速度：米每秒还是像素每秒？

Question

Apple 文档指出物理体的速度以米每秒为单位测量。

这是真的吗？像素和米之间有关系吗？如果有的话是什么？

或者这是文档中的错误，它实际上是每秒像素数？

Answer 1

我写了一个小程序来尝试解决这个问题，并得到了 135 分的答案。

https://stackoverflow.com/a/37523818/1430420

编辑：这是任何人都可以检查我的数学是否错误的程序

//
//  GameScene.swift
//  gravityTest
//
//  Created by Steve Ives on 30/05/2016.
//  Copyright (c) 2016 Steve Ives. All rights reserved.
//

import SpriteKit

class GameScene: SKScene {

//    var elapsedTime: CFTimeInterval = 0.0
    var timeOfLastUpdate: CFTimeInterval = 0.0
    var updateCount = 0
    let sprite = SKSpriteNode(imageNamed:"Spaceship")

    override func didMove(to view: SKView) {
        /* Setup your scene here */

//        physicsWorld.gravity = CGVectorMake(-9.8, 0)

        let location = CGPoint(x: (scene?.frame.midX)!, y: (scene?.frame.midY)!)

        sprite.xScale = 0.5
        sprite.yScale = 0.5

        sprite.physicsBody = SKPhysicsBody(rectangleOf: sprite.size)
        sprite.physicsBody?.isDynamic = true
        sprite.position = location

        self.addChild(sprite)
    }

    override func update(_ currentTime: CFTimeInterval) {
        /* Called before each frame is rendered */
                updateCount += 1
        if (currentTime - timeOfLastUpdate) < 1 {return}

//        if (timeOfLastUpdate - currentTime > 1.0) {
        print("\(updateCount) - Time : \(currentTime) \(currentTime-timeOfLastUpdate) Location is : \(sprite.position) Velocity is : \(sprite.physicsBody!.velocity)")
//        }

        timeOfLastUpdate = currentTime
    }
}

Answer 2

我使用游乐场来计算我的距离。我已经快150了

我们知道距离是 d = (gt^2)/2，其中 g 是重力，t 是时间。

d = (9.8m/t^2)(t^2)/2 d = (9.8m)/2 d = 4.9m

1秒内，我们将行驶4.9m

在我们的例子中，我们旅行了 765.9298706054688 点。

765.9298706054688/4.9 为我们提供了大约 156.312218491。

理想情况下，我们会行驶 735 个点，不确定我的代码中的什么可能会导致 30 个点的差异，我怀疑这与 SKAction 有关

//: A SpriteKit based Playground

import PlaygroundSupport
import SpriteKit

class GameScene: SKScene {

    private var testNode = SKSpriteNode(color: .red, size: CGSize(width: 20, height: 20))
    private var flag = false
    private var time : CGFloat = 0
    private var beginTest = false
    override func didMove(to view: SKView) {

    }

    @objc static override var supportsSecureCoding: Bool {
        // SKNode conforms to NSSecureCoding, so any subclass going
        // through the decoding process must support secure coding
        get {
            return true
        }
    }

    func touchDown(atPoint pos : CGPoint) {
    }



    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        testNode = SKSpriteNode(color: .red, size: CGSize(width: 20, height: 20))
        let physicsBody = SKPhysicsBody(rectangleOf: testNode.size)
        physicsBody.angularDamping = 0
        physicsBody.linearDamping = 0
        physicsBody.friction = 0
        testNode.physicsBody = physicsBody
        let action = SKAction.customAction(withDuration: 2)
        {
            node,elapsedTime in
            if elapsedTime >= 1 && !self.flag && self.time == 0{
                self.flag = true
                self.time = elapsedTime
            }
        }
        testNode.run(action)
        beginTest = true
    }


    override func update(_ currentTime: TimeInterval) {
        if beginTest{
            beginTest = false
            addChild(testNode)
        }
    }
    override func didFinishUpdate() {

    }


    override func didApplyConstraints() {
        if flag{
            let position = CGPoint(x:testNode.position.x,y:-testNode.position.y)

            print("Node traveled \(position.y) points over \(time) seconds with a velocity of \(testNode.physicsBody!.velocity)")

            let pointsPerMeter = -testNode.physicsBody!.velocity.dy/9.8
            print("Node traveled \(pointsPerMeter) points per meter over \(time) seconds with a velocity of \(testNode.physicsBody!.velocity)")
            flag = false
            testNode.removeFromParent()
            time = 0
        }
    }
}

// Load the SKScene from 'GameScene.sks'
let sceneView = SKView(frame: CGRect(x:0 , y:0, width: 640, height: 480))
if let scene = GameScene(fileNamed: "GameScene") {
    // Set the scale mode to scale to fit the window
    scene.scaleMode = .aspectFill

    // Present the scene
    sceneView.presentScene(scene)
}

PlaygroundSupport.PlaygroundPage.current.liveView = sceneView

节点在 1.0 秒内移动了 765.9298706054688 个点，速度为 (0.0, -1494.4998779296875)
节点在 1.0 秒内每米移动 152.49998754384566 点，速度为 (0.0, -1494.4998779296875)

Answer 3

给任何人和我自己的答案

在苹果 SKSpriteKit 中没有什么可以真正定义像素，这是我的问题中的用词不当。另一方面，一个点是明确定义的。

每个人可能已经知道这一点，但是如果您将 SKPhysicsBody 附加到节点并分配以下属性：

    seg.physicsBody?.isDynamic = true
    seg.physicsBody?.affectedByGravity = false
    seg.physicsBody?.allowsRotation = false
    seg.physicsBody?.usesPreciseCollisionDetection = true
    seg.physicsBody?.mass = 0
    seg.physicsBody?.restitution = 1
    seg.physicsBody?.linearDamping = 0

然后应用 200 的速度 dy 分量会导致对象以每秒 200 点的速度移动。这对我来说，移除所有外力后，物理世界将转化为 1m/s = 1point/s。