SKPhysics: Made a rope, why does it break?

Question

I've been experimenting with sprite kit a little bit, building a prototype for an idea I have. I've been connecting a string of physics bodies together using an SKPhysicsJointPin, to make a rope (actually more like a bike chain, but it's good enough). Also in the scene are a number of balls, and I can drop them when I tap them. This leads to the following:

However, when I drop more balls, the chain seems to be unable to handle it, and 'breaks':

Here is a movie showing the phenomenon

What's happening? The documentation never suggests SKPhysicsJointPin has a limited maximum strength or elasticity or similar. Is this a 'bug' in sprite kit, or am I using the wrong approach?

Answer 1

I faced a similar elasticity bug with a rope simulation and could finally come up with a workaround.

Here's my rope interface:

#import <SpriteKit/SpriteKit.h>

@interface ALRope : NSObject

@property(nonatomic, readonly) NSArray *ropeRings;

@property(nonatomic) int ringCount;

@property(nonatomic) CGFloat ringScale;

@property(nonatomic) CGFloat ringsDistance;

@property(nonatomic) CGFloat jointsFrictionTorque;

@property(nonatomic) CGFloat ringsZPosition;

@property(nonatomic) CGPoint startRingPosition;

@property(nonatomic) CGFloat ringFriction;

@property(nonatomic) CGFloat ringRestitution;

@property(nonatomic) CGFloat ringMass;


@property(nonatomic) BOOL shouldEnableJointsAngleLimits;

@property(nonatomic) CGFloat jointsLowerAngleLimit;

@property(nonatomic) CGFloat jointsUpperAngleLimit;



-(instancetype)initWithRingTexture:(SKTexture *)ringTexture;


-(void)buildRopeWithScene:(SKScene *)scene;

-(void)adjustRingPositions;

-(SKSpriteNode *)startRing;

-(SKSpriteNode *)lastRing;

@end

Rope Implementation:

#import "ALRope.h"

@implementation ALRope

{
    SKTexture *_ringTexture;

    NSMutableArray *_ropeRings;
}

static CGFloat const RINGS_DISTANCE_DEFAULT = 0;

static CGFloat const JOINTS_FRICTION_TORQUE_DEFAULT = 0;

static CGFloat const RING_SCALE_DEFAULT = 1;

static int const RING_COUNT_DEFAULT = 30;

static CGFloat const RINGS_Z_POSITION_DEFAULT = 1;

static BOOL const SHOULD_ENABLE_JOINTS_ANGLE_LIMITS_DEFAULT = NO;

static CGFloat const JOINT_LOWER_ANGLE_LIMIT_DEFAULT = -M_PI / 3;

static CGFloat const JOINT_UPPER_ANGLE_LIMIT_DEFAULT = M_PI / 3;

static CGFloat const RING_FRICTION_DEFAULT = 0;

static CGFloat const RING_RESTITUTION_DEFAULT = 0;

static CGFloat const RING_MASS_DEFAULT = -1;


-(instancetype)initWithRingTexture:(SKTexture *)ringTexture
{
    if(self = [super init]) {
        _ringTexture = ringTexture;

        //apply defaults
        _startRingPosition = CGPointMake(0, 0);
        _ringsDistance = RINGS_DISTANCE_DEFAULT;
        _jointsFrictionTorque = JOINTS_FRICTION_TORQUE_DEFAULT;
        _ringScale = RING_SCALE_DEFAULT;
        _ringCount = RING_COUNT_DEFAULT;
        _ringsZPosition = RINGS_Z_POSITION_DEFAULT;
        _shouldEnableJointsAngleLimits = SHOULD_ENABLE_JOINTS_ANGLE_LIMITS_DEFAULT;
        _jointsLowerAngleLimit = JOINT_LOWER_ANGLE_LIMIT_DEFAULT;
        _jointsUpperAngleLimit = JOINT_UPPER_ANGLE_LIMIT_DEFAULT;
        _ringFriction = RING_FRICTION_DEFAULT;
        _ringRestitution = RING_RESTITUTION_DEFAULT;
        _ringMass = RING_MASS_DEFAULT;
    }
    return self;
}


-(void)buildRopeWithScene:(SKScene *)scene
{
    _ropeRings = [NSMutableArray new];
    SKSpriteNode *firstRing = [self addRopeRingWithPosition:_startRingPosition underScene:scene];

    SKSpriteNode *lastRing = firstRing;
    CGPoint position;
    for (int i = 1; i < _ringCount; i++) {
        position = CGPointMake(lastRing.position.x, lastRing.position.y - lastRing.size.height - _ringsDistance);
        lastRing = [self addRopeRingWithPosition:position underScene:scene];
    }

    [self addJointsWithScene:scene];
}

-(SKSpriteNode *)addRopeRingWithPosition:(CGPoint)position underScene:(SKScene *)scene
{
    SKSpriteNode *ring = [SKSpriteNode spriteNodeWithTexture:_ringTexture];
    ring.xScale = ring.yScale = _ringScale;
    ring.position = position;
    ring.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:ring.size.height / 2];
    ring.physicsBody.allowsRotation = YES;
    ring.physicsBody.friction = _ringFriction;
    ring.physicsBody.restitution = _ringRestitution;
    if(_ringMass > 0) {
        ring.physicsBody.mass = _ringMass;
    }

