SpriteKit PhysicsWorld coordinate system, weird runtime joint anchor

Question

Two likely related things here:

1) I can draw a box and add to child from my SKScene impelmentation file with self, self.scene, and myWorld, but not with an SKSprite node's scene property.

SKSpriteNode *bla = [SKSpriteNode spriteNodeWithColor:[UIColor redColor] size:CGSizeMake(100, 100)];
[self.scene addChild:bla]; // If I use bla.scene, it doesn't work. self, self.scene, myworld all do work though.
bla.position = CGPointMake(0, 0);
bla.zPosition = 999;

2) I've seen the related questions here and here, but I'm trying to add a joint during gameplay (grabbing a rope). This method gets called after doing some sorting in `didBeginContact:

-(void)monkey:(SKPhysicsBody *)monkeyPhysicsBody didCollideWithRope:(SKPhysicsBody *)ropePhysicsBody atPoint:(CGPoint)contactPoint
{
    if (monkeyPhysicsBody.joints.count == 0) {
        // Create a new joint between the player and the rope segment
        CGPoint convertedRopePosition = [self.scene convertPoint:ropePhysicsBody.node.position fromNode:ropePhysicsBody.node.parent];
        SKPhysicsJointPin *jointPin = [SKPhysicsJointPin jointWithBodyA:playerPhysicsBody bodyB:ropePhysicsBody anchor:convertedRopePosition];
        jointPin.upperAngleLimit = M_PI/4;
        jointPin.shouldEnableLimits = YES;
        [self.scene.physicsWorld addJoint:jointPin];
    }
}

I've got showPhyiscs enabled on the scene, so I can see that the joint is ending up in a totally wacky place. Unfortunately, I don't know how to apply the linked solutions since I'm not adding the SKSpriteNodes in this method, they already exist, so I can't just flip the order of position and physicsBody.

I've tried every permutation I could for both of the convertPoint methods. Nothing worked. My best guess is that physicsWorld is using some wacky coordinate system.

score 2 · Answer 1 · answered Nov 29 '14 at 06:58

Method members of SKPhysicsWorld that relate to position (CGPoint) or frame (CGRect) are to be in scene coordinates. Scene coordinates reference the point {0,0} as the bottom left corner and is consistent throughout SpriteKit.

The scene property of your object named bla will be nil when bla is first created and is set by the scene when added to it.

[bla.scene addChild:bla]; // this won't do anything as scene will be nil when first created

It looks as though convertedRopePosition is being assigned an incorrect value because the second member you're passing into, - (CGPoint)convertPoint:(CGPoint)point fromNode:(SKNode *)node , is the scene when it should be another node in the same node tree as this node. where this node is the caller (in this case the SKScene subclass).

Try replacing the line-

CGPoint convertedRopePosition = [self.scene convertPoint:ropePhysicsBody.node.position fromNode:ropePhysicsBody.node.parent];

with-

CGPoint convertedRopePosition = [self convertPoint:ropePhysicsBody.node.position fromNode:ropePhysicsBody.node];

Thanks for fast reply! `bla.scene` not working now makes sense. The convertPoint thing still didn't work though. Like I said, I've tried every permutation I could think of. Using your line of code, the joint still appears in some random spot. See screenshot: https://www.dropbox.com/s/dd4zqjo9at6sjr2/iOS%20Simulator%20Screen%20Shot%20Nov%2029%2C%202014%2C%201.40.30%20PM.png?dl=0 — James Paul Mason, Nov 29 '14 at 20:42

score 0 · Answer 2 · answered Dec 27 '14 at 18:00

I came up with a janky work around for this problem. It turns out that the coordinate system offset for physicsWorld was likely due to an anchor difference. Changing the anchors of every related thing made no difference, and you can't change the anchor of the physicsWorld directly, so I ended up adding half of the scene width and half of the scene height to the anchor position of my joint. That got it to show in the right place and behave normally.

score 0 · Accepted Answer · edited May 23 '17 at 10:28

This problem persisted once side scrolling was factored in. I've posted other questions here with this same problem but I'll include th

I added the following convenience method to my GameScene.m. It essentially takes the place of the seemingly useless convertTo built in method.

-(CGPoint)convertSceneToFrameCoordinates:(CGPoint)scenePoint
{
    CGFloat xDiff = myWorld.position.x - self.position.x;
    CGFloat yDiff = myWorld.position.y - self.position.y;
    return CGPointMake(scenePoint.x + self.frame.size.width/2 + xDiff, scenePoint.y + self.frame.size.height/2 + yDiff);
}

I use this method to add joints. It handles all of the coordinate system transformations that need to be dealt with that lead to the issue raised in this question. For example, the way I add joints

    CGPoint convertedRopePosition = [self convertSceneToFrameCoordinates:ropePhysicsBody.node.position];
    SKPhysicsJointPin *jointPin = [SKPhysicsJointPin jointWithBodyA:monkeyPhysicsBody bodyB:ropePhysicsBody anchor:convertedRopePosition];
    jointPin.upperAngleLimit = M_PI/4;
    jointPin.shouldEnableLimits = YES;
    [self.scene.physicsWorld addJoint:jointPin];

SpriteKit PhysicsWorld coordinate system, weird runtime joint anchor

3 Answers3