CollisionHandlers

[marciokoko]

I have a layer searching for many collision types. I already have one collision handler set:

in createSpace:

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    cpSpaceAddCollisionHandler(space, kCollisionTypeA, kCollisionTypeB, (cpCollisionBeginFunc)begin, NULL, NULL, NULL, NULL);

Then above it all, in the layer class:

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static void
postStepRemove(cpSpace *space, cpShape *shape, void *unused)
{

    cpSpaceRemoveBody(space, shape->body);
    cpBodyFree(shape->body);
    
    cpSpaceRemoveShape(space, shape);
    cpShapeFree(shape);

}
static cpBool begin(cpArbiter *arb, cpSpace *space, void *ignore){
    CP_ARBITER_GET_SHAPES(arb, aShape, bShape);
    cpSpaceAddPostStepCallback(space, (cpPostStepFunc)postStepRemove, bShape, NULL);
    return cpTrue;
}

but I want to add another one:

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// TO BE IGNORED
    cpSpaceAddCollisionHandler(space, kCollisionTypeC, kCollisionTypeD, begin, NULL, NULL, NULL, NULL);
// TO BE postStepRemove like A-B
    cpSpaceAddCollisionHandler(space, kCollisionTypeE, kCollisionTypeA, begin, NULL, NULL, NULL, NULL);

My question is, how to I add a second or n-th collision handler?

[slembcke]

I'm not sure I understand the question, but you can register as many collision handlers as you want. Call cpSpaceAddCollisonHandler() with the callbacks you want for each one.

[marciokoko]

Is there any situation in which you might imagine all groundShapes loading their ability to hold up GameObjects? All of a sudden after adding the collision handler for the shield and the one for the new meteor, after the player touches the shield, everything falls down and thru the bottom ground even!

For that to happen I would have to somehow cancel all groundShapes collision type, right?

[slembcke]

Yes, it's not an uncommon issue. When you allow two infinite mass objects to collide for instance, it will cause divide by zero issues that will basically corrupt the physics state of both bodies. For the ground, this will cause the collision response to fail against it.

If compile and run Chipmunk in debug mode, there should be assertions that catch the error that cause it though.

[marciokoko]

Im creating them using mass of 1.0f.


[self addBoxBodyAndShapeWithLocation:location
size:size space:theSpace mass:1.0 e:0.0 u:0.5
collisionType:kCollisionTypeShield canRotate:TRUE];

oh...i just noticed one of the bodies isn't created with this method at all!? Does that mean its created by default with infinite mass?

[slembcke]

There is no default mass in Chipmunk. Are you using SpaceManager? I'm not very familiar with that. How are you creating the "other" body? Is it using SpaceManager code? What does that do?

[marciokoko]

No, Im basing off of RW code for the Space Viking app, but anyway, here is the code:

In the actionLayer I call:

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hopper = [[[CPHopper alloc] initWithLocation:ccp(100,100) space:space groundBody:groundBody] autorelease];

The hopper & shield are a subclass of CPSprite and have this in their init method to create a body associated with a sprite:

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        [self addBoxBodyAndShapeWithLocation:location size:size space:theSpace mass:1.0 e:0.0 u:0.5 collisionType:kCollisionTypeViking canRotate:TRUE];

Where CPSprite has the respective method to create cpBody:

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- (void)addBoxBodyAndShapeWithLocation:(CGPoint)location
                                  size:(CGSize)size
                                 space:(cpSpace *)theSpace
                                  mass:(cpFloat)mass
                                     e:(cpFloat)e
                                     u:(cpFloat)u
                         collisionType:(cpCollisionType)collisionType
                             canRotate:(BOOL)canRotate {
    space = theSpace;
    
    float moment = INFINITY;
    if (canRotate) {
        moment = cpMomentForBox(mass, size.width, size.height);
    }
    
    body = cpBodyNew(mass, moment);
    body->p = location;
    cpSpaceAddBody(space, body);
    
    shape = cpBoxShapeNew(body, size.width, size.height);
    shape->e = e;
    shape->u = u;
    shape->collision_type = collisionType;
    shape->data = self;
    cpSpaceAddShape(space, shape);
}

So is mass = 1.0 mean infinity? Meaning no mass is 0 and infinity is 1?

[slembcke]

No, 1.0 mass means 1.0 mass (of whatever units you decided you are using). You need to use the INFINITY constant to set it to infinity.

[marciokoko]

Ok could you tell me how to put chipmunk in debug mode so i can figure out what's going on?

[slembcke]

Using the debug build configuration from Xcode should be sufficient.

It will print out something like the following when in debug mode:

Initializing cpSpace - Chipmunk v6.0.3 (Debug Enabled)
Compile with -DNDEBUG defined to disable debug mode and runtime assertion checks

So as long as you don't define NDEBUG (the same symbol used to disable other system provided assert() statements), Chipmunk will be running in debug mode.