晕渲地貌【译】

原文链接及内容

原文链接：https://openlayers.org/en/latest/examples/shaded-relief.html

本例使用ol/source/Raster生成数据。栅格数据源接受任意数量的输入数据源(基于切片或图像)，并在输入数据上运行一个操作管道，最终操作的返回值用作输出数据源的数据。
在本例中，使用单个平铺高程数据源作为输入。晕渲地貌是在一个单一的“图像”操作中计算的。通过在栅格数据源上设置operationType: 'image'，对每个输入数据源使用ImageData对象调用该操作，也可以使用通用data对象调用操作。在这个例子中，来自上述输入的太阳仰角和方位角数据被分配给这个数据对象，并在着色操作中访问。着色操作返回一个ImageData对象数组。当栅格数据源被图像层使用时，将渲染管道中最后一个操作返回的第一个ImageData对象。

main.js代码如下：

import Map from 'ol/Map.js';
import View from 'ol/View.js';
import {Image as ImageLayer, Tile as TileLayer} from 'ol/layer.js';
import {OSM, Raster, XYZ} from 'ol/source.js';

/**
 * Generates a shaded relief image given elevation data.  Uses a 3x3
 * neighborhood for determining slope and aspect.
 * @param {Array<ImageData>} inputs Array of input images.
 * @param {Object} data Data added in the "beforeoperations" event.
 * @return {ImageData} Output image.
 */
function shade(inputs, data) {
  const elevationImage = inputs[0];
  const width = elevationImage.width;
  const height = elevationImage.height;
  const elevationData = elevationImage.data;
  const shadeData = new Uint8ClampedArray(elevationData.length);
  const dp = data.resolution * 2;
  const maxX = width - 1;
  const maxY = height - 1;
  const pixel = [0, 0, 0, 0];
  const twoPi = 2 * Math.PI;
  const halfPi = Math.PI / 2;
  const sunEl = (Math.PI * data.sunEl) / 180;
  const sunAz = (Math.PI * data.sunAz) / 180;
  const cosSunEl = Math.cos(sunEl);
  const sinSunEl = Math.sin(sunEl);
  let pixelX,
    pixelY,
    x0,
    x1,
    y0,
    y1,
    offset,
    z0,
    z1,
    dzdx,
    dzdy,
    slope,
    aspect,
    cosIncidence,
    scaled;
  function calculateElevation(pixel) {
    // The method used to extract elevations from the DEM.
    // In this case the format used is
    // red + green * 2 + blue * 3
    //
    // Other frequently used methods include the Mapbox format
    // (red * 256 * 256 + green * 256 + blue) * 0.1 - 10000
    // and the Terrarium format
    // (red * 256 + green + blue / 256) - 32768
    //
    return pixel[0] + pixel[1] * 2 + pixel[2] * 3;
  }
  for (pixelY = 0; pixelY <= maxY; ++pixelY) {
    y0 = pixelY === 0 ? 0 : pixelY - 1;
    y1 = pixelY === maxY ? maxY : pixelY + 1;
    for (pixelX = 0; pixelX <= maxX; ++pixelX) {
      x0 = pixelX === 0 ? 0 : pixelX - 1;
      x1 = pixelX === maxX ? maxX : pixelX + 1;

      // determine elevation for (x0, pixelY)
      offset = (pixelY * width + x0) * 4;
      pixel[0] = elevationData[offset];
      pixel[1] = elevationData[offset + 1];
      pixel[2] = elevationData[offset + 2];
      pixel[3] = elevationData[offset + 3];
      z0 = data.vert * calculateElevation(pixel);

      // determine elevation for (x1, pixelY)
      offset = (pixelY * width + x1) * 4;
      pixel[0] = elevationData[offset];
      pixel[1] = elevationData[offset + 1];
      pixel[2] = elevationData[offset + 2];
      pixel[3] = elevationData[offset + 3];
      z1 = data.vert * calculateElevation(pixel);

      dzdx = (z1 - z0) / dp;