    [scene addChild:ring];
    [_ropeRings addObject:ring];
    return ring;
}

-(void)addJointsWithScene:(SKScene *)scene
{
    for (int i = 1; i < _ropeRings.count; i++) {
        SKSpriteNode *nodeA = [_ropeRings objectAtIndex:i-1];
        SKSpriteNode *nodeB = [_ropeRings objectAtIndex:i];
        SKPhysicsJointPin *joint = [SKPhysicsJointPin jointWithBodyA:nodeA.physicsBody
                                                               bodyB:nodeB.physicsBody
                                                              anchor:CGPointMake(nodeA.position.x,
                                                                                 nodeA.position.y - (nodeA.size.height + _ringsDistance) / 2)];
        joint.frictionTorque = _jointsFrictionTorque;
        joint.shouldEnableLimits = _shouldEnableJointsAngleLimits;
        if(_shouldEnableJointsAngleLimits) {
            joint.lowerAngleLimit = _jointsLowerAngleLimit;
            joint.upperAngleLimit = _jointsUpperAngleLimit;
        }
        [scene.physicsWorld addJoint:joint];
    }
}

//workaround for elastic effect should be called from didSimulatePhysics
-(void)adjustRingPositions
{
    //based on zRotations of all rings and the position of start ring adjust the rest of the rings positions starting from top to bottom
    for (int i = 1; i < _ropeRings.count; i++) {
        SKSpriteNode *nodeA = [_ropeRings objectAtIndex:i-1];
        SKSpriteNode *nodeB = [_ropeRings objectAtIndex:i];
        CGFloat thetaA = nodeA.zRotation - M_PI / 2,
        thetaB = nodeB.zRotation + M_PI / 2,
        jointRadius = (_ringsDistance + nodeA.size.height) / 2,
        xJoint = jointRadius * cosf(thetaA) + nodeA.position.x,
        yJoint = jointRadius * sinf(thetaA) + nodeA.position.y,
        theta = thetaB - M_PI,
        xB = jointRadius * cosf(theta) + xJoint,
        yB = jointRadius * sinf(theta) + yJoint;
        nodeB.position = CGPointMake(xB, yB);
    }
}

-(SKSpriteNode *)startRing
{
    return _ropeRings[0];
}

-(SKSpriteNode *)lastRing
{
    return [_ropeRings lastObject];
}

@end

Scene code to showcase how to use the Rope:

#import "ALRopeDemoScene.h"
#import "ALRope.h"

@implementation ALRopeDemoScene
{
    __weak SKSpriteNode *_branch;

    CGPoint _touchLastPosition;

    BOOL _branchIsMoving;

    ALRope *_rope;
}

-(id)initWithSize:(CGSize)size {
    if (self = [super initWithSize:size]) {
        /* Setup your scene here */

        self.backgroundColor = [SKColor colorWithRed:0.2 green:0.5 blue:0.6 alpha:1.0];

        [self buildScene];
    }
    return self;
}

-(void) buildScene {
    SKSpriteNode *branch = [SKSpriteNode spriteNodeWithImageNamed:@"Branch"];
    _branch = branch;
    branch.position = CGPointMake(CGRectGetMaxX(self.frame) - branch.size.width / 2,
                                  CGRectGetMidY(self.frame) + 200);
    branch.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(2, 10)];
    branch.physicsBody.dynamic = NO;
    [self addChild:branch];
    _rope = [[ALRope alloc] initWithRingTexture:[SKTexture textureWithImageNamed:@"rope_ring"]];

    //configure rope params if needed
    //    _rope.ringCount = ...;//default is 30
    //    _rope.ringScale = ...;//default is 1
    //    _rope.ringsDistance = ...;//default is 0
    //    _rope.jointsFrictionTorque = ...;//default is 0
    //    _rope.ringsZPosition = ...;//default is 1
    //    _rope.ringFriction = ...;//default is 0
    //    _rope.ringRestitution = ...;//default is 0
    //    _rope.ringMass = ...;//ignored unless mass > 0; default -1
    //    _rope.shouldEnableJointsAngleLimits = ...;//default is NO
    //    _rope.jointsLowerAngleLimit = ...;//default is -M_PI/3
    //    _rope.jointsUpperAngleLimit = ...;//default is M_PI/3

    _rope.startRingPosition = CGPointMake(branch.position.x - 100, branch.position.y);//default is (0, 0)

    [_rope buildRopeWithScene:self];

    //attach rope to branch
    SKSpriteNode *startRing = [_rope startRing];
    CGPoint jointAnchor = CGPointMake(startRing.position.x, startRing.position.y + startRing.size.height / 2);
    SKPhysicsJointPin *joint = [SKPhysicsJointPin jointWithBodyA:branch.physicsBody bodyB:startRing.physicsBody anchor:jointAnchor];
    [self.physicsWorld addJoint:joint];
}

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    CGPoint location = [touch locationInNode:self];
    if(CGRectContainsPoint(_branch.frame, location)) {
        _branchIsMoving = YES;
        _touchLastPosition = location;
    }
}

-(void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    _branchIsMoving = NO;

}

-(void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{

    if(_branchIsMoving) {
        UITouch *touch = [touches anyObject];
        CGPoint location = [touch locationInNode:self],
        branchCurrentPosition = _branch.position;
        CGFloat dx = location.x - _touchLastPosition.x,
        dy = location.y - _touchLastPosition.y;
        _branch.position = CGPointMake(branchCurrentPosition.x + dx, branchCurrentPosition.y + dy);
        _touchLastPosition = location;
    }
}

-(void)didSimulatePhysics
{
    //workaround for elastic effect
    [_rope adjustRingPositions];
}


@end

Notice the [rope adjustRingPositions] call from [scene didSimulatePhysics]. That was the actual workaround for the elastic bug.

Complete demo code is here. I hope this helps!