SpriteKit 中的速度单位答案

【问题标题】：Velocity units in SpriteKitSpriteKit 中的速度单位
【发布时间】：2014-01-05 11:52:22
【问题描述】：

我正在使用 SpriteKit 发射弹丸。使用-(void)applyImpulse:(CGVector)impulse 给射弹节点一个脉冲。

我想计算弹丸在撞击地面之前的飞行距离，知道弹丸节点物理体的速度矢量和重力加速度就可以轻松完成（见Wikipedia: Range of a Projectile）。

但不清楚该距离的单位是什么，以及它们如何转换为SKScene 中的距离。 SKPhysicsBody 的速度属性的 documentation 没有提到单位。我目前正在运行一些测试以找出单位并映射到 SKScene。

有人已经弄清楚了吗？

【问题讨论】：

标签： ios game-physics sprite-kit

【解决方案1】：

单位可能是 CGPoints/秒

我做了一些实验，通过打印位置变化/时间变化并将其与物理体的速度属性进行比较。这是我的代码：

func moveInDirection(direction:CGVector, timeInterval:NSTimeInterval){

    physicsBody?.linearDamping = damping
    let changeInX = position.x-previous_whale_position.x
    let changeInY = position.y-previous_whale_position.y
    let x_Velocity = changeInX/CGFloat(timeInterval)
    let y_Velocity = changeInY/CGFloat(timeInterval)
    println("the m/svelocity      \(physicsBody!.velocity.dx), \(physicsBody!.velocity.dy)")
    println("the cgpointvelocity  \(x_Velocity), \(y_Velocity)")
    println()
    physicsBody!.applyImpulse(direction*accelerationMultiplier)
    physicsBody!.velocity = physicsBody!.velocity.limitToHypot(maxVelocity)


