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Better tile level of detail for alternate projections #11271

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Better tile level of detail for alternate projections #11271

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62 changes: 33 additions & 29 deletions src/geo/transform.js
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,7 @@

import LngLat from './lng_lat.js';
import LngLatBounds from './lng_lat_bounds.js';
import MercatorCoordinate, {mercatorXfromLng, mercatorYfromLat, mercatorZfromAltitude, latFromMercatorY, MAX_MERCATOR_LATITUDE} from './mercator_coordinate.js';
import MercatorCoordinate, {mercatorXfromLng, mercatorYfromLat, mercatorZfromAltitude, latFromMercatorY, lngFromMercatorX, MAX_MERCATOR_LATITUDE} from './mercator_coordinate.js';
import {getProjection} from './projection/index.js';
import tileTransform from '../geo/projection/tile_transform.js';
import Point from '@mapbox/point-geometry';
Expand All @@ -28,6 +28,17 @@ import type {FeatureDistanceData} from '../style-spec/feature_filter/index.js';
const NUM_WORLD_COPIES = 3;
const DEFAULT_MIN_ZOOM = 0;

const TILE_OUTLINE = (() => {
const n = 8;
const line = [];
for (let i = 0; i < n; i++) line.push([i / n, 0]);
for (let i = 0; i < n; i++) line.push([1, i / n]);
for (let i = 0; i < n; i++) line.push([1 - i / n, 1]);
for (let i = 0; i < n; i++) line.push([0, 1 - i / n]);
line.push([0, 0]);
return line;
})();

type RayIntersectionResult = { p0: vec4, p1: vec4, t: number};
type ElevationReference = "sea" | "ground";

Expand Down Expand Up @@ -726,31 +737,25 @@ class Transform {

const scaleAdjustment = getScaleAdjustment(this);

const relativeScaleAtMercatorCoord = mc => {
// Calculate how scale compares between projected coordinates and mercator coordinates.
// Returns a length. The units don't matter since the result is only
// used in a ratio with other values returned by this function.

// Construct a small square in Mercator coordinates.
const offset = 1 / 40000;
const mcEast = new MercatorCoordinate(mc.x + offset, mc.y, mc.z);
const mcSouth = new MercatorCoordinate(mc.x, mc.y + offset, mc.z);

// Convert the square to projected coordinates.
const ll = mc.toLngLat();
const llEast = mcEast.toLngLat();
const llSouth = mcSouth.toLngLat();
const p = this.locationCoordinate(ll);
const pEast = this.locationCoordinate(llEast);
const pSouth = this.locationCoordinate(llSouth);

// Calculate the size of each edge of the reprojected square
const dx = Math.hypot(pEast.x - p.x, pEast.y - p.y);
const dy = Math.hypot(pSouth.x - p.x, pSouth.y - p.y);

// Calculate the size of a projected square that would have the
// same area as the reprojected square.
return Math.sqrt(dx * dy) * scaleAdjustment / offset;
const relativeScaleOfTile = (it) => {
// Calculate how tile size compares between projected coordinates and mercator coordinates.
let area = 0;
let x = 0;
let y = 0;
const z2 = 1 << it.zoom;
const inc = Math.min(z2, 8); // use lower resolution outline on higher tiles

for (let i = 0; i < TILE_OUTLINE.length; i += inc) {
const [mx, my] = TILE_OUTLINE[i];
const lng = lngFromMercatorX((it.x + mx) / z2);
const lat = latFromMercatorY((it.y + my) / z2);
const {x: x1, y: y1} = this.projection.project(lng, lat);
area += (x * y1 - x1 * y) * z2 * z2 / 2;
x = x1;
y = y1;
}

return scaleAdjustment * Math.sqrt(area);
};

const aabbForTile = (z, x, y, wrap, min, max) => {
Expand Down Expand Up @@ -858,10 +863,9 @@ class Transform {
if (!isMercator && actualZ <= 5) {
// In other projections, not all tiles are the same size.
// Account for the tile size difference by adjusting the distToSplit.
// Adjust by the ratio of the area at the tile center to the area at the map center.
// Adjust by the ratio of the area of the projected tile to mercator tile.
// Adjustments are only needed at lower zooms where tiles are not similarly sized.
const numTiles = Math.pow(2, it.zoom);
const relativeScale = relativeScaleAtMercatorCoord(new MercatorCoordinate((it.x + 0.5) / numTiles, (it.y + 0.5) / numTiles));
const relativeScale = relativeScaleOfTile(it);
// Fudge the ratio slightly so that all tiles near the center have the same zoom level.
tileScaleAdjustment = relativeScale > 0.85 ? 1 : relativeScale;
}
Expand Down