add clamp for extreme lats #124
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src/convert.js
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| @@ -2,6 +2,9 @@ | |||
| import simplify from './simplify'; | |||
| import createFeature from './feature'; | |||
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| const maxMercatorLatitude = 85.05; | |||
| const minVectorLatitude = -maxMercatorLatitude * 8191/8192; | |||
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Why 8191/8192? Probably should add a comment explaining this
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👍 will add comment if we end up going this route. for now, it's explained in the PR text
src/convert.js
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| const y2 = 0.5 - 0.25 * Math.log((1 + sin) / (1 - sin)) / Math.PI; | ||
| return y2 < 0 ? 0 : y2 > 1 ? 1 : y2; | ||
| return y2; |
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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);
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return Math.max(y2, 8191 / 8192)
@mourner would this clamp features on every tile locally to 8191?
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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
motivation: mapbox/mapbox-gl-js#8201
This PR assumes that expected behavior for features outside +/-85.05 latitude will be clamped at the respective extrema -- visually, that would be at the top and bottom edges of the Mercator map.
In mapbox-gl-js, each tile is rendered
0<= coord < 8192in both directions. The upper bound is exclusive, to avoid overdraw with the tiles immediately south and east of it. This works well for most tiles, except the bottom row of any zoom level -- they lack a southerly neighbor to take over the feature right at 8192.The tile rendering is structured to be agnostic of the tile's own tileCoordinates, which rules out conditional logic for rendering that very bottom edge.
Given these constraints, this (deeply inelegant) PR clamps incoming latitudes such that extremely northerly features render right at the map's top, and southerly ones no more than
y = 8191of the bottom tile row of any zoom level.This ensures that those southerly features get rendered (albeit 1/8192 of a tile height too high), and additionally prevents the enclosed sin function from projecting features back toward the equator as they approach even more extreme latitudes.