}

这是日志中的一些输出

the m/svelocity      -419.904, -114.774
the cgpointvelocity  -411.577, -112.497

the m/svelocity      -410.107, -134.854
the cgpointvelocity  -522.235, -171.725

the m/svelocity      -406.494, -155.742
the cgpointvelocity  -413.777, -158.531

the m/svelocity      -395.372, -182.136
the cgpointvelocity  -396.967, -182.874

the m/svelocity      -389.263, -194.851
the cgpointvelocity  -397.426, -198.937

the m/svelocity      -384.932, -203.273
the cgpointvelocity  -388.112, -204.951

the m/svelocity      -383.476, -206.006
the cgpointvelocity  -390.179, -209.609

the m/svelocity      -382.996, -206.897
the cgpointvelocity  -389.749, -210.543

the m/svelocity      -382.839, -207.188
the cgpointvelocity  -385.678, -208.726

the m/svelocity      -382.787, -207.283
the cgpointvelocity  -386.198, -209.131

the m/svelocity      -382.77, -207.314
the cgpointvelocity  -386.386, -209.272

the m/svelocity      -382.765, -207.325
the cgpointvelocity  -387.465, -209.871

the m/svelocity      -382.763, -207.328
the cgpointvelocity  -387.539, -209.915

the m/svelocity      -382.762, -207.329
the cgpointvelocity  -388.23, -210.291

the m/svelocity      -385.052, -203.045
the cgpointvelocity  -396.597, -209.134

the m/svelocity      -404.03, -171.534
the cgpointvelocity  -300.013, -127.373

the m/svelocity      -411.441, -130.726
the cgpointvelocity  -522.956, -166.157

the m/svelocity      -422.1, -106.413
the cgpointvelocity  -427.514, -107.779

the m/svelocity      -425.415, -92.2759
the cgpointvelocity  -430.583, -93.3979

the m/svelocity      -431.763, -55.4343
the cgpointvelocity  -451.494, -57.9693

the m/svelocity      -438.175, -25.8145
the cgpointvelocity  -327.677, -19.3046

the m/svelocity      -435.305, 1.48222
the cgpointvelocity  -431.963, 1.46786

the m/svelocity      -431.347, 17.6925
the cgpointvelocity  -555.525, 22.7858

the m/svelocity      -434.409, 27.9586
the cgpointvelocity  -435.048, 27.9997

the m/svelocity      -432.63, 48.2073
the cgpointvelocity  -437.134, 48.7081

the m/svelocity      -431.843, 54.8057
the cgpointvelocity  -440.961, 55.9637

the m/svelocity      -430.199, 66.49
the cgpointvelocity  -440.097, 68.0202

the m/svelocity      -432.777, 73.2641
the cgpointvelocity  -321.332, 54.3979

the m/svelocity      -424.901, 76.3717
the cgpointvelocity  -540.091, 97.0769

the m/svelocity      -427.745, 80.7875
the cgpointvelocity  -433.12, 81.8004

the m/svelocity      -424.632, 95.811
the cgpointvelocity  -431.081, 97.2632

the m/svelocity      -423.5, 100.698
the cgpointvelocity  -429.958, 102.232

the m/svelocity      -423.118, 102.293
the cgpointvelocity  -434.132, 104.954

the m/svelocity      -426.516, 103.671
the cgpointvelocity  -316.766, 76.9931

the m/svelocity      -419.455, 102.133
the cgpointvelocity  -533.238, 129.839

the m/svelocity      -422.936, 103.04
the cgpointvelocity  -429.117, 104.546

the m/svelocity      -409.127, 99.6756
the cgpointvelocity  -415.2, 101.156

the m/svelocity      -395.769, 96.4212
the cgpointvelocity  -399.771, 97.3951

the m/svelocity      -382.847, 93.273
the cgpointvelocity  -387.945, 94.5152

the m/svelocity      -370.347, 90.2276
the cgpointvelocity  -374.075, 91.1383

the m/svelocity      -358.255, 87.2816
the cgpointvelocity  -363.762, 88.6238

the m/svelocity      -346.558, 84.4319
the cgpointvelocity  -350.016, 85.275

the m/svelocity      -335.243, 81.6751
the cgpointvelocity  -350.701, 85.4411

the m/svelocity      -326.999, 79.6668
the cgpointvelocity  -242.241, 59.0151

the m/svelocity      -313.709, 76.4288
the cgpointvelocity  -404.319, 98.5007

the m/svelocity      -305.995, 74.5495
the cgpointvelocity  -225.667, 54.9776

the m/svelocity      -293.558, 71.5194
the cgpointvelocity  -380.591, 92.7216

the m/svelocity      -286.339, 69.7608
the cgpointvelocity  -217.244, 52.9272

the m/svelocity      -276.99, 67.4831
the cgpointvelocity  -273.834, 66.7158

the m/svelocity      -265.732, 64.7403
the cgpointvelocity  -338.328, 82.4244

the m/svelocity      -257.056, 62.6265
the cgpointvelocity  -265.84, 64.7653

the m/svelocity      -250.735, 61.0865
the cgpointvelocity  -188.673, 45.9667

the m/svelocity      -242.548, 59.092
the cgpointvelocity  -242.195, 59.0063

the m/svelocity      -232.69, 56.6902
the cgpointvelocity  -302.26, 73.6413

the m/svelocity      -226.968, 55.2963
the cgpointvelocity  -166.956, 40.6744

the m/svelocity      -217.743, 53.0488
the cgpointvelocity  -287.069, 69.9373

the m/svelocity      -212.389, 51.7444
the cgpointvelocity  -158.363, 38.5817

the m/svelocity      -205.455, 50.0549
the cgpointvelocity  -204.966, 49.9354

the m/svelocity      -197.104, 48.0204
the cgpointvelocity  -254.568, 62.0196

the m/svelocity      -192.257, 46.8396
the cgpointvelocity  -143.343, 34.9233

the m/svelocity      -184.443, 44.9358
the cgpointvelocity  -234.465, 57.1213

the m/svelocity      -178.421, 43.4686
the cgpointvelocity  -179.961, 43.8436

the m/svelocity      -172.595, 42.0494
the cgpointvelocity  -177.962, 43.3537

the m/svelocity      -168.351, 41.0154
the cgpointvelocity  -125.254, 30.5169

the m/svelocity      -161.509, 39.3484
the cgpointvelocity  -204.98, 49.9368

the m/svelocity      -156.235, 38.0636
the cgpointvelocity  -159.304, 38.8082

the m/svelocity      -151.134, 36.8208
the cgpointvelocity  -156.913, 38.2251

the m/svelocity      -147.418, 35.9154
the cgpointvelocity  -109.618, 26.7046

the m/svelocity      -142.605, 34.7428
the cgpointvelocity  -143.35, 34.9249

the m/svelocity      -136.809, 33.3307
the cgpointvelocity  -176.614, 43.0291

the m/svelocity      -133.445, 32.5111
the cgpointvelocity  -99.8049, 24.3138

the m/svelocity      -128.021, 31.1897
the cgpointvelocity  -165.016, 40.2001

the m/svelocity      -124.873, 30.4228
the cgpointvelocity  -93.041, 22.6676

所以我得出结论，距离单位几乎是 cgpoints/second.. 当前速度似乎不能很好地代表实际速度。这可能是由于测量是在不同的时间间隔内进行的，在物理模拟期间测量 m/s 速度，而在更新函数期间测量 cgpoint/second 速度。然而，平均而言，我无法解释的 m/s 速度似乎较低。可能发生不精确的舍入

【讨论】：

【解决方案2】：

来自 SpriteKit 编程指南：

捏造数字 虽然知道 Sprite Kit 很有用计量国际单位制项目，担心精确的数字并不那么重要。有没有没关系你的火箭飞船重 1 公斤或 1,000,000 公斤，只要质量与游戏中使用的其他物理值一致。通常，比例比实际值更重要用过。

所以质量的单位是公斤，冲量的单位是牛顿秒，扭矩的单位是牛顿，但只要它们在您的游戏中保持一致，就没有那么重要了。

【讨论】：

感谢您的回答，但它并没有真正回答我的问题。我在编程指南中也读过这部分，说实话觉得有点奇怪。由于脉冲以牛顿秒为单位，因此速度将以 m/s 为单位。我猜秒映射到模拟秒，但不清楚仪表是如何映射到屏幕上的像素的。
实际上，扭矩的单位应该是牛顿米而不是牛顿。嗯，从技术上讲是米牛顿，因为它的 r 交叉 F 和叉积是不可交换的。
“担心精确的数字并不那么重要” 傻瓜——整个游戏物理工程世界都以完全“真实”的米和公斤为单位工作。如果GT3之类的团队成员去苹果公司工作，那会很有趣。开什么玩笑。