cocos2d for iPhone

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

1. 

   Omegas7
   Member

   So this time I will be entering the world of physics =D

   Well, this looks complex, so let's see if I get it:
   First I make a world object, like this:
   

   b2Vec2 gravity;
   gravity.Set(0.0f, -10.0f);
   bool doSleep = true;
   world = new b2World(gravity, doSleep);

   So what is b2Vec2? Is it a class? Then shouldn't gravity be a pointer and have an asterisk *?
   Set is a method from b2Vec2, and how come when I write gravity. and press esc, I only get 2 completion choices: "x,y"?

   // Debug Draw functions
   m_debugDraw = new GLESDebugDraw( PTM_RATIO );
   world->SetDebugDraw(m_debugDraw);
   
   uint32 flags = 0;
   flags += b2DebugDraw::e_shapeBit;
   //flags += b2DebugDraw::e_jointBit;
   //flags += b2DebugDraw::e_aabbBit;
   //flags += b2DebugDraw::e_pairBit;
   //flags += b2DebugDraw::e_centerOfMassBit;
   m_debugDraw->SetFlags(flags);

   I am 100% clueless. What is GLESDebugDraw? What is uint32?

   You see this line?
   b2Body* groundBody = world->CreateBody(&groundBodyDef);
   The ->... What's that? Another way to call a method?

   It seems b2BodyDef has a property called userData. Exactly what is it for?

   I know that sprites linked to box2d bodies are just puppets and they will just reflect what happens to said bodies in the simulation, so it is useless to try to change the sprites' positions or make them run actions... So the solution would be to do all that to the box2d body itself, but I was unable to do so =/

   There are quite a few nice tutorials about how to make certain games with box2d, but none of them really explains in depth all the magic behind the code...

   Posted 1 year ago #
2. 

   XyrisKenn
   Member

   Hi, the OpenGLES functions aren't related to Box2D actions, and needn't be changed. The documentation on Box2D.org is up-to-date now, and the Cocos2D for iphone has 'testbed' examples. They are great fun :) Good luck,

   Posted 1 year ago #
3. 

   MikeSz
   Member

   well... seems like you're not new to Box2D but to C/C++ ;)

   1) b2Vec2 is a type that consists of 2 floats. exactly like CGPoint actually. first float is x, second is y

   2) Gravity is not initialized as pointer to b2Vec2 but just as a b2Vec2. it is equivalent of CGPoint gravity. so you access you via .

   3) GLESDebugDraw is a class that... draws debug data like lines, squares, circles etc.

   4) uint32 is unsigned int32, which means its lowest number is 0, not -2^31

   5) -> is for pointers. if you trace the CreateBody function it makes body via a new call. to access pointer's properties you use ->

   6) userData is a pointer to... well, any data you wish to attach to this body. in most cases it will be CCNode or CCsprite

   7) sprites are not "puppets" - they will not rotate and transform according to a body just by themselves. you need to tell them to do so, that's why you use the userData pointer

   my recommendation is - try to find a more general book on C/C++, cause most of your problems don't come from not undestanding box2D at all...

   Posted 1 year ago #
4. 

   steg90
   Member

   Got beaten to answering your questions, but I'll add mine anyway!

   When using pointers, i.e dereferencing, you use ->
   As world is being dereferenced, you use -> as in the code you gave. If you use an object that is on the stack use the dot operator...

   When using the new keyword, objects are allocated on the heap, b2Vec2 is a structure, it is used as a column vector and has a few useful vector methods such as Normalize, LengthSquared, Length and also has some operator overloads so you can add, subtract the vectors. Take a look at b2Math.h for the structure.

   GLESDebugDraw, allows any box2d bodies to be shown, i.e. outlined, useful to show where your box2d bodies are (debug).

   uint32 is an Unsigned 32 bit integer.

   UserData is used to store user data as it states, such as a CCSprite, useful for checking when a collision has happened, get the user data and do whatever.

