One-Way Markers by mbostock

Inspired by Tom MacWright’s previous take on one-way line markers, this example adopts a technique from my path tween example: uniform sampling via getPointAtLength. A rendered line is resampled such that each control point is equally spaced along the length of the line. Thus, the desired number of markers are rendered.

index.html

<!DOCTYPE html>
<meta charset="utf-8">
<style>

#base-path {
  fill: none;
  stroke: #000;
  stroke-width: 1.5px;
}

#marker-path {
  fill: none;
  stroke: none;
  stroke-width: 1.5px;
  marker-mid: url(#marker);
}

</style>
<svg width="960" height="500">
  <marker id="marker"
      viewBox="0 0 6 6"
      refY="3"
      markerWidth="7"
      markerHeight="7"
      orient="auto">
    <path d="M0,3v-3l6,3l-6,3z"></path>
  </marker>
  <path id="base-path"></path>
  <path id="marker-path"></path>
</svg>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var z = 0.5;

var svg = d3.select("svg"),
    width = +svg.attr("width"),
    height = +svg.attr("height");

var x = d3.scale.linear()
    .domain([-1.3, 1.3])
    .range([0, width]);

var y = d3.scale.linear()
    .domain([-1.3, 1.3])
    .range([0, height]);

var line = d3.svg.line()
    .interpolate("basis");

var basePath = d3.select("#base-path"),
    markerPath = d3.select("#marker-path");

d3.timer(function() {
  if ((z += 0.001) > 2) z = 0.5;
  var path = line(d3.range(0, 12 + .1, .2).map(function(t) { return [x(Math.cos(t * z)), y(Math.sin(t))]; }));
  basePath.attr("d", path);
  markerPath.attr("d", path).attr("d", resample(20));
});

// Uniform sampling based on specified pixel distance.
function resample(dx) {
  return function() {
    var path = this,
        l = path.getTotalLength(),
        t = [0], i = 0, dt = dx / l;
    while ((i += dt) < 1) t.push(i);
    t.push(1);
    return line(t.map(function(t) {
      var p = path.getPointAtLength(t * l);
      return [p.x, p.y];
    }));
  };
}

</script>