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POChipmunk Physics Positioning
 

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  1. POChipmunk Physics Positioning
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    <p>I've looked around quite a bit and haven't been able to find a concise answer to this question. For my Cocos2D game I have integrated the Chipmunk physics engine. On initialization, I setup the boundaries of the 'playing field' by establishing a series of bodies and static shapes as follows:</p> <pre><code>- (void)initPlayingField { // Physics parameters CGSize fieldSize = _model.fieldSize; CGFloat radius = 2.0; CGFloat elasticity = 0.3; CGFloat friction = 1.0; // Bottom CGPoint lowerLeft = ccp(0, 0); CGPoint lowerRight = ccp(fieldSize.width, 0); [self addStaticBodyToSpace:lowerLeft finish:lowerRight radius:radius elasticity:elasticity friction:friction]; // Left CGPoint topLeft = ccp(0, fieldSize.height); [self addStaticBodyToSpace:lowerLeft finish:topLeft radius:radius elasticity:elasticity friction:friction]; // Right CGPoint topRight = ccp(fieldSize.width, fieldSize.height); [self addStaticBodyToSpace:lowerRight finish:topRight radius:radius elasticity:elasticity friction:friction]; // Top [self addStaticBodyToSpace:topLeft finish:topRight radius:radius elasticity:elasticity friction:friction]; } </code></pre> <p>This setup works, except for one thing-- the positions of the boundaries appear to be off by a significant amount of pixels to the right. After initializing a player, and sending him towards the boundary walls, it does not bounce off the edges of the screen (as I would expect by setting up the bounding box at 0,0), but rather some number of pixels (maybe 30-50) inwards from the edges of the screen. What gives? </p> <p>I initially thought the issue came from where I was rendering the player sprite, but after running it through the debugger, everything looks good. Are there special protocols to follow when using chipmunk's positions to render sprites? Do I need to multiply everything by some sort of PIXELS_PER_METER? Would it matter if they are in a sprite sheet? A sprite batch node? Here is how I set the sprite position in my view: </p> <pre><code>Player *player = [[RootModel sharedModel] player]; self.playerSprite.position = player.position; self.playerSprite.rotation = player.rotation; </code></pre> <p>And here is how I set the position in my Model:</p> <pre><code>- (void)update:(CGFloat)dt { self.position = self.body-&gt;p; self.rotation = self.body-&gt;a; } </code></pre> <p>(Yes, I know it's redundant to store the position because chipmunk does that inherently.)</p> <p>Also, bonus question for for those who finished reading the post. How might you move a body at a constant velocity in whatever direction it is facing, only allowing it to turn left or right at any given time, but never slow down?</p>
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