Point-Along-Path Interpolation

Demonstrates how to use the getTotalLength and getPointAtLength methods on SVG path elements to interpolate a point along an existing path, and how to animate the stroke using the stroke-dasharray style. Built with D3. Forked (and tweaked) from Mike Bostock's original.

<!DOCTYPE html>
<html>
  <head>
    <meta http-equiv="Content-Type" content="text/html;charset=utf-8"/>
    <title>Point-Along-Path Interpolation</title>
    <script type="text/javascript" src="https://cdn.jsdelivr.net/npm/d3@2.4.2/d3.js"></script>
    <style type="text/css">

path {
  cursor: pointer;
  fill: #eee;
  stroke: #666;
  stroke-width: 1.5px;
}

circle {
  fill: steelblue;
  stroke: white;
  stroke-width: 1.5px;
}

</style>
  </head>
  <body>
    <script type="text/javascript">

var w = 960,
    h = 500;

var svg = d3.select("body").append("svg:svg")
    .attr("width", w)
    .attr("height", h)
  .append("svg:g")
    .attr("transform", "translate(" + w / 2 + "," + h / 2 + ")");

var path = svg.append("svg:path")
    .attr("d", d3.svg.arc()
    .innerRadius(h / 4)
    .outerRadius(h / 3)
    .startAngle(0)
    .endAngle(Math.PI));

var circle = svg.append("svg:circle")
    .attr("r", 6.5)
    .attr("transform", "translate(0," + -h / 3 + ")");

var draw = 0;

function transition() {
  circle.transition()
      .duration(5000)
      .attrTween("transform", translateAlong(path.node()));
  path.transition()
      .duration(5000)
      .styleTween("stroke-dasharray", strokeAlong(path.node()))
      .each("end", transition);
  draw = 1 - draw;
}

transition();

// Returns an attrTween for translating along the specified path element.
function translateAlong(path) {
  var l = path.getTotalLength();
  return function(d, i, a) {
    return function(t) {
      var p = path.getPointAtLength(t * l);
      return "translate(" + p.x + "," + p.y + ")";
    };
  };
}

// Returns an attrTween for stroking along the specified path element.
function strokeAlong(path) {
  var l = path.getTotalLength();
  return function(d, i, a) {
    return function(t) {
      var stroked = t * l, not = l - stroked;
      return (draw ? '' : '0,') + stroked + "," + not;
    };
  };
}

</script>
  </body>
</html>

Modified http://mbostock.github.com/d3/d3.js?2.4.6 to a secure url

https://mbostock.github.com/d3/d3.js?2.4.6