Selecting Countries on a Canvas Globe by syntagmatic

index.html

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

body {
  background: #fcfcfa;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script>

var width = 960,
    height = 600,
    speed = 1e-2,
    start = Date.now();

var sphere = {type: "Sphere"};

var projection = d3.geo.orthographic()
    .scale(height / 2.1)
    .translate([width / 2, height / 2])
    .clipAngle(90)
    .precision(.5);

var graticule = d3.geo.graticule();

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(projection)
    .context(context);

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

var hiddenContext = hiddenCanvas.node().getContext("2d");

var hiddenProjection = d3.geo.equirectangular()
    .translate([width / 2, height / 2])
    .scale(width / 7);

var hiddenPath = d3.geo.path()
    .projection(hiddenProjection)
    .context(hiddenContext);

d3.json("world-110m.json", function(error, topo) {
  var selected = false;

  var land = topojson.feature(topo, topo.objects.land),
      borders = topojson.feature(topo, topo.objects.countries),
      grid = graticule();

  var fillToCountry = {};
  var i = borders.features.length;
  while (i--) {
    hiddenContext.beginPath();
    hiddenPath(borders.features[i]);
    hiddenContext.fillStyle = "rgb(" + i + ",0,0)";
    hiddenContext.fill();
  }

  d3.timer(function() {
    projection.rotate([speed * (Date.now() - start), -15]);

    context.clearRect(0, 0, width, height);

    context.beginPath();
    path(sphere);
    context.lineWidth = 3;
    context.strokeStyle = "#000";
    context.stroke();

    context.beginPath();
    path(sphere);
    context.fillStyle = "#fff";
    context.fill();

    context.beginPath();
    path(land);
    context.fillStyle = "#222";
    context.fill();

    context.beginPath();
    path(borders);
    context.lineWidth = .5;
    context.strokeStyle = "#fff";
    context.stroke();

    context.beginPath();
    path(grid);
    context.lineWidth = .5;
    context.strokeStyle = "rgba(119,119,119,.5)";
    context.stroke();

    if (selected !== false) {
      context.beginPath();
      path(borders.features[selected]);
      context.fillStyle = "#0ad";
      context.fill();
    }
  });

  canvas
    .on("mousemove", select)
    .on("touchstart", select);

  function select() {
    var pos = d3.mouse(this);
    var latlong = projection.invert(pos);
    var hiddenPos = hiddenProjection(latlong);
    if (hiddenPos[0] > -1) {
      var p = hiddenContext.getImageData(hiddenPos[0], hiddenPos[1], 1, 1).data;
      selected = p[0];
      context.beginPath();
      path(borders.features[selected]);
      context.fillStyle = "#0ad";
      context.fill();
    } else {
      selected = false;
    }
  };
});

d3.select(self.frameElement).style("height", (2*height) + "px");

</script>