1 d3.geo.greatArc = function() {

     2   var source = d3_geo_greatArcSource,

     3       target = d3_geo_greatArcTarget,

     4       precision = 6 * d3_geo_radians;

     5 

     6   function greatArc() {

     7     var a = typeof source === "function" ? source.apply(this, arguments) : source,

     8         b = typeof target === "function" ? target.apply(this, arguments) : target,

     9         i = d3_geo_greatArcInterpolate(a, b),

    10         dt = precision / i.d,

    11         t = 0,

    12         coordinates = [a];

    13     while ((t += dt) < 1) coordinates.push(i(t));

    14     coordinates.push(b);

    15     return {

    16       type: "LineString",

    17       coordinates: coordinates

    18     };

    19   }

    20 

    21   // Length returned in radians; multiply by radius for distance.

    22   greatArc.distance = function() {

    23     var a = typeof source === "function" ? source.apply(this, arguments) : source,

    24         b = typeof target === "function" ? target.apply(this, arguments) : target;

    25      return d3_geo_greatArcInterpolate(a, b).d;

    26   };

    27 

    28   greatArc.source = function(x) {

    29     if (!arguments.length) return source;

    30     source = x;

    31     return greatArc;

    32   };

    33 

    34   greatArc.target = function(x) {

    35     if (!arguments.length) return target;

    36     target = x;

    37     return greatArc;

    38   };

    39 

    40   // Precision is specified in degrees.

    41   greatArc.precision = function(x) {

    42     if (!arguments.length) return precision / d3_geo_radians;

    43     precision = x * d3_geo_radians;

    44     return greatArc;

    45   };

    46 

    47   return greatArc;

    48 };

    49 

    50 function d3_geo_greatArcSource(d) {

    51   return d.source;

    52 }

    53 

    54 function d3_geo_greatArcTarget(d) {

    55   return d.target;

    56 }

    57 

    58 function d3_geo_greatArcInterpolate(a, b) {

    59   var x0 = a[0] * d3_geo_radians, cx0 = Math.cos(x0), sx0 = Math.sin(x0),

    60       y0 = a[1] * d3_geo_radians, cy0 = Math.cos(y0), sy0 = Math.sin(y0),

    61       x1 = b[0] * d3_geo_radians, cx1 = Math.cos(x1), sx1 = Math.sin(x1),

    62       y1 = b[1] * d3_geo_radians, cy1 = Math.cos(y1), sy1 = Math.sin(y1),

    63       d = interpolate.d = Math.acos(Math.max(-1, Math.min(1, sy0 * sy1 + cy0 * cy1 * Math.cos(x1 - x0)))),

    64       sd = Math.sin(d);

    65 

    66   // From http://williams.best.vwh.net/avform.htm#Intermediate

    67   function interpolate(t) {

    68     var A = Math.sin(d - (t *= d)) / sd,

    69         B = Math.sin(t) / sd,

    70         x = A * cy0 * cx0 + B * cy1 * cx1,

    71         y = A * cy0 * sx0 + B * cy1 * sx1,

    72         z = A * sy0       + B * sy1;

    73     return [

    74       Math.atan2(y, x) / d3_geo_radians,

    75       Math.atan2(z, Math.sqrt(x * x + y * y)) / d3_geo_radians

    76     ];

    77   }

    78 

    79   return interpolate;

    80 }