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add clamp for extreme lats #124

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add clamp for extreme lats #124

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0fa52e5
add clamp for lats beyond +/-90
May 1, 2019
71ae3da
remove extraneous line
May 1, 2019
5c5f1e8
differentiate variables for clarity
May 1, 2019
4af468c
lint
May 1, 2019
10dfcfd
apply differential clamping
May 2, 2019
2405a96
lint
May 2, 2019
acba247
clamp latitude after projection
peterqliu Aug 6, 2019
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8 changes: 6 additions & 2 deletions src/convert.js
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,9 @@
import simplify from './simplify';
import createFeature from './feature';

const maxMercatorLatitude = 85.05;
const minVectorLatitude = -maxMercatorLatitude * 8191/8192;

@ryanhamley ryanhamley May 2, 2019

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Why 8191/8192? Probably should add a comment explaining this

@peterqliu peterqliu May 2, 2019

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👍 will add comment if we end up going this route. for now, it's explained in the PR text


// converts GeoJSON feature into an intermediate projected JSON vector format with simplification data

export default function convert(data, options) {
Expand Down Expand Up @@ -136,7 +139,8 @@ function projectX(x) {
}

function projectY(y) {
const sin = Math.sin(y * Math.PI / 180);
const clampedY = y < minVectorLatitude ? minVectorLatitude : y > maxMercatorLatitude ? maxMercatorLatitude : y;
const sin = Math.sin(clampedY * Math.PI / 180);
const y2 = 0.5 - 0.25 * Math.log((1 + sin) / (1 - sin)) / Math.PI;
return y2 < 0 ? 0 : y2 > 1 ? 1 : y2;
return y2;

@mourner mourner May 2, 2019 •
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Nit: If we do the 8191 / 8192 hack, I think it should follow after the projection (not before). Currently (looking at tests) the clamp is 1px off (3094 instead of 3095). E.g. we do return Math.max(y2, 8191 / 8192);

@peterqliu peterqliu May 2, 2019

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return Math.max(y2, 8191 / 8192)

@mourner would this clamp features on every tile locally to 8191?

@mourner mourner May 2, 2019

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@peterqliu no, since this is a global projection — it projects latitude into 0..1 range (0 is near the north pole, 1 is near the south one). Tile-aware projection happens further down the line

}