Add collision exceptions.

[dimforge]

Add collision exceptions.

[sebcrozet]

Thank you for your feedbacks @kakoeimon and @dpmkl. So I think we are talking here about letting the user setup some arbitrary filter rule for collision pairs. I think this is something we will want to add, yes.
We will also need #2 which is more efficient but not as versatile.

[sebcrozet]

What do you mean exactly by "collision exceptions"? How is this different from #2 from the end-user point of view?

[dpmkl]

I would reckon he means something like the collision filter callback in Box2D. https://github.com/erincatto/box2d/blob/master/src/dynamics/b2_world_callbacks.cpp#L28
This could come in handy for 2D Platformer mechanics, where you are able to jump up through platforms but don't fall down through them.

[kakoeimon]

The difference is that you may want an object not to collide with a particular object and not the whole group.
For example a bullet will collide with all the objects but not with the player who shooted it.

[jpetkau]

[Edit: oh, I guess this is a dup of #2 if my interpretation is correct.]

Some examples:

• Chain or cloth: allow each link to interpenetrate its neighbors, but collide normally with everything else
• 2D physics: often you want to ignore self-collision (e.g. in a side-view game with a walking character, the character's legs should be able to cross each other).
• (Sort of the same): 2D game with pseudo-depth in Z: objects should be able to pass in front of/behind other layers, but still hit their own layer.
• Bullet / grenade that ignores shooter / thrower but can hit everything else. [From @kakoeimon's reply]

The most general form of this would be an arbitrary predicate saying whether a pair of objects can collide or not. But simple filter bitmasks are faster and useful enough for many cases. Examples:

• In Box2d each object has a 32-bit "category" and a 32-bit "mask", and collisions require obj1.cat& obj2.mask && obj2.cat & obj1.mask: http://www.iforce2d.net/b2dtut/collision-filtering
• In NVidia's flex there's a 32-bit "phase", with a 24-bit "group" and a 1-bit "flag" indicating whether a particle should collide with other particles with the same group.

[kakoeimon]

It is not a dup of #2
Think about it as a bullet/granade that can do a frendly fire but cannot damage the shooter/thrower.
This is not just for 2D, neither masks and layers are.
Those two #2 and this are essential in game making. Especialy if there are no plans to expose to the user checks just before the collision detection.
If there are plans to expose to the users a way to make checks before the actual collision detection then there is no need to implement this, neither #2 and the user can implement his own checkings.

[jpetkau]

I'm not sure how what you're describing differs from "an arbitrary predicate saying whether a pair of objects can collide or not"?

It is not a dup of #2 ...
Think about it as a bullet/granade that can do a friendly fire but cannot damage the shooter/thrower.
This is not just for 2D, neither masks and layers are.

I wasn't saying it's only for 2D (e.g. the cloth example only makes sense in 3D). I was just giving some examples of when it's useful, and two of them happened to be for 2D.

[kakoeimon]

Yes without the "But" It does not differ.