Only Integrating Box2D collision detection in my 2d engine?

Question

I have integrated box2d in my engine, ( Debug Draw, etc. ) and with a world I can throw in some 2d squares/rectangles etc.

I saw this post, where the user is basically not using a world for collision detection, however the user doesn't explain anything about how he's using the manifold (b2Manifold), etc.

Another post, is in the cocos2d forum, ( scroll down to the user Lam in the third reply )

Could anyone help me a bit with this?, basically want to add collision detection without the need of using b2World ,etc etc.

Thanks a lot!

Answer 1

Is there any reason you can't use a b2World? Just because you use it, doesn't mean you have to use the physics simulations, unless you're severely performance limited.

See this example on using Box2D for collision only in Cocos2d. Maybe you can apply something similar to your project: http://www.raywenderlich.com/606/how-to-use-box2d-for-just-collision-detection-with-cocos2d-iphone

Answer 2

The code in both of those posts seems complete to me. You begin with two polygon shapes and their corresponding transforms and from b2CollidePolygons you get back a contact manifold.

The b2Manifold is just a collection of points at which the two shapes' boundaries intersect. In the posted code the author uses that if the manifold's point count is greater than zero there is an overlap. Depending on the nature of the shapes there can be different number of intersection points when they overlap.

Note that the collision handling of the b2World is quite robust. Beyond simple collision queries it has broad-phase culling, ray-casting, a spatial tree, begin-contact and end-contact events, efficient memory management to name some things. There are classes in the API for most of these tasks but I can't say I'm familiar with using them manually. Could be worth a look.