SpriteKit - physicsBody not respecting edge

Question

I have this object that has a physicsBody created from a path.

The whole scene area ( = the whole screen) itself is the boundary from where this object should not escape.

This scene's boundary is defined by

self.physicsBody = [SKPhysicsBody bodyWithEdgeLoopFromRect:self.frame];

Everything works almost as expected. The object collides and bounces on the edges and is constrained by them most part of the time, but if you insist several times, firing the object with high impulse to the edge the object will eventually do not respect the edge and escape, what is probably one more SpriteKit bug.

The object's class init is like this:

self.physicsBody.dynamic = YES;

CGFloat offsetX = self.frame.size.width/2.0f;
CGFloat offsetY = self.frame.size.height/2.0f;

CGMutablePathRef path = CGPathCreateMutable();

CGPathMoveToPoint(path, NULL, 60 - offsetX, 79 - offsetY);
CGPathAddLineToPoint(path, NULL, 96 - offsetX, 69 - offsetY);
CGPathAddLineToPoint(path, NULL, 124 - offsetX, 44 - offsetY);
CGPathAddLineToPoint(path, NULL, 120 - offsetX, 23 - offsetY);
CGPathAddLineToPoint(path, NULL, 85 - offsetX, 0 - offsetY);
CGPathAddLineToPoint(path, NULL, 62 - offsetX, 3 - offsetY);
CGPathAddLineToPoint(path, NULL, 37 - offsetX, 19 - offsetY);
CGPathAddLineToPoint(path, NULL, 12 - offsetX, 29 - offsetY);
CGPathAddLineToPoint(path, NULL, 3 - offsetX, 39 - offsetY);
CGPathAddLineToPoint(path, NULL, 7 - offsetX, 58 - offsetY);
CGPathAddLineToPoint(path, NULL, 27 - offsetX, 46 - offsetY);
CGPathAddLineToPoint(path, NULL, 36 - offsetX, 61 - offsetY);

CGPathCloseSubpath(path);

self.physicsBody = [SKPhysicsBody bodyWithPolygonFromPath:path];
self.physicsBody.restitution = 0.0f;
self.physicsBody.usesPreciseCollisionDetection = YES;

What measures can I do to detect that and prevent the object from leaving the screen?

thanks.

Have you been able to fix the issue? I encountered similar issue with circle body and the edge. After a some attempts applying force, the object escapes the edge and can never get back to the screen... Not sure if it's really SpriteKit's bug. — Khanh
Yeah, but that didn't apply to my case. I have circle physics body and the problem is still happening... — Khanh
@Khanh SpriteKit is buggy as hell. I have filled myself about 15 bug reports directly to Apple just for SpriteKit six months ago, but Apple don't give a crap to solve that. But what about the collision shape of the other object? — Duck
Yeah, agree with that. I have only 2 polygon-based shapes which I'm sure to have followed the guidelines. In the end, I have a little patch to check if it has reached the edge then cancel the velocity... — Khanh
if the circle shape is giving you problems try to create a shape that resembles a circle and see what happens. — Duck

LearnCocos2D LearnCocos2D · Accepted Answer · 2014-02-13T13:22:01

I didn't trace the points but it looks like you have either a non-convex polygon or one where the vertices do not follow counter-clockwise winding, or both. Make sure the shape is convex and vertex order (winding) is in counter-clockwise order.

If this shape is intended to be a static (non-moving) body only for collision detection you can use bodyWithEdgeLoop or bodyWithEdgeChain where winding and convex shape don't matter.

PS: don't forget to release the path:

CGPathRelease(path);

SpriteKit - physicsBody not respecting edge

1 Answers