cocos2d for iPhone

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

1. 

   cocoBavan
   Member

   I implemented collision detection using Box2D and found out that I have kinds messed up things. Though I wanted a Circle shape as the colliding body, it collides as if it is a square. I don;t know why.

   First I read ,

   • http://www.raywenderlich.com/606/how-to-use-box2d-for-just-collision-detection-with-cocos2d-iphone

   and the code provided was working with no problem.

   • http://www.raywenderlich.com/downloads/Box2DCollision.zip

   But, after reading the description in the b2ContactListener class file I was not happy about implementing it that way. The class file comments said

   /// Implement this class to get contact information. You can use these results for
   /// things like sounds and game logic. You can also get contact results by
   /// traversing the contact lists after the time step. However, you might miss
   /// some contacts because continuous physics leads to sub-stepping.
   /// Additionally you may receive multiple callbacks for the same contact in a
   /// single time step.
   /// You should strive to make your callbacks efficient because there may be
   /// many callbacks per time step.
   /// @warning You cannot create/destroy Box2D entities inside these callbacks.

   Therefore, I tend to use b2Body->GetContactList() in the update method as you can find it in the example below.

   • http://www.4shared.com/file/s-5BEeG_/Box2DCollision.htmlBox2DCollision.zip

   There I find this trouble of Circle being inferred as Square. (I really don't know what an b2ContactEdge is)

   I can implement a more sophisticated listener class, but I'm afraid that it may kill my 3G's processor. :)

   Did any of you encounter this scenario? What could be the solution?

   ps: I don't know much about this copy right things.. The example files had some copy right information. Hope, Ray won;t sue me for using his file here and modifying it.

   Posted 1 year ago #
2. 

   Dragyn
   Member

   I haven't looked at your code, but I believe your circle/square problem is how you defined the body and shape for your object. Check out Ray's Intro to box2D tutorial here: http://www.raywenderlich.com/457/intro-to-box2d-with-cocos2d-tutorial-bouncing-balls

   You'll see in the section "The Box2D World In Practice" where the body and shape is defined as a circle. This particular tutorial creates a bouncing ball, and it definitely works well for circle-based collision detection.

   (If you modify his sample code to create multiple balls, you can keep yourself entertained for hours bouncing them around the screen. Okay, maybe it's just me. I like simple entertainment.)

   Posted 1 year ago #
3. 

   Jeff
   Member

   A quick check of your code doesn't seem to include any check for contact->IsTouching().
   Are you possibly assuming because you have a contact in your contact list that this also immediately implies a collision?

   I am pretty sure (check b2Contact docs) that the system generates contacts even when bounding boxes overlap but the contact instance only involves collision if the contact has contact points and is touching.

   /// The class manages contact between two shapes. A contact exists for each overlapping
   /// AABB in the broad-phase (except if filtered). Therefore a contact object may exist
   /// that has no contact points.
   class b2Contact

   Posted 1 year ago #
4. 

   cocoBavan
   Member

   Dragyn, Thank you.

   I have tested the Circle thing with implementing our own b2ContactListener and of-course it works. Ray does it that way. But he has admitted its limitations in one of his comments.

   I had the same situation in one of my games. What I ended up doing was have the monster object keep track of the bullets that had already collided with it, and ignore the collision if the bullet had already hit it.

   In one of his comments he says,

   Box2D will tell the contact listener which objects collide each frame, which we store into an array, and then march through the array to process the objects that are colliding.
   We have to keep checking each frame, since the objects are moving they could collide (or not) each frame.

   I am really not sure about that too.

   Posted 1 year ago #
5. 

   cocoBavan
   Member

   Ah.. Jeff :D I am very stupid.. Thank you. Thank you. It works.
   I should have read contact class first. .. Anyway, thank you all. :)

   Posted 1 year ago #

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