Define a new "Sphere" GeoJSON type.

This allows us to generate the clip outline for whole-earth projections
with arbitrary rotations.

Internally, this is interpreted as a polygon with null coordinates.

d3.js

@@ -5366,6 +5366,9 @@
     Polygon: function(polygon) {
       this.polygon(polygon.coordinates);
     },
+    Sphere: function() {
+      this.polygon(null);
+    },
     object: function(object) {
       return d3_geo_typeObjects.hasOwnProperty(object.type) ? this[object.type](object) : this.geometry(object);
     },
@@ -5388,7 +5391,8 @@
     MultiPoint: 1,
     MultiPolygon: 1,
     Point: 1,
-    Polygon: 1
+    Polygon: 1,
+    Sphere: 1
   };
   var d3_geo_typeObjects = {
     Feature: 1,
@@ -5618,25 +5622,29 @@
   }
   function d3_geo_circleClipPolygon(coordinates, context, clipLine, interpolate) {
     var subject = [], clip = [], segments = [], buffer = d3_geo_circleBufferSegments(clipLine), draw = [], visibleArea = 0, invisibleArea = 0, invisible = false;
-    coordinates.forEach(function(ring) {
-      var x = buffer(ring, context), ringSegments = x[1], segment, n = ringSegments.length;
-      if (!n) {
-        invisible = true;
-        invisibleArea += x[0][0];
-        return;
-      }
-      if (x[0][0] !== false) {
-        visibleArea += x[0][0];
-        draw.push(segment = ringSegments[0]);
-        var point = segment[0], n = segment.length - 1, i = 0;
-        context.moveTo(point[0], point[1]);
-        while (++i < n) context.lineTo((point = segment[i])[0], point[1]);
-        context.closePath();
-        return;
-      }
-      if (n > 1 && x[0][1]) ringSegments.push(ringSegments.pop().concat(ringSegments.shift()));
-      segments = segments.concat(ringSegments.filter(d3_geo_circleSegmentLength1));
-    });
+    if (coordinates) {
+      coordinates.forEach(function(ring) {
+        var x = buffer(ring, context), ringSegments = x[1], segment, n = ringSegments.length;
+        if (!n) {
+          invisible = true;
+          invisibleArea += x[0][0];
+          return;
+        }
+        if (x[0][0] !== false) {
+          visibleArea += x[0][0];
+          draw.push(segment = ringSegments[0]);
+          var point = segment[0], n = segment.length - 1, i = 0;
+          context.moveTo(point[0], point[1]);
+          while (++i < n) context.lineTo((point = segment[i])[0], point[1]);
+          context.closePath();
+          return;
+        }
+        if (n > 1 && x[0][1]) ringSegments.push(ringSegments.pop().concat(ringSegments.shift()));
+        segments = segments.concat(ringSegments.filter(d3_geo_circleSegmentLength1));
+      });
+    } else {
+      visibleArea = -4 * π;
+    }
     if (!segments.length) {
       if (visibleArea < 0 || invisible && invisibleArea < 0) {
         var moved = false;

src/geo/circle.js

@@ -197,36 +197,41 @@ function d3_geo_circleClipPolygon(coordinates, context, clipLine, interpolate) {
       visibleArea = 0,
       invisibleArea = 0,
       invisible = false;
-  coordinates.forEach(function(ring) {
-    var x = buffer(ring, context),
-        ringSegments = x[1],
-        segment,
-        n = ringSegments.length;
 
-    if (!n) {
-      invisible = true;
-      invisibleArea += x[0][0];
-      return;
-    }
+  if (coordinates) {
+    coordinates.forEach(function(ring) {
+      var x = buffer(ring, context),
+          ringSegments = x[1],
+          segment,
+          n = ringSegments.length;
 
-    // No intersections.
-    if (x[0][0] !== false) {
-      visibleArea += x[0][0];
-      draw.push(segment = ringSegments[0]);
-      var point = segment[0],
-          n = segment.length - 1,
-          i = 0;
-      context.moveTo(point[0], point[1]);
-      while (++i < n) context.lineTo((point = segment[i])[0], point[1]);
-      context.closePath();
-      return;
-    }
+      if (!n) {
+        invisible = true;
+        invisibleArea += x[0][0];
+        return;
+      }
 
-    // Rejoin connected segments.
-    if (n > 1 && x[0][1]) ringSegments.push(ringSegments.pop().concat(ringSegments.shift()));
+      // No intersections.
+      if (x[0][0] !== false) {
+        visibleArea += x[0][0];
+        draw.push(segment = ringSegments[0]);
+        var point = segment[0],
+            n = segment.length - 1,
+            i = 0;
+        context.moveTo(point[0], point[1]);
+        while (++i < n) context.lineTo((point = segment[i])[0], point[1]);
+        context.closePath();
+        return;
+      }
 
-    segments = segments.concat(ringSegments.filter(d3_geo_circleSegmentLength1));
-  });
+      // Rejoin connected segments.
+      if (n > 1 && x[0][1]) ringSegments.push(ringSegments.pop().concat(ringSegments.shift()));
+
+      segments = segments.concat(ringSegments.filter(d3_geo_circleSegmentLength1));
+    });
+  } else {
+    visibleArea = -4 * π; // whole sphere
+  }
 
   if (!segments.length) {
     if (visibleArea < 0 || invisible && invisibleArea < 0) {

src/geo/type.js

@@ -50,6 +50,10 @@ var d3_geo_typeDefaults = {
     this.polygon(polygon.coordinates);
   },
 
+  Sphere: function() {
+    this.polygon(null);
+  },
+
   // dispatch for any GeoJSON object type
   object: function(object) {
     return d3_geo_typeObjects.hasOwnProperty(object.type)
@@ -86,7 +90,8 @@ var d3_geo_typeGeometries = {
   MultiPoint: 1,
   MultiPolygon: 1,
   Point: 1,
-  Polygon: 1
+  Polygon: 1,
+  Sphere: 1
 };
 
 var d3_geo_typeObjects = {

test/geo/path-test.js

@@ -418,6 +418,10 @@ suite.addBatch({
           }
         }
       },
+      "Sphere": function(path) {
+        path({type: "Sphere"});
+        assert.deepEqual(testContext.buffer().filter(function(d) { return d.type === "moveTo"; }), [{type: "moveTo", x: 87, y: 700}]);
+      }
     },
     "clipAngle(170)": {
       topic: function() {