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

Printer DataMatrix in Objective-C 12: Physics Engines

CHAPTER 12: Physics Engines
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} return processCollision; } static void contactEnd(cpArbiter* arbiter, cpSpace* space, void* data) { cpShape* shapeA; cpShape* shapeB; cpArbiterGetShapes(arbiter, &shapeA, &shapeB); CCSprite* spriteA = (CCSprite*)shapeA->data; CCSprite* spriteB = (CCSprite*)shapeB->data; if (spriteA != nil && spriteB != nil) { spriteA.color = ccWHITE; spriteB.color = ccWHITE; } }
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The contactBegin method should return YES if the collision should be processed normally. By returning NO or 0 from this method, you can also ignore collisions. In order to get to the sprites, you first have to get the shapes from the cpArbiter, which just like b2Contact holds the contact information. Via the cpArbiterGetShapes method and passing two shapes as out parameters, you get the colliding shapes from which you can then retrieve the individual CCSprite pointers. If they are both valid, their color can be changed. As with Box2D, these callbacks don t get called by themselves. In the HelloWorldScene init method, right after the space is created, you must add the collision handlers using the cpSpaceAddCollisionHandler method:
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unsigned int defaultCollisionType = 0; cpSpaceAddCollisionHandler(space, defaultCollisionType, defaultCollisionType, &contactBegin, NULL, NULL, &contactEnd, NULL);
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The default collision type for shapes is 0, and because I don t care about filtering collisions, both collision type parameters are set to 0. You can assign each body s shape an integer value to its collision_type property and then add collision handlers that are only called if bodies of matching collision types collide. This is called filtering collisions, and is described in the Chipmunk manual, at http://files.slembcke.net/chipmunk/release/ChipmunkLatest-Docs/#cpShape. The next four parameters are pointers to C callback methods for the four collision stages: begin, pre-solve, post-solve, and separation (the same as the EndContact event in Box2D). These serve the same purpose as the corresponding callbacks in Box2D. Most of the time you ll only be interested in the begin and separation events. I pass NULL for pre-solve and post-solve, as I m not interested in handling these. You can use these methods to influence the collision or to retrieve the collision force in the post-solve step. The final parameter is an arbitrary data pointer you can pass on to the callback methods if you need it. I don t, so I set it to NULL as well.
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CHAPTER 12: Physics Engines
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With that, you have a working collision callback mechanism.
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Joints for Chipmunks
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The Chipmunk example project also needs its own implementation of addSomeJoinedBodies. The setup is more verbose than for Box2D, as shown in Listing 12 14. You ll recognize most of the code as setting up static and dynamic bodies if you find that code familiar, feel free to skip to the end where the joints are created.
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Listing 12 14. Creating Three Bodies Connected with Joints -(void) addSomeJoinedBodies:(CGPoint)pos { float mass = 1.0f; float moment = cpMomentForBox(mass, TILESIZE, TILESIZE); float halfTileSize = TILESIZE * 0.5f; int numVertices = 4; CGPoint vertices[] = { CGPointMake(-halfTileSize, -halfTileSize), CGPointMake(-halfTileSize, halfTileSize), CGPointMake(halfTileSize, halfTileSize), CGPointMake(halfTileSize, -halfTileSize), }; // Create a static body cpBody* staticBody = cpBodyNew(INFINITY, INFINITY); staticBody->p = pos; CGPoint offset = CGPointZero; cpShape* shape = cpPolyShapeNew(staticBody, numVertices, vertices, offset); cpSpaceAddStaticShape(space, shape); // Create three new dynamic bodies float posOffset = 1.4f; pos.x += TILESIZE * posOffset; cpBody* bodyA = cpBodyNew(mass, moment); bodyA->p = pos; cpSpaceAddBody(space, bodyA); shape = cpPolyShapeNew(bodyA, numVertices, vertices, offset); shape->data = [self addRandomSpriteAt:pos]; cpSpaceAddShape(space, shape); pos.x += TILESIZE * posOffset; cpBody* bodyB = cpBodyNew(mass, moment); bodyB->p = pos; cpSpaceAddBody(space, bodyB); shape = cpPolyShapeNew(bodyB, numVertices, vertices, offset); shape->data = [self addRandomSpriteAt:pos]; cpSpaceAddShape(space, shape); pos.x += TILESIZE * posOffset; cpBody* bodyC = cpBodyNew(mass, moment);
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CHAPTER 12: Physics Engines
bodyC->p = pos; cpSpaceAddBody(space, bodyC); shape = cpPolyShapeNew(bodyC, numVertices, vertices, offset); shape->data = [self addRandomSpriteAt:pos]; cpSpaceAddShape(space, shape); // Create the cpConstraint* cpConstraint* cpConstraint* joints and add the constraints to the space constraint1 = cpPivotJointNew(staticBody, bodyA, staticBody->p); constraint2 = cpPivotJointNew(bodyA, bodyB, bodyA->p); constraint3 = cpPivotJointNew(bodyB, bodyC, bodyB->p);
cpSpaceAddConstraint(space, constraint1); cpSpaceAddConstraint(space, constraint2); cpSpaceAddConstraint(space, constraint3); }
In this example I m creating a pivot joint with cpPivotJointNew, which is the same as the b2RevoluteJoint used in the Box2D example. Each joint is created with the two bodies that should be connected to each other, and one of the bodies center position as the anchor point. The cpPivotJointNew method returns a cpConstraint pointer, which you ll have to add to the space using the cpSpaceAddConstraint method.
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