SpriteKit球失去所有能量击球墙，restitution=1

Question

如果我向y方向施加1的冲量，球就会在没有失去任何能量的情况下来回反弹。然而，如果初始冲量为0.5或以下，当球撞到墙上时，它会立即失去所有的能量。为什么会发生这种情况？我对SKPhysicsBody类的属性有很好的理解。试试这段代码，看看你的电脑上是否有同样的行为。

-(id)initWithSize:(CGSize)size {
if (self = [super initWithSize:size]) {
    /* Setup your scene here */
    self.physicsWorld.gravity = CGVectorMake(0.0f, 0.0f);
    SKPhysicsBody* borderBody = [SKPhysicsBody bodyWithEdgeLoopFromRect:self.frame];
    self.physicsBody = borderBody;
    self.physicsBody.friction = 0.0f;
    self.backgroundColor = [SKColor colorWithRed:0.7 green:0.7 blue:0.7 alpha:1.0];

    SKShapeNode *ball = [SKShapeNode node];
    CGMutablePathRef pathToDraw = CGPathCreateMutable();
    [ball setStrokeColor:[UIColor blackColor]];
    CGPathMoveToPoint(pathToDraw, NULL, 0, 0);
    CGPathAddEllipseInRect(pathToDraw, NULL, CGRectMake(-16, -16, 32, 32));
    ball.path = pathToDraw;

    ball.position = CGPointMake(size.width / 2, size.height / 2);
    ball.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:ball.frame.size.width/2];
    ball.physicsBody.friction = 0.0f;
    ball.physicsBody.restitution = 1.0f;
    ball.physicsBody.linearDamping = 0.0f;
    ball.physicsBody.allowsRotation = NO;

    [self addChild:ball];

    [ball.physicsBody applyImpulse:CGVectorMake(0, 0.5)];
}
return self;
}

Answer 1

当碰撞速度足够小时(如您的CGVector of (0, 0.5))，雪碧套件物理引擎下的Box2d将将碰撞计算为非弹性(即，好像restitution为0，消除了任何弹力)，并且节点不会弹出。

根据Box2d 文档，这是为了防止抖动。

在Box2d源代码中，甚至有以下一行：

/// A velocity threshold for elastic collisions. Any collision with a relative linear
/// velocity below this threshold will be treated as inelastic.
#define b2_velocityThreshold            

你的冲动应该在这个门槛之上才能得到尊重。