## Implementing shape groups with layers

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### Implementing shape groups with layers

Hi

I'm currently transferring some collision/physics code from bullet to chipmunk, but I'm having a hard time with the cpShape::group and cpShape::layers attribute. Currently I've got the following system:

• A shape can be added to zero or more groups. When a shape is created, it doesn't belong to any group and thus doesn't collide with any other shape.
• A group can be set to collide with zero or more other groups.
• A group can collide with itself, but doesn't have to.

So a typical scenario would be: 3 Groups: Players, enemies and bullets. The player shapes collide with bullet shapes, the enemy shapes collide with bullet shapes and the player shapes can collide with other players (to get some kind of shared power or something).

The problem is: How do I map this scenario to the groups and layers? I don't want transitivity as in: The players collide with the bullets, the bullets collide with the enemies, but the enemies should not collide with the players.
Phillemann

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Joined: Fri Feb 19, 2010 9:18 am

### Re: Implementing shape groups with layers

You can use layers as rules. Think of each layer as a rule that two things collide, and you get to define 32 rules. (more if you redefine cpLayers to be a bigger int type)

Your scenario has two rules and needs two bits. Players collide with bullets (1), and Enemies collide with bullets (2). Players would use 1 for their layers, enemies 2 and bullets 3 (1 | 2). Players don't collide with enemies because 1 & 2 is 0. You'll need to use Chipmunk groups or collision callbacks to exclude collisions within a rule type if you don't want them to collide.

Make sense?
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