   To move a box2d body and the sprite attached to it:

   for(b2Body *b = world->GetBodyList(); b; b=b->GetNext())
   	{
   		void *object = b->GetUserData();
   		if(object!= NULL) {
   			if( [object isKindOfClass:[CCSprite class] ] == YES)
   			{
   				CCSprite *sprite = (CCSprite*)b->GetUserData();
   				switch(sprite.tag)
   				{
   					case kPlayerTag:  // for player physics method
   					{
   						sprite.position = ccp(b->GetPosition().x * PTM_RATIO, b->GetPosition().y * PTM_RATIO);
   						sprite.rotation = -1 * CC_RADIANS_TO_DEGREES(b->GetAngle());
   					}
   					break;
                                    }
                            }
                     }
           }
   
   // etc...

   Hope this helps some.

   Steve

   Posted 1 year ago #
5. 

   Omegas7
   Member

   Hey, thanks there!

   @steg90: What I mean, what if I want to give an action like jump to a certain sprite?

   Posted 1 year ago #
6. 

   robodude666
   Member

   Sorry I'm late to the party. Spent most of the day working on my Physics II final exam.

   I'll toss in my 2 cents for a few of the questions... for the hell of it.

   Correction: You're new to Box2D and C/C++ :P.

   Also, when combining Box2D into any rendering world the Sprite does become a puppet. The sprite by itself will not do anything unless it is told to be moved. That's why you have that big loop going through all bodies in the world.

   GLESDebugDraw is a class included with the template for a Cocos2d+Box2d project. You'll find it in GLES-Render.[h/m]. Box2D has a DebugDraw system (via the b2DebugDraw class) that is capable of drawing the outlines, center of mass, joints, AABB bounding box, etc for bodies/shapes in the world. Because Box2D is just a physics engine and not a rendering engine it has no ability to draw anything. Instead, it has a class called b2DebugDraw which has methods that are called to draw particular shapes for different things. This class is extended into GLESDebugDraw and the specified shapes/lines are drawn with OpenGL ES (hence GLES).

   uint32 is a datatype, much like int and float. The name is very descriptive and tells you everything you need to know. You can break it down into 3 parts: u int 32. Theudenotes that it is unsigned, or has no sign (positive). Theintdenotes that it holds integers (whole numbers). The32denotes that it is 32-bits wide. Putting this all together, auint32can hold a value between 0 and (2^32)-1, or 4,294,967,295. An int32, or a regular int can hold between (-2^31) to (2^31)-1. By default data types are signed. All of the data types from smallest to largest are: nibble (4bit), byte/char (8-bit), word/short (16-bit), long/int (32-bit), doubleword/longword/int64 (64-bit). You'll find that not all of them are pre-defined for you, but they are the common types. Generally speakingintanduint32` will suffice most needs.

   Box2D is, like you may know, in the C++ world. In the C++ world, and in any programming language really, resources should be kept to a minimum. This is why Box2D uses a "Def" system. The b2BodyDef is a struct which contains data, or a definition regarding a b2Body. You'll discover that all shapes and joints also have a Def struct associated to them. The b2BodyDef struct consists of several values which get passed to the world in order to create a new body. The CreateBody method in the b2World class is called a Factory as it produces an instance of a new class. The definition that gets passed in describes this new body that we want created.

   You are correct in assuming all of Cocos2D's actions will not work when using Box2D. This is because we are using Box2D to move our objects around. We <em>can</em> apply a CCMoveTo action on a sprite, but its position will be reset the next time the tick method is called and the sprite is moved to the body's location. If you want a sprite to move to a particular location you'll need to apply a force large enough to the body to move it to the desired location.

   Last but not least, the UserData features of Box2D are extremely rich. They allow you to store a pointer to any object you want. This is VERY helpful when trying to associate something with the body. In the template project, for example, a sprite is associated to the body via the UserData. While this method works, you'll find that in larger projects you'll end up writing custom classes and using them as your userdata.

   -robodude666

   Posted 1 year ago #

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