satellite.js

import {geoProjectionMutator as projectionMutator} from "d3-geo";
import {acos, asin, atan2, cos, degrees, radians, sin, sqrt} from "./math.js";

function satelliteVerticalRaw(P) {
  function forward(lambda, phi) {
    var cosPhi = cos(phi),
        k = (P - 1) / (P - cosPhi * cos(lambda));
    return [
      k * cosPhi * sin(lambda),
      k * sin(phi)
    ];
  }

  forward.invert = function(x, y) {
    var rho2 = x * x + y * y,
        rho = sqrt(rho2),
        sinc = (P - sqrt(1 - rho2 * (P + 1) / (P - 1))) / ((P - 1) / rho + rho / (P - 1));
    return [
      atan2(x * sinc, rho * sqrt(1 - sinc * sinc)),
      rho ? asin(y * sinc / rho) : 0
    ];
  };

  return forward;
}

export function satelliteRaw(P, omega) {
  var vertical = satelliteVerticalRaw(P);
  if (!omega) return vertical;
  var cosOmega = cos(omega),
      sinOmega = sin(omega);

  function forward(lambda, phi) {
    var coordinates = vertical(lambda, phi),
        y = coordinates[1],
        A = y * sinOmega / (P - 1) + cosOmega;
    return [
      coordinates[0] * cosOmega / A,
      y / A
    ];
  }

  forward.invert = function(x, y) {
    var k = (P - 1) / (P - 1 - y * sinOmega);
    return vertical.invert(k * x, k * y * cosOmega);
  };

  return forward;
}

export default function() {
  var distance = 2,
      omega = 0,
      m = projectionMutator(satelliteRaw),
      p = m(distance, omega);

  // As a multiple of radius.
  p.distance = function(_) {
    if (!arguments.length) return distance;
    return m(distance = +_, omega);
  };

  p.tilt = function(_) {
    if (!arguments.length) return omega * degrees;
    return m(distance, omega = _ * radians);
  };

  return p
      .scale(432.147)
      .clipAngle(acos(1 / distance) * degrees - 1e-6);
}