itowns/lib/Parser/GTXParser.js

A JS/WebGL framework for 3D geospatial data visualization

import * as THREE from 'three';
import { Extent } from '@itowns/geographic';
import GeoidGrid from "../Core/Geographic/GeoidGrid.js";
export const BYTES_PER_DOUBLE = 8;
export const BYTES_PER_FLOAT = 4;

/**
 * The `GTXParser` module provides a `[parse]{@link module:GTXParser.parse}` method. This method takes the content of a
 * GTX file in, and returns a {@link GeoidGrid}. The {@link GeoidGrid} contains all the necessary attributes and
 * methods to access the GTX data in iTowns.
 *
 * @module GTXParser
 */
export default {
  /**
   * Parses a GTX file content and returns a corresponding {@link GeoidGrid}.
   *
   * @param   {ArrayBuffer}   gtx                             The content of the GTX file to parse.
   * @param   {Object}        options                         An object gathering the optional parameters to pass to
                                                              * the parser.
   * @param   {Object}        [options.in={}]                 Information on the GTX data.
   * @param   {string}        [options.in.crs='EPSG:4326']    The Coordinates Reference System (CRS) of the GTX data.
                                                              * It must be a geographic CRS, and must be given as an
                                                              * EPSG code.
   * @param   {string}        [options.in.dataType='float']   The encoding of geoid height data within the GTX file.
                                                              * Must be `'float'` or `'double'`.
   *
   * @returns {Promise<GeoidGrid>}    A promise resolving with a {@link GeoidGrid}, which contains all the necessary
                                      * attributes and methods to access GTX file data.
   */
  parse(gtx) {
    let options = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : {
      in: {}
    };
    const dataType = options.in.dataType || 'float';
    if (!['float', 'double'].includes(dataType)) {
      throw new Error('`dataType` parameter is incorrect for GTXParser.parse method. ' + 'This parameter must be either `double` or `float`.');
    }

    // ---------- GET METADATA FROM THE FILE : ----------

    const headerView = new DataView(gtx, 0, 40);
    const metadata = {
      minX: headerView.getFloat64(8),
      minY: headerView.getFloat64(0),
      stepX: headerView.getFloat64(24),
      stepY: headerView.getFloat64(16),
      nColumns: headerView.getInt32(36),
      nRows: headerView.getInt32(32)
    };

    // ---------- BUILD A DATA VIEWER : ----------

    const dataView = new DataView(gtx, 40);

    // ---------- CREATE A GeoidGrid FOR THE GIVEN FILE DATA : ----------

    // formula for the max longitude : maxLongitude = minLongitude + deltaLongitude * (nColumns - 1)
    const maxX = metadata.minX + metadata.stepX * (metadata.nColumns - 1);
    // formula for the max latitude : maxLatitude = minLatitude + deltaLatitude * (nRows - 1)
    const maxY = metadata.minY + metadata.stepY * (metadata.nRows - 1);
    const dataExtent = new Extent(options.in.crs || 'EPSG:4326', metadata.minX, maxX, metadata.minY, maxY);
    const dataStep = new THREE.Vector2(metadata.stepX, metadata.stepY);
    return Promise.resolve(new GeoidGrid(dataExtent, dataStep, (verticalIndex, horizontalIndex) => {
      // formula to get the index of a geoid height from a latitude and longitude indexes is :
      // ``(nColumns * latIndex + lonIndex) * nBytes``, where nBytes stands for the number of bytes geoid
      // height data are encoded on.
      if (dataType === 'float') {
        return dataView.getFloat32((metadata.nColumns * verticalIndex + horizontalIndex) * BYTES_PER_FLOAT);
      } else if (dataType === 'double') {
        return dataView.getFloat64((metadata.nColumns * verticalIndex + horizontalIndex) * BYTES_PER_DOUBLE);
      }
    }));
  }
};