77 lines
2 KiB
JavaScript
77 lines
2 KiB
JavaScript
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import {Adder} from "d3-array";
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import {atan2, cos, quarterPi, radians, sin, tau} from "./math.js";
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import noop from "./noop.js";
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import stream from "./stream.js";
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export var areaRingSum = new Adder();
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// hello?
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var areaSum = new Adder(),
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lambda00,
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phi00,
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lambda0,
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cosPhi0,
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sinPhi0;
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export var areaStream = {
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point: noop,
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lineStart: noop,
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lineEnd: noop,
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polygonStart: function() {
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areaRingSum = new Adder();
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areaStream.lineStart = areaRingStart;
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areaStream.lineEnd = areaRingEnd;
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},
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polygonEnd: function() {
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var areaRing = +areaRingSum;
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areaSum.add(areaRing < 0 ? tau + areaRing : areaRing);
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this.lineStart = this.lineEnd = this.point = noop;
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},
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sphere: function() {
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areaSum.add(tau);
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}
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};
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function areaRingStart() {
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areaStream.point = areaPointFirst;
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}
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function areaRingEnd() {
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areaPoint(lambda00, phi00);
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}
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function areaPointFirst(lambda, phi) {
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areaStream.point = areaPoint;
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lambda00 = lambda, phi00 = phi;
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lambda *= radians, phi *= radians;
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lambda0 = lambda, cosPhi0 = cos(phi = phi / 2 + quarterPi), sinPhi0 = sin(phi);
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}
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function areaPoint(lambda, phi) {
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lambda *= radians, phi *= radians;
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phi = phi / 2 + quarterPi; // half the angular distance from south pole
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// Spherical excess E for a spherical triangle with vertices: south pole,
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// previous point, current point. Uses a formula derived from Cagnoli’s
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// theorem. See Todhunter, Spherical Trig. (1871), Sec. 103, Eq. (2).
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var dLambda = lambda - lambda0,
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sdLambda = dLambda >= 0 ? 1 : -1,
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adLambda = sdLambda * dLambda,
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cosPhi = cos(phi),
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sinPhi = sin(phi),
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k = sinPhi0 * sinPhi,
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u = cosPhi0 * cosPhi + k * cos(adLambda),
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v = k * sdLambda * sin(adLambda);
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areaRingSum.add(atan2(v, u));
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// Advance the previous points.
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lambda0 = lambda, cosPhi0 = cosPhi, sinPhi0 = sinPhi;
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}
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export default function(object) {
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areaSum = new Adder();
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stream(object, areaStream);
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return areaSum * 2;
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}
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