Back-facing Hemisphere

Testing a prototype new graphics pipeline for d3.geo. The red lines are the front hemisphere, while the gray lines are the back hemisphere. Back-hemisphere clipping is achieved by first clipping around the antipode of the front hemisphere, and then applying an additional rotation.

<!DOCTYPE html>
<meta charset="utf-8">
<body>
<script src="d3.js"></script>
<script>

var width = 960,
    height = 960,
    scale = width / 2 - 4;

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

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

var graticule = d3.geo.graticule()();

d3.timer(function(elapsed) {
  var renderBack = d3.geo.pipeline()
      .source(d3.geo.jsonSource)
      .pipe(d3.geo.rotate, elapsed * .0002 + Math.PI, -elapsed * .0001, 0)
      .pipe(d3.geo.clipCircle, Math.PI / 2)
      .pipe(d3.geo.rotate, Math.PI, 0, 0)
      .pipe(d3.geo.project, d3.geo.orthographic, .3 / scale)
      .sink(d3.geom.contextSink, context)

var renderFront = d3.geo.pipeline()
      .source(d3.geo.jsonSource)
      .pipe(d3.geo.rotate, elapsed * .0002, elapsed * .0001, 0)
      .pipe(d3.geo.clipCircle, Math.PI / 2)
      .pipe(d3.geo.project, d3.geo.orthographic, .3 / scale)
      .sink(d3.geom.contextSink, context);

context.clearRect(0, 0, width, height);
  context.save();
  context.translate(width / 2, height / 2);
  context.scale(scale, -scale);

context.beginPath();
  renderBack(graticule);
  context.lineWidth = 1 / scale;
  context.strokeStyle = "#999";
  context.stroke();

context.beginPath();
  renderFront(graticule);
  context.lineWidth = 1.5 / scale;
  context.strokeStyle = "red";
  context.stroke();

context.restore();
});

d3.select(self.frameElement).style("height", height + "px");

</script>