cocos2d for iPhone

A fast, easy to use, free, and community supported 2D game engine

1. 

   degborta
   Member

   I am having some trouble understanding how to accomplish what I need :)

   I am using Box2d and have a world and a ball object added to the world. I am able to drag this ball via touch and mousejoints.

   The problem I am having is that I don't want the ball to be affected by the world gravity. I need to be able to move/drag it freely across the world.

   I have tried setting the balls density to 0, but then it becomes a static shape so I can't find the fixture anymore, so I am not able to drag it..

   Any ideas are greatly appreciated, as I am very new to both cocos2d and obj-c

   Posted 1 year ago #
2. 

   patrickC
   Member

   When you say you don't want the ball affected by gravity, do you also imply no other forces?
   When you drag the ball and then "drop" it with a speed, do you want it to bounce around and do stuff or should it immediately stop?

   As a cheap "hack", you could try applying an opposite impulse/force to your ball body when it's not being dragged.
   Or any other way of nullifying all forces currently acting on a body after the step call.
   Or, if it's not to be simulated at all, perhaps a dynamic body isn't the best choice.

   Posted 1 year ago #
3. 

   degborta
   Member

   PatrickC: Thanks for your reply. I dont want the ball to be affected at all, no other forces. When I drop the ball I want it to stop immediately..

   I tried applying opposite impulse/force before I posted, but I couldn't get it working really, I guess I have to do it on each world step. I just applied it once when I created the object.

   I really don't know what is the best choice. I figured I'd try getting this to work just for the collisiondetection. But I appreciate any other ideas that I could use instead of the dynamic body.

   Posted 1 year ago #
4. 

   PhilM
   Member

   Could you set it as a static shape, and handle the touch detection using manual rect checks instead?

   I do that for some dynamic box2d objects that are too small to touch in Forest Folk. Not ideal, as it complicates the touch code a little, but works fine. I presume it would work with static shapes too.

   Posted 1 year ago #
5. 

   degborta
   Member

   If the definition of static shape is that it has a density of 0 then I am not able to touch/drag the object any longer, or have I misunderstood something?

   Posted 1 year ago #
6. 

   PhilM
   Member

   ah, I've just spotted this in the old 2.0 box2d manual
   "Static bodies don't collide with other static bodies and are immovable."

   So if you went that way, you'd probably need to destroy it on touch began, and recreate on touch end. Which might not be great if your doing this often.

   Perhaps its better to stick with your other attempt. I would guess it would need updating every step to counteract the effects the physics would have on it, rather than just the once. Not tried it myself though. The box2d forums might have some relevant info.

   Good luck with it!

   Posted 1 year ago #
7. 

   Nalin
   Member

   The best way of doing this is to avoid using the built in gravity completely

   Instead you build a custom timestep loop which only applies gravity to the objects you choose.

   I did this in CardBlanche http://www.cardblanchegame.com which uses Box2D and Cocos. If you view the video it shows that cards that you touch and drag around stop instantly and are unaffected by gravity

   HTH

   Posted 1 year ago #

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