Chipmunk: excluding bodies from space's gravity

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Chipmunk: excluding bodies from space's gravity

(9 posts) (6 voices)

Started 2 years ago by pfg2009
Latest reply from pfg2009

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pfg2009
Member

Hi!
I'm playing with chipmunk and I'm trying to model objects that occupy the same cpSpace, some of which respond to gravity and some of which don't. But I'm not sure how best to model it.

Imagine a scenario:
* some terrain outline (a static body that doesn't ever move)
* a boulder (an active body that does move, collides with other objects in the space, and responds to some global gravity)
* a bird (an active body that does move, collides with other objects, but doesn't get affected by gravity)

In order for all these objects to collide and bounce off of each other correctly, I believe they all need to be part of the same cpSpace object. However, if the cpSpace has a global gravity force defined, it will apply it to all objects, including the bird, which is not what I want. What's the best way to go around this problem?

1) nix the gravity vector from the cpSpace object and apply gravity manually using cpBodyApplyForce to all objects that need it
2) apply a negative gravity force to all objects that should defy gravity (ie. bird) but keep the global gravity vector in cpSpace
3) ??? is there a better way? it seems clumsy to me to do this manually, but I don't see anything simpler.

Posted 2 years ago #
vutronic
Member

For the bird you can create a shape with a static body and add the shape to space. Example, cpBody * staticBody = cpBodyNew(INFINITY, INFINITY ) ;

Create your bird object and add a referece of its Chipmunk's shape to the bird. When the position of your bird is updated you can set the position of its shape.

Posted 2 years ago #
pfg2009
Member

That solution makes sense if I wanted to manually update the position of the bird for every frame. However, I was hoping to still be able to apply other forces to the bird, such as:

(void) flyRight() {
cpBodyApplyImpulse(birdBody, cpv(10, 0), cpvzero); // or something similar
}

Also, if I define the bird as a static object with infinite mass and moment, other objects will bounce off of it, but it will not bounce off of other objects. Ideally, I want the bird to behave like every other movable object in the space, just not have it be affected by gravity.

Posted 2 years ago #
crmagicxxx
Member

What I think the best method for you is to override default VelocityFunc for that body. Check latest chipmunk demos (especially planet.c) to see how they implemented gravity on some body. It's fairly simple to understand once you get the concept... It also enables you various different things, like changing the damping for particular body, not the whole system, etc...

Posted 2 years ago #
pfg2009
Member

I will look in that direction. Thank you!

Posted 2 years ago #
illinnovations
Member

Not sure if this will help you, but you can try just not adding the cpBody to the space. Then you can just use the shape for collision detection.

Posted 2 years ago #
kalx
Member

I had to do something similar and went with the 2nd option you listed. I applied a force vector to negate gravity on certain objects. You can calculate the force pretty easily using F = MA.

Posted 2 years ago #

I did what kalx suggested... I have sharks in my game, and I didn't want the sharks to sink when they swam.... i just wanted them to swim side to side... so when I created the body, i apply the opposite force of gravity to the shark...

body = cpBodyNew(200, INFINITY);
body->p = cpv(160, LEVEL_SECTION_HEIGHT - 60);
cpSpaceAddBody([[GameState instance] getSpace], body);
shape = cpCircleShapeNew(body, 15.0, cpvzero);
shape->e = 0.1;
shape->u = 0.2;
shape->data = self;
shape->collision_type = COLLISION_SHARK;
shape->layers = 0xFFFFFFFE;
cpSpaceAddShape([[GameState instance] getSpace], shape);

// here is my gravity offset...
cpBodyApplyForce(body, cpv(GRAVITY_HORIZONTAL, GRAVITY_VERTICAL * -1 * 200), cpvzero);

Posted 2 years ago #

pfg2009
Member

I ended up going with option #2 as well. It seemed cleanest and I only had a handful of objects to modify. If there were lots, I'd probably override the default velocity function. Thanks for all the feedback!

Posted 1 year ago #

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