.net barcode generator library open source 12: Physics Engines in Objective-C

Creator Data Matrix ECC200 in Objective-C 12: Physics Engines

CHAPTER 12: Physics Engines
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You should try to keep the dimensions of objects in your world as close to 1 meter as much as possible. That is not to say that you can t have objects that are smaller than 0.1 meters or larger than 10 meters, but you may run into glitches and strange behavior if you create relatively small or large bodies. The PTM_RATIO is defined like this:
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#define PTM_RATIO 32
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It is used to define that 32 pixels on the screen equal 1 meter in Box2D. A box-shaped body that s 32 pixels wide and high will be 1 meter wide and high. A body that s 4 4 pixels in size will be 0.125 0.125 meters in Box2D, while a relatively huge object of 256 256 pixels will be 8 8 meters in Box2D. The PTM_RATIO allows you to scale the size of Box2D objects down to the dimensions within which Box2D works best, and a PTM_RATIO of 32 is a good compromise for a screen area that may be as large as 1024 768 pixels on the iPad.
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Converting Points
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Note that the b2Vec2 struct is different from CGPoint, which means you cannot use a CGPoint where a b2Vec2 is required, and vice versa. In addition, Box2D points need to be converted to meters and back to pixels. To avoid making any mistakes, such as forgetting to convert from or to meters, or simply making a typo and using the x coordinate twice, it s highly recommended to wrap this repetitive code into convenience methods like these:
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-(b2Vec2) toMeters:(CGPoint)point { return b2Vec2(point.x / PTM_RATIO, point.y / PTM_RATIO); } -(CGPoint) toPixels:(b2Vec2)vec { return ccpMult(CGPointMake(vec.x, vec.y), PTM_RATIO); }
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This allows you to write the following code to easily convert between CGPoint and pixels to b2Vec2 and meters:
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CGPoint point = CGPointMake(100, 100); b2Vec2 vec = b2Vec2(200, 200); CGPoint pointFromVec; pointFromVec = [self toPixels:vec]; b2Vec2 vecFromPoint; vecFromPoint = [self toMeters:point];
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CHAPTER 12: Physics Engines
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Adding Boxes to the Box2D World
With a static body containing the objects within screen boundaries, all that s missing is something to be kept within the screen boundaries. How about little boxes, then I ve added David Gervais orthogonal tileset image dg_grounds32.png to the Resources folder of the PhysicsBox2D01 project. The tiles are 32 32 pixels, so they ll make perfect 1 1-meter boxes. Listing 12 3 is the code in the init method that adds the texture and creates a couple boxes. It also schedules the update method, which is needed to update the box sprite positions, and it enables touch so that the user can tap the screen to create a new box.
Listing 12 3. Adding an Initial Set of Boxes // Use the orthogonal tileset for the little boxes CCSpriteBatchNode* batch = [CCSpriteBatchNode batchNodeWithFile:@"dg_grounds32.png" capacity:150]; [self addChild:batch z:0 tag:kTagBatchNode]; // Add a few objects initially for (int i = 0; i < 11; i++) { [self addNewSpriteAt:CGPointMake(screenSize.width / 2, screenSize.height / 2)]; } [self scheduleUpdate]; self.isTouchEnabled = YES;
The addNewSpriteAt method shown in Listing 12 4 is part of the cocos2d Box2D application template project, but slightly modified to make use of all the tiles in the tileset.
Listing 12 4. Adding a New Dynamic Body with a Sprite -(void) addNewSpriteAt:(CGPoint)pos { CCSpriteBatchNode* batch = (CCSpriteBatchNode*)[self getChildByTag:kTagBatchNode]; int idx = CCRANDOM_0_1() * TILESET_COLUMNS; int idy = CCRANDOM_0_1() * TILESET_ROWS; CGRect tileRect = CGRectMake(TILESIZE * idx, TILESIZE * idy, TILESIZE, TILESIZE); CCSprite* sprite = [CCSprite spriteWithBatchNode:batch rect:tileRect]; sprite.position = pos; [batch addChild:sprite]; // Create a body definition and set it to be a dynamic body b2BodyDef bodyDef; bodyDef.type = b2_dynamicBody; bodyDef.position = [self toMeters:pos]; bodyDef.userData = sprite; b2Body* body = world->CreateBody(&bodyDef);
CHAPTER 12: Physics Engines
// Define a box shape and assign it to the body fixture b2PolygonShape dynamicBox; float tileInMeters = TILESIZE / PTM_RATIO; dynamicBox.SetAsBox(tileInMeters * 0.5f, tileInMeters * 0.5f); b2FixtureDef fixtureDef; fixtureDef.shape = &dynamicBox; fixtureDef.density = 0.3f; fixtureDef.friction = 0.5f; fixtureDef.restitution = 0.6f; body->CreateFixture(&fixtureDef); }
First, a sprite is created from the CCSpriteBatchNode by using CCSprite s spriteWithBatchNode initializer and supplying a CGRect that is 32 32 pixels in size, to randomly pick one of the tileset s tiles as the sprite s image. Then a body is created, but this time the b2BodyDef type property is set to b2_dynamicBody, which makes it a dynamic body that can move around and collide with other dynamic bodies. The previously created sprite is assigned to the userData property. Later on, when you are iterating over the bodies in the world, this allows you to quickly access the body s sprite. The body s shape is a b2PolygonShape set to a box shape that is half a meter in size. The SetAsBox method creates a box shape that is twice the given width and height, so the coordinates need to be divided by 2 or, as in this case, multiplied by 0.5f to create a box shape whose sides are 1 meter wide and high. The dynamic body also needs a fixture that contains the body s essential parameters first and foremost the shape, but also density, friction, and restitution which influence how the body moves and bounces around in the world. Consider the fixture to be a set of data used by bodies.
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