32 x 32.
6.  
7.     if (ignore)
8.     {
9.         m_LayerCollisionMatrix[layer1] &= ~(1 << layer2);
10.         m_LayerCollisionMatrix[layer2] &= ~(1 << layer1);
11.     }
12.     else
13.     {
14.         m_LayerCollisionMatrix[layer1] |= 1 << layer2;
15.         m_LayerCollisionMatrix[layer2] |= 1 << layer1;
16.     }
17.  
18.     <snip>
19. }