cocos2d for iPhone

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

1. 

   JohnnyStar
   Member

   Hey guys!
   I'm new for box2d! In box2d docs no description for queryAABB.. I've read a thread about mouse grab on this forum, but I need help with smth else)

   How to grab all bodies in AABB area, for example to apply impulse to all these bodies?

   I've tried smth like these, but it doesn't work( Please help me!)

   b2AABB aabb;
   aabb.lowerBound.Set(-1.0f+location.x, -1.0f+location.y);
   aabb.upperBound.Set(1.0f+location.x, 1.0f+location.y);
   b2Vec2 callPoint;
   callPoint.Set (location.x,location.y);
   QueryCallback callback(callPoint);
   world->QueryAABB(&callback, aabb);

   if (callback.m_fixture)
   {
   for(b2Fixture *fixture = callback.m_fixture; fixture; fixture = callback.m_fixture -> GetNext() ) {
   b2Body *body = fixture -> GetBody();
   b2Vec2 impulseV;
   impulseV.Set(0.0,20.0);
   body -> ApplyImpulse(impulseV, callPoint);
   }
   }

   //and class

   class QueryCallback : public b2QueryCallback
   {
   public:
   QueryCallback(const b2Vec2& point)
   {
   m_point = point;
   m_fixture = NULL;
   }

   bool ReportFixture(b2Fixture* fixture)
   {
   b2Body* body = fixture->GetBody();
   if (body->GetType() == b2_dynamicBody)
   {
   bool inside = fixture->TestPoint(m_point);
   if (inside)
   {
   m_fixture = fixture;

   // We are done, terminate the query.
   return false;
   }
   }

   // Continue the query.
   return true;
   }

   b2Vec2 m_point;
   b2Fixture* m_fixture;
   };

   Posted 2 years ago #
2. 

   Lam
   Member

   Your for loop is off.

   It'll never loop through the fixture list since your GetNext() is always returning the same item.

   I hope that hint will help you find the error. If not then just ask again.

   Posted 2 years ago #
3. 

   JohnnyStar
   Member

   @Lam
   the problem is that I can't grab anything at all, even if rewrite this code to grab only one fixture (without FOR loop) then nothing happen(

   Posted 2 years ago #
4. 

   Lam
   Member

   It's also probably because you need to do your coordinate conversion from an actual cocos2d units to the box2D units. How many pixels represents 1 meter in box2d.

   In the cocos2D-box2D template that Steve Oldmeadow wrote, you'll see a constant PTM_RATIO. It pretty much represents the conversion of pixels to meter, which is 32 pixels equates to 1 box2d meter.

   You have to take that conversion into account whenever you are passing coordinates back and forth between cocos2d and box2d.

   Previous example snippet:
   

   b2AABB aabb;
   aabb.lowerBound.Set(-1.0f+location.x, -1.0f+location.y);
   aabb.upperBound.Set(1.0f+location.x, 1.0f+location.y);

   The handling above is incorrect because the units are off. You can't add 'location', which is in pixels, to 1.0f. Not quite sure what 1.0f is but that arbitrary number is probably in box2d units.

   At least we know that an AABB is in meters and you're passing in pixel units so basically the mathematics is wrong since the units don't match.

   You should write up some functionality for returning an AABB that has properly converted coordinates based on pixel unit CGRects and also some function for returning converted b2Vec from CGPoints.

   An idea:
   

   // Grab the converted px/m coordinate representation of an axis aligned bounding box for a sprite bounding box
   b2AABB aabb = b2AABBFromCGRect([sprite boundingBox]);
   
   // Grab the converted b2Vec callpoint position maybe from the sprite position
   b2Vec2 callPoint = b2VecFromCCP(sprite.position);
   
   QueryCallback callback(callPoint);
   world->QueryAABB(&callback, aabb);

   Now you need to write those methods of course (here's an example of returning a proper b2Vec)
   

   static inline b2Vec2 b2VecFromCCP(CGPoint p, bool convert = true){
   	return convert? b2Vec2(p.x / PTM_RATIO, p.y / PTM_RATIO) : b2Vec2(p.x, p.y);
   }
   
   static inline CGPoint ccpFromb2Vec(b2Vec2 p, bool convert = true){
   	return convert? ccp(p.x * PTM_RATIO, p.y * PTM_RATIO) : ccp(p.x, p.y);
   }

   Posted 2 years ago #
5. 

   MikeSz
   Member

   old thread but I need a bit of help too :)

   just like JohnnyStar - I am looking for ALL objects within certain AABB

   However, QueryCallback can only contain a single fixture, so thats a no go here

   so what should I do here ? I can manually go through all the bodies and check their AABB with that 'certain' AABB, but that will be slow (and frankly speaking - that's not the way it should be handled...)

   ---

   EDIT: OK, did it :) checked the headers and found this:

   /// Called for each fixture found in the query AABB.
   /// @return false to terminate the quer"y.
   bool ReportFixture(b2Fixture* fixture)

   so basically return true there, and it will go through each and every object inside the AABB :) then do whatever you need with them. just store one of them into the m_fixture; so that you can check if any collision occured (for UI handling)

   Posted 2 years ago #

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