jasondavies/index.html

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

path {
  fill: none;
  stroke: #000;
}

circle {
  fill: none;
  stroke: #000;
  stroke-width: 3px;
}

</style>
<body>
<script src="http://d3js.org/d3.v2.min.js"></script>
<script src="https://raw.github.com/d3/d3-plugins/clip/geo/clip/clip.js"></script>
<script src="resample.js"></script>
<script>

var width = 400,
    height = 400;

var origin = [0, 0],
    velocity = [-.01, .01],
    t0 = Date.now();

var projection = d3.geo.azimuthal()
    .translate([width / 2, height / 2])
    .mode("orthographic")
    .origin(origin)
    .scale(180);

var clip = d3.geo.clip()
    .origin(origin);

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

var feature = {
  type: "Polygon",
  coordinates: [
    [[-45, 45], [45, 45],   [45, -45], [-45, -45], [-45, 45]],
    [[-40, 40], [-40, -40], [-5, -40], [-5, 40],   [-40, 40]],
    [[5, 40],   [5, -40],   [40, -40], [40, 40],   [5, 40]]
  ].map(d3.geo.resample)
};

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

var b = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height)
  .append("path")
    .style("stroke", "blue")
    .datum(feature);

d3.timer(function() {
  var t = Date.now() - t0,
      o = [origin[0] + t * velocity[0], origin[1] + t * velocity[1]];
  projection.origin(o);
  clip.origin(o);
  a.attr("d", function(d) { return path(clip(d)); });
  b.attr("d", path);
});

</script>

## resample.js
d3.geo.resample = (function() {
  var arc = d3.geo.greatArc().target(function(d) { return d; });

  // Resample coordinates, creating great arcs between each.
  function resample(coordinates) {
    var i = 0,
        n = coordinates.length,
        j,
        m,
        resampled = n ? [coordinates[0]] : coordinates,
        resamples,
        origin = arc.source();

    while (++i < n) {
      resamples = arc.source(coordinates[i - 1])(coordinates[i]).coordinates;
      for (j = 0, m = resamples.length; ++j < m;) resampled.push(resamples[j]);
    }

    arc.source(origin);
    return resampled;
  }

  return resample;
})();
	<!DOCTYPE html>
	<meta charset="utf-8">
	<style>

	path {
	fill: none;
	stroke: #000;
	}

	circle {
	fill: none;
	stroke: #000;
	stroke-width: 3px;
	}

	</style>
	<body>
	<script src="http://d3js.org/d3.v2.min.js"></script>
	<script src="https://raw.github.com/d3/d3-plugins/clip/geo/clip/clip.js"></script>
	<script src="resample.js"></script>
	<script>

	var width = 400,
	height = 400;

	var origin = [0, 0],
	velocity = [-.01, .01],
	t0 = Date.now();

	var projection = d3.geo.azimuthal()
	.translate([width / 2, height / 2])
	.mode("orthographic")
	.origin(origin)
	.scale(180);

	var clip = d3.geo.clip()
	.origin(origin);

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

	var feature = {
	type: "Polygon",
	coordinates: [
	[[-45, 45], [45, 45], [45, -45], [-45, -45], [-45, 45]],
	[[-40, 40], [-40, -40], [-5, -40], [-5, 40], [-40, 40]],
	[[5, 40], [5, -40], [40, -40], [40, 40], [5, 40]]
	].map(d3.geo.resample)
	};

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

	var b = d3.select("body").append("svg")
	.attr("width", width)
	.attr("height", height)
	.append("path")
	.style("stroke", "blue")
	.datum(feature);

	d3.timer(function() {
	var t = Date.now() - t0,
	o = [origin[0] + t * velocity[0], origin[1] + t * velocity[1]];
	projection.origin(o);
	clip.origin(o);
	a.attr("d", function(d) { return path(clip(d)); });
	b.attr("d", path);
	});

	</script>
	d3.geo.resample = (function() {
	var arc = d3.geo.greatArc().target(function(d) { return d; });

	// Resample coordinates, creating great arcs between each.
	function resample(coordinates) {
	var i = 0,
	n = coordinates.length,
	j,
	m,
	resampled = n ? [coordinates[0]] : coordinates,
	resamples,
	origin = arc.source();

	while (++i < n) {
	resamples = arc.source(coordinates[i - 1])(coordinates[i]).coordinates;
	for (j = 0, m = resamples.length; ++j < m;) resampled.push(resamples[j]);
	}

	arc.source(origin);
	return resampled;
	}

	return resample;
	})();