      // determine elevation for (pixelX, y0)
      offset = (y0 * width + pixelX) * 4;
      pixel[0] = elevationData[offset];
      pixel[1] = elevationData[offset + 1];
      pixel[2] = elevationData[offset + 2];
      pixel[3] = elevationData[offset + 3];
      z0 = data.vert * calculateElevation(pixel);

      // determine elevation for (pixelX, y1)
      offset = (y1 * width + pixelX) * 4;
      pixel[0] = elevationData[offset];
      pixel[1] = elevationData[offset + 1];
      pixel[2] = elevationData[offset + 2];
      pixel[3] = elevationData[offset + 3];
      z1 = data.vert * calculateElevation(pixel);

      dzdy = (z1 - z0) / dp;

      slope = Math.atan(Math.sqrt(dzdx * dzdx + dzdy * dzdy));

      aspect = Math.atan2(dzdy, -dzdx);
      if (aspect < 0) {
        aspect = halfPi - aspect;
      } else if (aspect > halfPi) {
        aspect = twoPi - aspect + halfPi;
      } else {
        aspect = halfPi - aspect;
      }

      cosIncidence =
        sinSunEl * Math.cos(slope) +
        cosSunEl * Math.sin(slope) * Math.cos(sunAz - aspect);

      offset = (pixelY * width + pixelX) * 4;
      scaled = 255 * cosIncidence;
      shadeData[offset] = scaled;
      shadeData[offset + 1] = scaled;
      shadeData[offset + 2] = scaled;
      shadeData[offset + 3] = elevationData[offset + 3];
    }
  }

  return {data: shadeData, width: width, height: height};
}

const elevation = new XYZ({
  url: 'https://{a-d}.tiles.mapbox.com/v3/aj.sf-dem/{z}/{x}/{y}.png',
  crossOrigin: 'anonymous',
  maxZoom: 13,
});

const raster = new Raster({
  sources: [elevation],
  operationType: 'image',
  operation: shade,
});

const map = new Map({
  target: 'map',
  layers: [
    new TileLayer({
      source: new OSM(),
    }),
    new ImageLayer({
      opacity: 0.3,
      source: raster,
    }),
  ],
  view: new View({
    extent: [-13675026, 4439648, -13580856, 4580292],
    center: [-13615645, 4497969],
    minZoom: 10,
    zoom: 13,
  }),
});

const controlIds = ['vert', 'sunEl', 'sunAz'];
const controls = {};
controlIds.forEach(function (id) {
  const control = document.getElementById(id);
  const output = document.getElementById(id + 'Out');
  control.addEventListener('input', function () {
    output.innerText = control.value;
    raster.changed();
  });
  output.innerText = control.value;
  controls[id] = control;
});

raster.on('beforeoperations', function (event) {
  // the event.data object will be passed to operations
  const data = event.data;
  data.resolution = event.resolution;
  for (const id in controls) {
    data[id] = Number(controls[id].value);
  }
});

界面布局文件index.html代码如下：

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Shaded Relief</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
      table.controls td {
        padding: 2px 5px;
      }
      table.controls td:nth-child(3) {
        text-align: right;
        min-width: 3em;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <table class="controls">
      <tr>
        <td><label for="vert">vertical exaggeration:</label></td>
        <td><input id="vert" type="range" min="1" max="5" value="1"/></td>
        <td><span id="vertOut"></span> x</td>
      </tr>
      <tr>
        <td><label for="sunEl">sun elevation:</label></td>
        <td><input id="sunEl" type="range" min="0" max="90" value="45"/></td>
        <td><span id="sunElOut"></span> °</td>
      </tr>
      <tr>
        <td><label for="sunAz">sun azimuth:</label></td>
        <td><input id="sunAz" type="range" min="0" max="360" value="45"/></td>
        <td><span id="sunAzOut"></span> °</td>
      </tr>
    </table>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://cdn.jsdelivr.net/npm/elm-pep@1.0.6/dist/elm-pep.js"></script>
    <script type="module" src="main.js"></script>
  </body>
</html>