Shape Tweening

See also the linear path tweening example.

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

path {
  fill: #ccc;
  stroke: #000;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var width = 960,
    height = 500;

var projection = d3.geo.albers()
    .rotate([120, 0])
    .center([0, 37.7])
    .scale(2700);

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

d3.json("california.json", function(polygon) {
  var coordinates0 = polygon.coordinates[0].map(projection),
      coordinates1 = circle(coordinates0),
      path = svg.append("path"),
      d0 = "M" + coordinates0.join("L") + "Z",
      d1 = "M" + coordinates1.join("L") + "Z";

loop();

function loop() {
    path
        .attr("d", d0)
      .transition()
        .duration(5000)
        .attr("d", d1)
      .transition()
        .delay(5000)
        .attr("d", d0)
        .each("end", loop);
  }
});

function circle(coordinates) {
  var circle = [],
      length = 0,
      lengths = [length],
      polygon = d3.geom.polygon(coordinates),
      p0 = coordinates[0],
      p1,
      x,
      y,
      i = 0,
      n = coordinates.length;

// Compute the distances of each coordinate.
  while (++i < n) {
    p1 = coordinates[i];
    x = p1[0] - p0[0];
    y = p1[1] - p0[1];
    lengths.push(length += Math.sqrt(x * x + y * y));
    p0 = p1;
  }

var area = polygon.area(),
      radius = Math.sqrt(Math.abs(area) / Math.PI),
      centroid = polygon.centroid(-1 / (6 * area)),
      angleOffset = -Math.PI / 2, // TODO compute automatically
      angle,
      i = -1,
      k = 2 * Math.PI / lengths[lengths.length - 1];

// Compute points along the circle’s circumference at equivalent distances.
  while (++i < n) {
    angle = angleOffset + lengths[i] * k;
    circle.push([
      centroid[0] + radius * Math.cos(angle),
      centroid[1] + radius * Math.sin(angle)
    ]);
  }

return circle;
}

</script>

https://d3js.org/d3.v3.min.js