@loaders.gl/mvt/src/lib/vector-tiler/proto-tile.ts

Loader for Mapbox Vector Tiles

// loaders.gl
// SPDX-License-Identifier: MIT
// Copyright (c) vis.gl contributors
// Forked from https://github.com/mapbox/geojson-vt under compatible ISC license

import type {ProtoFeature} from './features/proto-feature';

export type ProtoTile = {
  /** Processed features */
  protoFeatures: ProtoFeature[];
  /** if we slice further down, no need to keep source geometry */
  sourceFeatures: ProtoFeature[] | null;

  /** Properties */
  tags?: Record<string, string>;

  /** tile x coordinate */
  x: number;
  /** tile y coordinate */
  y: number;
  /** tile z coordinate */
  z: number;

  /** spatial extent */
  minX: number;
  /** spatial extent */
  maxX: number;
  /** spatial extent */
  minY: number;
  /** spatial extent */
  maxY: number;

  /** Whether this tile has been transformed */
  transformed: boolean;
  numPoints: number;
  numSimplified: number;
  /** Number of features in this tile */
  numFeatures: number;
};

export type CreateTileOptions = {
  maxZoom?: number;
  tolerance: number;
  extent: number;
  lineMetrics: boolean;
};

/**
 * Create a tile from features and tile index
 */
export function createProtoTile(
  features: ProtoFeature[],
  z,
  tx,
  ty,
  options: CreateTileOptions
): ProtoTile {
  const tolerance = z === options.maxZoom ? 0 : options.tolerance / ((1 << z) * options.extent);
  const tile: ProtoTile = {
    protoFeatures: [],
    sourceFeatures: null,
    numPoints: 0,
    numSimplified: 0,
    numFeatures: features.length,
    x: tx,
    y: ty,
    z,
    transformed: false,
    minX: 2,
    minY: 1,
    maxX: -1,
    maxY: 0
  };
  for (const feature of features) {
    addProtoFeature(tile, feature, tolerance, options);
  }
  return tile;
}

// eslint-disable-next-line complexity, max-statements
function addProtoFeature(
  tile: ProtoTile,
  feature: ProtoFeature,
  tolerance: number,
  options: CreateTileOptions
) {
  const geometry = feature.geometry;
  const type = feature.type;
  const simplifiedGeometry: number[] = [];

  tile.minX = Math.min(tile.minX, feature.minX);
  tile.minY = Math.min(tile.minY, feature.minY);
  tile.maxX = Math.max(tile.maxX, feature.maxX);
  tile.maxY = Math.max(tile.maxY, feature.maxY);

  let simplifiedType: 1 | 2 | 3;
  switch (type) {
    case 'Point':
    case 'MultiPoint':
      simplifiedType = 1;
      for (let i = 0; i < geometry.length; i += 3) {
        simplifiedGeometry.push(geometry[i], geometry[i + 1]);
        tile.numPoints++;
        tile.numSimplified++;
      }
      break;

    case 'LineString':
      simplifiedType = 2;
      addProtoLine(simplifiedGeometry, geometry, tile, tolerance, false, false);
      break;

    case 'MultiLineString':
      simplifiedType = 2;
      for (let i = 0; i < geometry.length; i++) {
        addProtoLine(simplifiedGeometry, geometry[i], tile, tolerance, false, i === 0);
      }
      break;

    case 'Polygon':
      simplifiedType = 3;
      for (let i = 0; i < geometry.length; i++) {
        addProtoLine(simplifiedGeometry, geometry[i], tile, tolerance, true, i === 0);
      }
      break;

    case 'MultiPolygon':
      simplifiedType = 3;
      for (let k = 0; k < geometry.length; k++) {
        const polygon = geometry[k];
        for (let i = 0; i < polygon.length; i++) {
          addProtoLine(simplifiedGeometry, polygon[i], tile, tolerance, true, i === 0);
        }
      }
      break;

    default:
      throw new Error(`Unknown geometry type: ${type}`);
  }

  if (simplifiedGeometry.length) {
    let tags: Record<string, unknown> | null = feature.tags || null;

    if (type === 'LineString' && options.lineMetrics) {
      tags = {};
      for (const key in feature.tags) {
        tags[key] = feature.tags[key];
      }
      // @ts-expect-error adding fields to arrays
      // eslint-disable-next-line camelcase
      tags.mapbox_clip_start = geometry.start / geometry.size;
      // @ts-expect-error adding fields to arrays
      // eslint-disable-next-line camelcase
      tags.mapbox_clip_end = geometry.end / geometry.size;
    }

    const tileFeature: ProtoFeature = {
      geometry: simplifiedGeometry,
      simplifiedType,
      // @ts-expect-error
      tags
    };
    if (feature.id !== null) {
      tileFeature.id = feature.id;
    }

    tile.protoFeatures.push(tileFeature);
  }
}

// eslint-disable-next-line max-params, max-statements
function addProtoLine(
  result: any[],
  geometry: any,
  tile: ProtoTile,
  tolerance: number,
  isPolygon: boolean,
  isOuter: boolean
): void {
  const sqTolerance = tolerance * tolerance;

  if (tolerance > 0 && geometry.size < (isPolygon ? sqTolerance : tolerance)) {
    tile.numPoints += geometry.length / 3;
    return;
  }

  const ring: number[] = [];

  for (let i = 0; i < geometry.length; i += 3) {
    if (tolerance === 0 || geometry[i + 2] > sqTolerance) {
      tile.numSimplified++;
      ring.push(geometry[i], geometry[i + 1]);
    }
    tile.numPoints++;
  }

  if (isPolygon) rewind(ring, isOuter);

  result.push(ring);
}

function rewind(ring: number[], clockwise?: boolean): void {
  let area = 0;
  for (let i = 0, j = ring.length - 2; i < ring.length; j = i, i += 2) {
    area += (ring[i] - ring[j]) * (ring[i + 1] + ring[j + 1]);
  }
  if (area > 0 === clockwise) {
    for (let i = 0, len = ring.length; i < len / 2; i += 2) {
      const x = ring[i];
      const y = ring[i + 1];
      ring[i] = ring[len - 2 - i];
      ring[i + 1] = ring[len - 1 - i];
      ring[len - 2 - i] = x;
      ring[len - 1 - i] = y;
    }
  }
}