Animate Projection Change

forked from mbostock's block: Orthographic to Equirectangular

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

path {
  fill: none;
  stroke-linejoin: round;
}

.sphere,
.graticule {
  stroke: #aaa;
}

.equator {
  stroke: red;
  stroke-width: 2px;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/topojson.v2.min.js"></script>
<script>

var width = 960,
    height = 500;

var projection = interpolatedProjection(
    d3.geo.orthographic()
        //.rotate([10, -10])
        .center([10, -2])
        .scale(220)
        .translate([width / 2, height / 2]),
    d3.geo.equirectangular()
        .scale(145)
        .translate([width / 2, height / 2]));

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

var graticule = d3.geo.graticule();

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

svg.append("path")
    .datum({type: "Sphere"})
    .attr("class", "sphere")
    .attr("d", path);

svg.append("path")
    .datum(graticule)
    .attr("class", "graticule")
    .attr("d", path);

d3.json("world-110m.json", function(error, world) {
  if (error) throw error;
  var countries = topojson.feature(world, world.objects.countries).features;
    // show countries and colour them on a scale
  svg.selectAll(".country")
      .data(countries)
      .enter().insert("path", ".graticule")
      .attr("class", "country")
      .attr("d", path)
});
  
svg.append("path")
    .datum({type: "LineString", coordinates: [[-180, 0], [-90, 0], [0, 0], [90, 0], [180, 0]]})
    .attr("class", "equator")
    .attr("d", path);

var feature = svg.selectAll("path");

animation();

function interpolatedProjection(a, b) {
  var projection = d3.geo.projection(raw).scale(1),
      center = projection.center,
      translate = projection.translate,
      α;

function raw(λ, φ) {
    var pa = a([λ *= 180 / Math.PI, φ *= 180 / Math.PI]), pb = b([λ, φ]);
    return [(1 - α) * pa[0] + α * pb[0], (α - 1) * pa[1] - α * pb[1]];
  }

projection.alpha = function(_) {
    if (!arguments.length) return α;
    α = +_;
    var ca = a.center(), cb = b.center(),
        ta = a.translate(), tb = b.translate();
    center([(1 - α) * ca[0] + α * cb[0], (1 - α) * ca[1] + α * cb[1]]);
    translate([(1 - α) * ta[0] + α * tb[0], (1 - α) * ta[1] + α * tb[1]]);
    return projection;
  };

delete projection.scale;
  delete projection.translate;
  delete projection.center;
  return projection.alpha(0);
}

</script>

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

https://d3js.org/topojson.v2.min.js