svg.js元素的动画旋转给出了意想不到的结果（可见“摇摆”）

Question

我正在使用svg.js来创建一个自行车骑手的动画。半完整版本：https://pedalfuriously.neocities.org/。在使用requestAnimationFrame创建的动画（而不是动画中内置的svg.js）中移动和旋转svg元素时，我遇到了一些问题。

如果你看看链接，并使用cadence滑块使骑手踏板非常快，然后快速翻转滑块一直回到零，你可以看到他的小腿以断开的方式“摇摆”。真正做到的是，基于与曲柄旋转的绝对关系（而不是采用一些增量时间值来确定在该帧上的移动），在每个帧中确定腿的位置。

我想我已经能够确认我的代码的哪个方面导致了问题。这是一个没有表现出确切行为的最小例子，但我想这说明了我认为应该负责的事情：

var draw = SVG("drawing").viewbox(0, 0, 400, 400)
var origin = {
  x: 70,
  y: 70
}
var length = 60

var blueLine = draw.group()
blueLine.line(0, 0, 0 + length, 0).move(origin.x, origin.y)
  .stroke({
    color: "#00f",
    width: 4
  })
blueLine.angle = 0

var greenLine = draw.group()
greenLine.line(0, 0, 0 + length, 0).move(origin.x, origin.y)
  .stroke({
    color: "#0f0",
    width: 4
  })
greenLine.angle = 0

var previous = 0
var dt = 0
var step = function(timestamp) {
  dt = timestamp - previous
  previous = timestamp
  blueLine.angle += 0.18 * dt
  blueLine.rotate(blueLine.angle, origin.x, origin.y)
  var endX = Math.cos(toRad(blueLine.angle)) * length
  var endY = Math.sin(toRad(blueLine.angle)) * length

  // Comment out this line, and rotation works fine
  greenLine.move(endX, endY)

  greenLine.angle = blueLine.angle - 10

  // Comment out this line, and movement works fine
  greenLine.rotate(greenLine.angle, origin.x, origin.y)

  // But they don't work together. If I both move and rotate 
  // the green line, it goes in this crazy huge arc, rather 
  // than rotating neatly around the end of the blue line 
  // as expected.
  window.requestAnimationFrame(step)
}
window.requestAnimationFrame(step)

function toRad(deg) {
  return deg * (Math.PI / 180)
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/svg.js/2.6.4/svg.js"></script>
<div id="drawing"></div>

我用实际代码注意到的其他事情是，如果我移动腿的位置，它会改变问题的严重程度，甚至完全停止它。如果臀部一直定位在自行车前部附近，问题就不那么严重了。此外，如果我禁用小腿旋转，则没有摇晃。在某些位置，即使在任何动作开始之前，小腿也会在负载时立即旋转出屏幕。

我希望得到一些关于我是否误解操纵元素的方式的指导，特别是在svg.js或者SVG中。

谢谢亲切的矢量图形专家！

这是腿的实际代码。 step（）函数可能是最相关的。不确定它是否有帮助：

Rider.Leg = function(foot, front, xOffset, yOffset) {
    var upper = front ? SVGE.upperLeg : SVGE.upperLegBack
    var lower = front ? SVGE.lowerLeg : SVGE.lowerLegBack
    this.foot = foot
    this.draw = foot.draw
    this.geo = {
        upper: {
            x: this.foot.pedal.gear.x + 150,
            y: this.foot.pedal.gear.y - 750,
            length: 396
        },
        lower: {
            length: 390
        }
    }
    this.upper = this.draw.group().svg(upper).move(this.geo.upper.x, this.geo.upper.y)
        .transform({ scale: 0.95, cx: 0, cy: 0 })
    this.lower = this.draw.group().svg(lower).move(this.geo.upper.x, this.geo.upper.y)
}

// Step function does not take in a time argument. Positioning of legs is based only on
// the absolute position of other elements, none of which jiggle.
Rider.Leg.prototype.step = function () {
    var angle = this.pedalAngle() - Math.PI
    var ha = this.scaleneAngle(this.geo.lower.length, this.geo.upper.length, this.pedalDistance())
    var ka = this.scaleneAngle(this.pedalDistance(), this.geo.lower.length, this.geo.upper.length)
    var x = this.geo.upper.length * Math.cos(ha + angle)
    var y = this.geo.upper.length * Math.sin(ha + angle)
    this.upper.rotate(Drive.toDeg(angle + ha), 0, 0)
    this.lower.move(this.geo.upper.x + x, + this.geo.upper.y + y)
    this.lower.rotate(Drive.toDeg(angle + ha + ka - Math.PI), 0, 0)

}

// Gets the distance between the hip joint and the pedal
Rider.Leg.prototype.pedalDistance = function () {
    var pos = this.foot.getPos()
    var xDist = this.geo.upper.x - pos.x
    var yDist = this.geo.upper.y - pos.y
    return Math.hypot(xDist, yDist)
}

// Gets the angle between the hip joint and the pedal 
Rider.Leg.prototype.pedalAngle = function () {
    var pos = this.foot.getPos()
    var xDist = this.geo.upper.x - pos.x
    var yDist = this.geo.upper.y - pos.y
    return Math.atan2(yDist, xDist)
}

Rider.Leg.prototype.scaleneAngle = function (a, b, c) {
    return Math.acos(((b * b) + (c * c) - (a * a)) / (2 * b * c))
}