Sparrow Forum

1. 

   cramhead
   Member

   Hi guys,

   I'm attempting make a 2D game using Sparrow and Box2d. I've had some initial success in getting the frameworks to play nice with one another, as described on http://mightyzeppelin.com/.
   When creating objects in Sparrow it's my impression that the position coordinates are the top, left of the displayed object, where as in Box2D I'm expecting that it's the center of the object.
   Is there quick and efficient mechanism to determine the center of the Sparrow object as it's rotating about?
   Many thanks,
   Marc

   Posted 1 year ago #
2. 

   Durrza
   Member

   Please do detail better your question
   centerx = image.width/2.0;
   centery = image.height/2.0;
   but surely it's not that simple? i'd guess you mean with image pivoting, image transformation, and you need the final coordinates in world space?

   Posted 1 year ago #
3. 

   matt62king
   Member

   I have had some luck with this method. I got it from a blog. It was originally set up for the UIKit and i just modified it to work with Sparrow. It has a few kinks such as the sprites will move in odd was depending on there original position. I'm sure if it comes from this method or some of the other ones. The collisions however are clean (Im using square boxes for now) so I assume the Box2D fixtures are being set correctly.

   - This for a stage set in landscape mode.
   - PTM_RATIO is defined as 16.
   - _content is a SPSprite that holds all the other SPSprites in the scene.

   + (void)addPhysicalBodyForSprite:(SPSprite *)sprite {
   b2BodyDef bodyDef;
   bodyDef.type = b2_dynamicBody;

   CGPoint p = CGPointMake((sprite.y-sprite.height/2.0), (sprite.x-sprite.width/2.0));
   CGPoint boxDimensions = CGPointMake(sprite.bounds.height/PTM_RATIO/2.0, sprite.bounds.width/PTM_RATIO/2.0);

   bodyDef.position.Set(p.x/PTM_RATIO, (480.0 - p.y)/PTM_RATIO);
   bodyDef.userData = sprite;

   // Tell the physics world to create the body
   b2Body *body = world->CreateBody(&bodyDef);

   // Define another box shape for our dynamic body.
   b2PolygonShape dynamicBox;

   dynamicBox.SetAsBox(sprite.height/PTM_RATIO/2.0, sprite.width/PTM_RATIO/2.0);

   // Define the dynamic body fixture.
   b2FixtureDef fixtureDef;
   fixtureDef.shape = &dynamicBox;
   fixtureDef.density = 3.0f;
   fixtureDef.friction = 0.3f;
   fixtureDef.restitution = 0.5f; // 0 is a lead ball, 1 is a super bouncy ball
   body->CreateFixture(&fixtureDef);

   // a dynamic body reacts to forces right away
   body->SetType(b2_dynamicBody);

   // we abuse the tag property as pointer to the physical body
   int i = [_content childIndex:sprite];
   i = (int)body;
   }

   Posted 1 year ago #
4. 

   ronaldmathies
   Moderator

   If you center the sprite you wouldn't be able to rotate nicely for example. So add an image to the sprite, center the image ( so the center of the image ends up on the top left of the sprite. Then position the box2d object on the top left of the sprite). And use the width and height of the image to define the dimensions of the dynamic box.

   Here is a sample of how i handle it: (displays a square block on the screen)

   The creation of the SPSprite:

   SPSprite *barSprite = [[[SPSprite alloc] init] autorelease];
   [barSprite setPosition:displayX y:displayY];

   SPImage *barImage = [SPImage imageWithContentsOfFile:@"bar.png"];
   [self setPostion:-([barImage width] / 2.0f) y:-([barImage height] / 2.0f)
   [barSprite addChild:barImage];

   The creation of the Box2D:

   // Define body definition
   b2BodyDef bodyDef;

   // The bar is static, it is manually controlled by the player.
   bodyDef.type = b2_staticBody;

   // Rotation is fixed.
   bodyDef.fixedRotation = true;

   float x = PX_TO_PTM(displayX - (BAR_WIDTH / 2.0f));
   float y = PX_TO_PTM(displayY - (BAR_HEIGHT / 2.0f));
   bodyDef.position.Set(x, y);

   b2PolygonShape polygonShape;

   float width = PX_TO_PTM(BAR_WIDTH / 2.0f);
   float height = PX_TO_PTM(BAR_HEIGHT / 2.0f);
   polygonShape.SetAsBox(width, height);

   // Create the fixture definition
   b2FixtureDef fixtureDef;
   fixtureDef.shape = &polygonShape;

   // Create the body
   b2Body *body = [self box2dWorld]->CreateBody(&bodyDef);
   body->CreateFixture(&fixtureDef);

   The method PX_TO_PTM is defined as follow:

   /**
   * Helper method for converting pixels to meters.
   */
   static inline float PX_TO_PTM(float px) {
   return px / PTM_RATIO;
   }

   Posted 1 year ago #

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