[解決済み] SVGで「スクイグリーライン」を描画する

function makeSquiggle(squigglePathId, followPathId, squiggleStep, squiggleAmplitude)
{
  var followPath = document.getElementById(followPathId);
  var pathLen = followPath.getTotalLength();

  // Adjust step so that there are a whole number of steps along the path
  var numSteps = Math.round(pathLen / squiggleStep);

  var pos = followPath.getPointAtLength(0);
  var newPath = "M" + [pos.x, pos.y].join(',');
  var side = -1;
  for (var i=1; i<=numSteps; i++)
  {
    var last = pos;
    var pos = followPath.getPointAtLength(i * pathLen / numSteps);

    // Find a point halfway between last and pos. Then find the point that is
    // perpendicular to that line segment, and is squiggleAmplitude away from
    // it on the side of the line designated by 'side' (-1 or +1).
    // This point will be the control point of the quadratic curve forming the
    // squiggle step.
    
    // The vector from the last point to this one
    var vector = {x: (pos.x - last.x),
                  y: (pos.y - last.y)};
    // The length of this vector
    var vectorLen = Math.sqrt(vector.x * vector.x + vector.y * vector.y);
    // The point halfwasy between last point and tis one
    var half = {x: (last.x + vector.x/2),
                y: (last.y + vector.y/2)};
    // The vector that is perpendicular to 'vector'
    var perpVector = {x: -(squiggleAmplitude * vector.y / vectorLen),
                      y: (squiggleAmplitude * vector.x / vectorLen)};
    // No calculate the control point position
    var controlPoint = {x: (half.x + perpVector.x * side),
                        y: (half.y + perpVector.y * side)};
    newPath += ("Q" + [controlPoint.x, controlPoint.y, pos.x, pos.y].join(','));
    // Switch the side (for next step)
    side = -side;
  }
  var squigglePath = document.getElementById(squigglePathId);
  squigglePath.setAttribute("d", newPath);
}


makeSquiggle("squiggle", "follow", 25, 20);

#follow {
  fill: none;
  stroke: grey;
  stroke-width: 2;
}

#squiggle {
  fill: none;
  stroke: red;
  stroke-width: 2;
}

<svg width="500" height="400">
  <path id="follow" d="M 50,300 C 100,100 300,0, 350,250 L 450,200"/>
  <path id="squiggle" d="M0,0"/>
</svg>