Dynamic Simplification by mbostock

This example demonstrates dynamic topology-preserving shape simplification using topojson.presimplify, a new feature in TopoJSON 1.3. The presimplify method computes the importance of each point using a configurable importance function; the default implementation uses Cartesian triangular area for Visvalingam simplification. Then, a custom geometry stream (d3.geo.transform) uses the previously-computed point importance to filter the geometry during rendering.

index.html

<!DOCTYPE html>
<meta charset="utf-8">
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script>

var width = 960,
    height = 500;

var area = 1, simplify = d3.geo.transform({
  point: function(x, y, z) {
    if (z >= area) this.stream.point(x, y);
  }
});

var canvas = d3.select("body").append("canvas")
    .attr("width", width)
    .attr("height", height);

var context = canvas.node().getContext("2d");

var path = d3.geo.path()
    .projection(simplify)
    .context(context);

d3.json("us.json", function(error, topo) {
  if (error) throw error;

  canvas
      .datum(topojson.mesh(topojson.presimplify(topo)))
    .transition()
      .duration(2500)
      .each(resimplify);
});

function render() {
  context.clearRect(0, 0, width, height);
  context.beginPath();
  canvas.each(path);
  context.stroke();
}

function resimplify() {
  var t = d3.select(this);
  (function repeat() {
    t = t.transition()
        .tween("area", areaTween(20))
      .transition()
        .tween("area", areaTween(.5))
        .each("end", repeat);
  })();
}

function areaTween(area1) {
  return function(d) {
    var i = d3.interpolate(area, area1);
    return function(t) {
      area = i(t);
      render();
    };
  };
}

</script>