Add Gilbert’s Two-World Perspective.

geo/projection/README.md

@@ -159,6 +159,7 @@ Projections:
 * equirectangular - [Equirectangular (Plate Carrée)](http://bl.ocks.org/mbostock/3757119)
 * fahey - [Fahey](http://bl.ocks.org/mbostock/4731228)
 * gallStereographic - [Gall’s stereographic](http://bl.ocks.org/mbostock/5234763)
+* gilbert - Gilbert’s two-world perspective
 * ginzburg4 - [Ginzburg IV](http://bl.ocks.org/mbostock/5288565)
 * ginzburg5 - [Ginzburg V](http://bl.ocks.org/mbostock/5288571)
 * ginzburg6 - [Ginzburg VI](http://bl.ocks.org/mbostock/5288577)

geo/projection/gilbert.js

@@ -0,0 +1,36 @@
+import "projection";
+
+function gilbert(λ, φ) {
+  return d3.geo.orthographic.raw(λ * .5, asin(Math.tan(φ * .5)));
+}
+
+gilbert.invert = function(x, y) {
+  var coordinates = d3.geo.orthographic.raw.invert(x, y);
+  coordinates[0] *= 2;
+  coordinates[1] = 2 * Math.atan(Math.sin(coordinates[1]));
+};
+
+function gilbertProjection() {
+  var p = projection(gilbert),
+      e = d3.geo.equirectangular().scale(degrees).translate([0, 0]),
+      o = d3.geo.orthographic();
+
+  p.stream = function(stream) {
+    stream = o.stream(stream);
+    var s = e.stream({
+      point: function(λ, φ) {
+        stream.point(λ * .5, asin(Math.tan(-φ * .5 * radians)) * degrees);
+      },
+      lineStart: function() { stream.lineStart(); },
+      lineEnd: function() { stream.lineEnd(); },
+      polygonStart: function() { stream.polygonStart(); },
+      polygonEnd: function() { stream.polygonEnd(); }
+    });
+    s.sphere = function() { stream.sphere(); };
+    return s;
+  };
+
+  return d3.rebind(p, o, "rotate", "scale", "translate", "clipAngle", "precision");
+}
+
+(d3.geo.gilbert = gilbertProjection).raw = gilbert;

geo/projection/index.js

@@ -23,6 +23,7 @@ import "eckert6";
 import "eisenlohr";
 import "fahey";
 import "gall-stereographic";
+import "gilbert";
 import "ginzburg4";
 import "ginzburg5";
 import "ginzburg6";