Fix for Bonne projection at 0° parallel.

denizk · Sep 27, 2011 · bbcaa27 · bbcaa27
1 parent 6a65451
commit bbcaa27
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Showing 5 changed files with 161 additions and 84 deletions.
diff --git a/d3.geo.js b/d3.geo.js
@@ -208,22 +208,24 @@ d3.geo.albersUsa = function() {
   return albersUsa.scale(lower48.scale());
 };
 d3.geo.bonne = function() {
-  var origin,
-      scale = 200,
+  var scale = 200,
       translate = [480, 250],
-      parallel, // 90 for Werner, 0 for Sinusoidal
-      p0,
-      ctp0,
-      x0,
-      y0;
+      x0, // origin longitude in radians
+      y0, // origin latitude in radians
+      y1, // parallel latitude in radians
+      c1; // cot(y1)
 
   function bonne(coordinates) {
-    var x1 = coordinates[0] * d3_geo_radians - x0,
-        y1 = coordinates[1] * d3_geo_radians - y0,
-        p = ctp0 + p0 - y1,
-        E = x1 * Math.cos(y1) / p,
-        x = p * Math.sin(E),
-        y = p * Math.cos(E) - ctp0;
+    var x = coordinates[0] * d3_geo_radians - x0,
+        y = coordinates[1] * d3_geo_radians - y0;
+    if (y1) {
+      var p = c1 + y1 - y, E = x * Math.cos(y) / p;
+      x = p * Math.sin(E);
+      y = p * Math.cos(E) - c1;
+    } else {
+      x *= Math.cos(y);
+      y *= -1;
+    }
     return [
       scale * x + translate[0],
       scale * y + translate[1]
@@ -232,28 +234,32 @@ d3.geo.bonne = function() {
 
   bonne.invert = function(coordinates) {
     var x = (coordinates[0] - translate[0]) / scale,
-        y = (coordinates[1] - translate[1]) / scale,
-        c = ctp0 + y,
-        p = Math.sqrt(x * x + c * c),
-        y1 = (ctp0 + p0 - p);
+        y = (coordinates[1] - translate[1]) / scale;
+    if (y1) {
+      var c = c1 + y, p = Math.sqrt(x * x + c * c);
+      y = c1 + y1 - p;
+      x = x0 + p * Math.atan2(x, c) / Math.cos(y);
+    } else {
+      y *= -1;
+      x /= Math.cos(y);
+    }
     return [
-      (x0 + p * Math.atan2(x, c) / Math.cos(y1)) / d3_geo_radians,
-      y1 / d3_geo_radians
+      x / d3_geo_radians,
+      y / d3_geo_radians
     ];
   };
 
+  // 90° for Werner, 0° for Sinusoidal
   bonne.parallel = function(x) {
-    if (!arguments.length) return parallel;
-    parallel = +x;
-    ctp0 = 1 / Math.tan(p0 = parallel * d3_geo_radians);
+    if (!arguments.length) return y1 / d3_geo_radians;
+    c1 = 1 / Math.tan(y1 = x * d3_geo_radians);
     return bonne;
   };
 
   bonne.origin = function(x) {
-    if (!arguments.length) return origin;
-    origin = x;
-    x0 = origin[0] * d3_geo_radians;
-    y0 = origin[1] * d3_geo_radians;
+    if (!arguments.length) return [x0 / d3_geo_radians, y0 / d3_geo_radians];
+    x0 = x[0] * d3_geo_radians;
+    y0 = x[1] * d3_geo_radians;
     return bonne;
   };
 
@@ -269,7 +275,7 @@ d3.geo.bonne = function() {
     return bonne;
   };
 
-  return bonne.origin([0, 0]).parallel(40);
+  return bonne.origin([0, 0]).parallel(45);
 };
 d3.geo.equirectangular = function() {
   var scale = 500,