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Filename : albers.js
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/**
 * echarts地图投射算法
 *
 * @desc echarts基于Canvas，纯Javascript图表库，提供直观，生动，可交互，可个性化定制的数据统计图表。
 * @author Kener (@Kener-林峰, kener.linfeng@gmail.com)
 *
 */
define(function() {
    // Derived from Tom Carden's Albers implementation for Protovis.
    // http://gist.github.com/476238
    // http://mathworld.wolfram.com/AlbersEqual-AreaConicProjection.html
    function _albers() {
        var radians = Math.PI / 180;
        var origin = [0, 0];            //[-98, 38],
        var parallels = [29.5, 45.5];
        var scale = 1000;
        var translate = [0, 0];         //[480, 250],
        var lng0;                       // radians * origin[0]
        var n;
        var C;
        var p0;
        
        function albers(coordinates) {
            var t = n * (radians * coordinates[0] - lng0);
            var p = Math.sqrt(
                        C - 2 * n * Math.sin(radians * coordinates[1])
                    ) / n;
            return [
                scale * p * Math.sin(t) + translate[0],
                scale * (p * Math.cos(t) - p0) + translate[1]
            ];
        }

        albers.invert = function (coordinates) {
            var x = (coordinates[0] - translate[0]) / scale;
            var y = (coordinates[1] - translate[1]) / scale;
            var p0y = p0 + y;
            var t = Math.atan2(x, p0y);
            var p = Math.sqrt(x * x + p0y * p0y);
            return [
                (lng0 + t / n) / radians,
                Math.asin((C - p * p * n * n) / (2 * n)) / radians
            ];
        };

        function reload() {
            var phi1 = radians * parallels[0];
            var phi2 = radians * parallels[1];
            var lat0 = radians * origin[1];
            var s = Math.sin(phi1);
            var c = Math.cos(phi1);
            lng0 = radians * origin[0];
            n = 0.5 * (s + Math.sin(phi2));
            C = c * c + 2 * n * s;
            p0 = Math.sqrt(C - 2 * n * Math.sin(lat0)) / n;
            return albers;
        }

        albers.origin = function (x) {
            if (!arguments.length) {
                return origin;
            }
            origin = [+x[0], +x[1]];
            return reload();
        };

        albers.parallels = function (x) {
            if (!arguments.length) {
                return parallels;
            }
            parallels = [+x[0], +x[1]];
            return reload();
        };

        albers.scale = function (x) {
            if (!arguments.length) {
                return scale;
            }
            scale = +x;
            return albers;
        };

        albers.translate = function (x) {
            if (!arguments.length) {
                return translate;
            }
            translate = [+x[0], +x[1]];
            return albers;
        };

        return reload();
    }
    
    return _albers;
});