mbostock/.block

## .block
license: gpl-3.0
redirect: https://observablehq.com/@d3/orthographic-to-equirectangular

## index.html
<!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>

var width = 960,
    height = 500;

var projection = interpolatedProjection(
    d3.geo.orthographic()
        .rotate([10, -10])
        .center([-10, 10])
        .scale(240)
        .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);

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 animation() {
  svg.transition()
      .duration(7500)
      .tween("projection", function() {
        return function(_) {
          projection.alpha(_);
          feature.attr("d", path);
        };
      })
    .transition()
      .duration(2500)
      .each("end", 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>

## thumbnail.png

      
    Raw
  

              thumbnail.png
	license: gpl-3.0
	redirect: https://observablehq.com/@d3/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>

	var width = 960,
	height = 500;

	var projection = interpolatedProjection(
	d3.geo.orthographic()
	.rotate([10, -10])
	.center([-10, 10])
	.scale(240)
	.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);

	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 animation() {
	svg.transition()
	.duration(7500)
	.tween("projection", function() {
	return function(_) {
	projection.alpha(_);
	feature.attr("d", path);
	};
	})
	.transition()
	.duration(2500)
	.each("end", 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>