缩放用 SKSpriteNodes 制作的钟摆

Question

我有一个带钟摆的工作时钟，但摆臂不能正确缩放。这是缩放之前的样子...

然后，在缩放之后……

钟面按比例缩小，摆臂（绿线）也按比例缩小，但看起来它围绕其中心点而不是钟面中心的支点进行缩放。我可以通过设置秋千的anchorPoint 来修复这种缩放，使其向一端而不是中心缩放...

swing.anchorPoint = CGPointMake(0, 0.5);

但是这样做会导致钟摆不摆动。 （我认为是因为物理力被施加到摇摆的anchorPoint 上，如果我将它移动到一端，它位于固定的销锚上）。

我怎样才能让手臂朝着支点缩放并仍然允许它摆动？

这是代码...

// in my SKScene
ClockFace *face = [ClockFace face]; // just an SKNode subclass
face.name = @"face";
face.position = CGPointMake(150, 150);
[self addChild:face];

// add the pendulum
SKSpriteNode *fulcrum = [SKSpriteNode spriteNodeWithColor:[UIColor redColor] size:CGSizeMake(5, 5)];
[face addChild:fulcrum];
fulcrum.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:fulcrum.frame.size];
fulcrum.physicsBody.dynamic = NO;

SKSpriteNode *swing = [SKSpriteNode spriteNodeWithColor:[UIColor greenColor] size:CGSizeMake(190, 2)];
[face addChild:swing];
swing.position = CGPointMake(95, 0);
swing.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:swing.frame.size];

SKPhysicsJointPin *pinJoint = [SKPhysicsJointPin jointWithBodyA:fulcrum.physicsBody bodyB:swing.physicsBody anchor:face.position];
[self.physicsWorld addJoint:pinJoint];

Answer 1

在 Swift 3 中这个例子：

    let nodeSize = CGSize(width: 10, height: 10)
    let node = SKSpriteNode(color: .red, size: nodeSize)
    node.physicsBody = SKPhysicsBody(rectangleOf: nodeSize)
    node.physicsBody?.isDynamic = false
    self.addChild(node)

    let node2Size = CGSize(width: 60, height: 8)
    let node2 = SKSpriteNode(color: .green, size: node2Size)
    node2.position = CGPoint(x: 5, y: 0)
    node2.anchorPoint = CGPoint(x: 0.0, y: 0.5) // <- New Line
    node2.physicsBody = SKPhysicsBody(rectangleOf: node2Size)
    node2.physicsBody?.mass = 1.0
    self.addChild(node2)

    // Scale Line
    node2.run(SKAction.repeatForever(SKAction.sequence([
        SKAction.scale(to: 0.2, duration: 1.0),
        SKAction.scale(to: 1.5, duration: 0.5),
        ])))
// Anchor Point
        let a = SKPhysicsJointPin.joint(withBodyA: node.physicsBody! , bodyB: node2.physicsBody!, anchor: CGPoint(x: 0.0, y: 0.0))
            self.physicsWorld.add(a)

Answer 2

缩放之所以奇怪是因为你如何做你的锚点。想象一个带有图钉的橡皮筋。锚点是图钉所在的位置。现在进行规模化。只需抓住橡皮筋并拉动即可。当图钉位于中间时，您会发现很容易均匀地拉伸两侧（这就是您在 y 轴上所做的）。当您将它放在橡皮筋的左侧时，您将只能缩放右侧（这就是您想要做的）

现在使用 PhysicsBody，您需要根据精灵锚点调整身体锚点。为此，您需要做一些数学运算：

let centerPoint = CGPointMake(sprite.size.width / 2 - (sprite.size.width * sprite.anchorPoint.x), sprite.size.height / 2 - (sprite.size.height * sprite.anchorPoint.y))
 sprite.physicsBody = SKPhysicsBody(rectangleOfSize: sprite.size, center: centerPoint)

使用 centerPoint 可以让您移动身体中心所在的位置。

// in my SKScene
ClockFace *face = [ClockFace face]; // just an SKNode subclass
face.name = @"face";
face.position = CGPointMake(150, 150);
[self addChild:face];

// add the pendulum
SKSpriteNode *fulcrum = [SKSpriteNode spriteNodeWithColor:[UIColor redColor] size:CGSizeMake(5, 5)];
[face addChild:fulcrum];
fulcrum.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:fulcrum.frame.size];
fulcrum.physicsBody.dynamic = NO;

SKSpriteNode *swing = [SKSpriteNode spriteNodeWithColor:[UIColor greenColor] size:CGSizeMake(190, 2)];
[face addChild:swing];
swing.anchorPoint = CGPointMake(0,0.5);
CGPoint *centerPoint = CGPointMake(swing.size.width / 2 - (swing.size.width * swing.anchorPoint.x), swing.size.height / 2 - (swing.size.height * swing.anchorPoint.y));

swing.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:swing.frame.size center:centerPoint];

SKPhysicsJointPin *pinJoint = [SKPhysicsJointPin jointWithBodyA:fulcrum.physicsBody bodyB:swing.physicsBody anchor:face.position];
[self.physicsWorld addJoint:pinJoint];