SpriteKit Physics Joints corrupt for moving scene

Question

I've created a SpriteKit Scene with physics bodys. This is working quite well. I'm also scrolling the scene around, using the Apple best practice with a "camera node".

My scenes node tree is like that:

Scene
|- World
   |- Camera node
   |- Game nodes with physics bodys

My Problem is, during the game I create new nodes with new joints (mostly fixed joints) dynamically. At that point, the world node has a different position than in the beginning (eg.: at start (0,0), during the game (1000,500)..) Now, the anchorpoint for the fixed joints is not at the position I want it to, it's somewhere in the scene.

I assume, the coordinate system of the scene is different to the coordinate system of the physics engine.

For the anchor Point, I use the following conversion:

[world.scene convertPoint:node.position fromNode:node.parent];

I want to set the anchorpoint at the position of "node". After moving the world node, the calculation of the anchorpoint is not working anymore.

Thanks for your help!

Greetz - Nekro

Update:

Code to center scene on the Player

- (void)centerOnNode:(SKNode*)node {
    CGPoint cameraPositionInScene = [node.scene convertPoint:node.position fromNode:node.parent];
    node.parent.position = CGPointMake(node.parent.position.x - cameraPositionInScene.x, node.parent.position.y - cameraPositionInScene.y);
}

Answer 1

There are two requirements to using joints with SpriteKit physics:

1. Define the joints in world (scene) space.
2. Don't move the world.

From your question and comments, it sounds like you've got #1 right: When the joint is between nodes that aren't direct children of the scene, you need to convert coordinates to scene space before using them to define a joint.

However, SpriteKit physics doesn't handle it well when you try to move the root coordinate system. So, implementing a "camera" by moving the world causes trouble for the joints. Instead:

• If you can afford to target only iOS 9.0 / OS X 10.11 / tvOS 9.0 and up, use SKCameraNode instead of moving the world. It's easier, too!

  _{(If you're wondering about whether you can support only iOS 9, check the adoption rate statistics, and compare to your development schedule... if you won't be shipping for awhile, iOS 9 might be the "-1" OS version by the time you're done. Remember also there's a high correlation between enthusiastic customers and early adopters.)}

• If you're supporting iOS 8 / OS X 10.10 still, try adding an extra layer to your scene: fake a camera by moving a node that is a child of your scene and the parent of all your game content.