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Chipmunk Physics • View topic - body and shapes

body and shapes

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body and shapes

Postby pward84020 » Mon Oct 22, 2012 1:10 pm

I am a bit confused about the api.

When you create a shape, it is automatically associated with a body ( you actually need to pass in the body to the shape create function, like cpShape * circleShape = cpCircleShapeNew( body, 10.0, cpvzero);

So, if the body has already been added to space... why do you need to add the shapes also ? Wouldn't they be automatically added since they are part of the body ?

cpBody *ballBody = cpBodyNew(100.0, INFINITY);
cpSpaceAddBody(space, ballBody);

cpShape *ballShape = cpCircleShapeNew(ballBody, 20.0, cpvzero);
cpSpaceAddShape(space, ballShape); // WHY IS THIS REQUIRED ?

- What would it mean if you create a body, add it to space, create shapes on it.. but never add the shapes ?

- When you remove / destroy the body , are the shapes also removed / destroyed ?

- Is it better to create the body, then create all the shapes on it, and then add it to space ?

- Do you only need to add the shape to space if the body is already in space ? ( ie, kinda an update on the bodys shape list )

Thanks.
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Re: body and shapes

Postby slembcke » Mon Oct 22, 2012 2:16 pm

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Re: body and shapes

Postby pward84020 » Mon Oct 22, 2012 2:42 pm

Thank you for your speedy and lengthy reply !!!!

I think I am getting a better understanding of things now.


TO RECAP:

I was assuming that since shapes are created with a body, that the body 'owned' them, and would free them when it was destroyed. Which is not the case. I did not realize you could create a shape with a NULL body object ( which you can then add the shape to space, and it would be a static shape since it has not body to interact with gravity, forces, etc )

So, whatever I add to space ( be it bodies, shapes, etc ), I must also remove, and destroy. And, of course, it is fatal to remove a body and destroy it without first removing and destroying its shapes ( since the shapes will be referencing the body still ).

So, to destroy a remove and destroy a body, it is probably best procedure to iterate over its shapes, and remove/destroy them first, and then destroy the body.

Also, shapes cannot be shared ( a shape cannot be belong to more than 1 body ).

Thanks for the information.
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Re: body and shapes

Postby slembcke » Mon Oct 22, 2012 8:26 pm

You cannot add a shape with a null body to a space. You can create the shape, and then set it's body (or change it's body) before adding it to the space though.

Also, you don't want to iterate a body and remove it's shapes. For the same reasons you generally can't iterate arrays or lists while removing their items, you can't remove Chipmunk objects from callbacks. The code that manages the physics objects or uses them should be responsible for tracking them. You just need to keep a list. You *can* use post-step callbacks to handle that, but it's awkward and doesn't really solve the problem of how you would track objects that aren't currently active at the time when you want to destroy them.

Shapes cannot be shared no. They don't use a lot of memory though, so it's not really much of an issue. It's hard to use up a lot of memory for 2D collision detection.
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