112 lines
2.5 KiB
JavaScript
112 lines
2.5 KiB
JavaScript
/// S2 Geometry functions
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// the regional scoreboard is based on a level 6 S2 Cell
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// - https://docs.google.com/presentation/d/1Hl4KapfAENAOf4gv-pSngKwvS_jwNVHRPZTTDzXXn6Q/view?pli=1#slide=id.i22
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// at the time of writing there's no actual API for the intel map to retrieve scoreboard data,
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// but it's still useful to plot the score cells on the intel map
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// the S2 geometry is based on projecting the earth sphere onto a cube, with some scaling of face coordinates to
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// keep things close to approximate equal area for adjacent cells
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// to convert a lat,lng into a cell id:
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// - convert lat,lng to x,y,z
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// - convert x,y,z into face,u,v
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// - u,v scaled to s,t (i,j?) with quadratic formula
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// - s,t (i,j?) converted to a position along a Hubbert space-filling curve
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// - combine face,position to get the cell id
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(function() {
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window.S2 = {};
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S2.LatLngToXYZ = function(latLng) {
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var d2r = L.LatLng.DEG_TO_RAD;
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var phi = latLng.lat*d2r;
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var theta = latLng.lng*d2r;
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var cosphi = Math.cos(phi);
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return [Math.cos(theta)*cosphi, Math.sin(theta)*cosphi, Math.sin(phi)];
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};
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var largestAbsComponent = function(xyz) {
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var temp = [Math.abs(xyz[0]), Math.abs(xyz[1]), Math.abs(xyz[2])];
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if (temp[0] > temp[1]) {
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if (temp[0] > temp[2]) {
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return 0;
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} else {
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return 1;
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}
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} else {
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if (temp[1] > temp[2]) {
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return 1;
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} else {
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return 2;
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}
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}
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};
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var faceXYZToUV = function(face,xyz) {
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var u,v;
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switch (face) {
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case 0: u = xyz[1]/xyz[0]; v = xyz[2]/xyz[0]; break;
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case 1: u = -xyz[0]/xyz[1]; v = xyz[2]/xyz[1]; break;
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case 2: u = -xyz[0]/xyz[2]; v = -xyz[1]/xyz[2]; break;
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case 3: u = xyz[2]/xyz[0]; v = xyz[1]/xyz[0]; break;
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case 4: u = xyz[2]/xyz[1]; v = -xyz[0]/xyz[1]; break;
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case 5: u = -xyz[1]/xyz[2]; v = -xyz[0]/xyz[2]; break;
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default: throw {error: 'Invalid face'}; break;
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}
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return [u,v];
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}
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var singleSTtoUV = function(st) {
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if (st >= 0.5) {
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return (1/3.0) * (4*st*st - 1);
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} else {
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return (1/3.0) * (1 - (4*(1-st)*(1-st)));
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}
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}
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var singleUVtoST = function(uv) {
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if (uv >= 0) {
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return 0.5 * Math.sqrt (1 + 3*uv);
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} else {
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return 1 - 0.5 * Math.sqrt (1 - 3*uv);
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}
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}
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S2.XYZToFaceUV = function(xyz) {
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var face = largestAbsComponent(xyz);
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if (xyz[face] < 0) {
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face += 3;
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}
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uv = faceXYZToUV (face,xyz);
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return [face, uv];
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};
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S2.UVToST = function(uv) {
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return [singleUVToST(uv[0]), singleUVToST(uv[1])];
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};
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var singleSTtoIJ = function(st) {
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var ij = Math.floor(st * kMaxSize);
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return Math.max(0, Math.min(kMaxSize-1, ij));
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};
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})();
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