自定义 LERC 图层

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const LercLayer = BaseTileLayer.createSubclass({
  properties: {
    urlTemplate: null,
    minElevation: 0,
    maxElevation: 4000
  }

  //override required methods
});

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// fetch tiles visible in the view
fetchTile: function (level, row, col, options) {
  const url = this.getTileUrl(level, row, col);

  // requested encoded elevation information
  // the signal option ensures that obsolete requests are aborted
  return geosceneRequest(url, {
    responseType: "array-buffer",
    signal: options && options.signal
  })
  .then((response) => {
    // create a canvas to draw the processed image
    const canvas = document.createElement("canvas");
    const context = canvas.getContext("2d");
    const width = this.tileInfo.size[0];
    const height = this.tileInfo.size[1];

    canvas.width = width;
    canvas.height = height;

    // uncompress raw elevation values in lerc format
    // into a pre-allocated array of elevation values.
    const lerc = LercDecode.decode(response.data, { noDataValue: 0 });

    // Array of elevation values width*height.
    const pixels = lerc.pixels[0];

    // stats for min, max and no data
    // values for uncompressed elevation
    const stats = lerc.statistics[0];

    // set the min and max elevation values set
    // by the layer
    const min = this.minElevation;
    const max = this.maxElevation;
    const noDataValue = stats.noDataValue;

    // Create a new blank image data object with the specified
    // dimensions.
    const imageData = context.createImageData(width, height);

    // get Uint8ClampedArray representing a one-dimensional array
    // containing the data in the RGBA order, with integer values
    // between 0 and 255 (included).
    const data = imageData.data;

    const factor = 256 / (max - min);
    let value = 0;
    let j;

    // Loop through elevation array from lerc to generate an
    // image that will be displayed.
    // `pixels` is a flat array of color values and alpha
    // [r, g, b, a, r, g, b, a, ...]
    // We need to iterate through elevation and assign color
    // and alpha values respectively.
    for (let i = 0; i < width * height; i++) {
      // map tile size is 256x256. Elevation values have a
      // tile size of 257 so we skip the last elevation
      // whenever "i" is incremented by 256 to jump to the next row.
      j = i + Math.floor(i / width);
      // read the elevation value at the given index from the elevation
      // array and multiply it by the factor. This will define
      // the shade of yellow color for the pixel.
      value = (pixels[j] - min) * factor;

      // create RGBA value for the pixel
      data[i * 4] = value;
      data[i * 4 + 1] = value;
      data[i * 4 + 2] = 0;
      data[i * 4 + 3] = pixels[i] === noDataValue ? 0 : value;
    }

    // The elevation change image and ready for display
    context.putImageData(imageData, 0, 0);

    return canvas;
  });
}