Versor Dragging II

Inspired by ivyywang and Jason Davies. See the orthographic version.

<!DOCTYPE html>
<canvas width="960" height="600"></cavnas>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="https://unpkg.com/topojson-client@2"></script>
<script src="versor.js"></script>
<script>

var canvas = d3.select("canvas"),
    width = canvas.property("width"),
    height = canvas.property("height"),
    context = canvas.node().getContext("2d");

var projection = d3.geoMercator()
    .translate([width / 2, height / 2])
    .precision(0.1);

var path = d3.geoPath()
    .projection(projection)
    .context(context);

canvas.call(d3.drag()
    .on("start", dragstarted)
    .on("drag", dragged));

var render = function() {},
    v0, // Mouse position in Cartesian coordinates at start of drag gesture.
    r0, // Projection rotation as Euler angles at start.
    q0; // Projection rotation as versor at start.

function dragstarted() {
  v0 = versor.cartesian(projection.invert(d3.mouse(this)));
  r0 = projection.rotate();
  q0 = versor(r0);
}

function dragged() {
  var v1 = versor.cartesian(projection.rotate(r0).invert(d3.mouse(this))),
      q1 = versor.multiply(q0, versor.delta(v0, v1)),
      r1 = versor.rotation(q1);
  projection.rotate(r1);
  render();
}

d3.json("https://unpkg.com/world-atlas@1/world/110m.json", function(error, world) {
  if (error) throw error;

var land = topojson.feature(world, world.objects.land);

render = function() {
    context.fillStyle = "#fff", context.fillRect(0, 0, width, height);
    context.beginPath(), path(land), context.fillStyle = "#000", context.fill();
  };

render();
});

</script>

https://d3js.org/d3.v4.min.js

https://unpkg.com/topojson-client@2