概述

对于栅格数据，为提升网络的传输效率，通过一定的计算公式将栅格数据压缩成一个灰度图，在通过客户端进行解析渲染是常见的一种手段。本文将结合canvas实现此功能。

效果

灰度图转换 地图叠加效果

测试数据

测试数据来源于ventusky上的温度图，渲染的色标也用了该网站上面的。

实现代码

  const canvas = document.getElementById('canvas');
  canvas.width = window.innerWidth
  canvas.height = window.innerHeight
  const ctx = canvas.getContext('2d');

  const legends = [{"v":50,"c":"rgb(43, 0, 1)"},{"v":40,"c":"rgb(107, 21, 39)"},{"v":30,"c":"rgba(190, 48, 102, 0.92)"},{"v":25,"c":"rgba(229, 109, 83, 0.92)"},{"v":20,"c":"rgba(234, 164, 62, 0.92)"},{"v":15,"c":"rgba(235, 215, 53, 0.92)"},{"v":10,"c":"rgba(190, 228, 61, 0.92)"},{"v":5,"c":"rgba(89, 208, 73, 0.92)"},{"v":0,"c":"rgba(75, 182, 152, 0.92)"},{"v":-5,"c":"rgba(62, 121, 198, 0.92)"},{"v":-10,"c":"rgba(85, 78, 177, 0.92)"},{"v":-15,"c":"rgba(36, 24, 106, 0.92)"},{"v":-20,"c":"rgba(145, 9, 145, 0.92)"},{"v":-30,"c":"rgba(255, 170, 255, 0.92)"},{"v":-40,"c":"rgba(238, 238, 238, 0.92)"}]

  class Palette {
    constructor(legends) {
      this._init(legends)
    }

    _init(legends) {
      // 创建canvas
      let canvas = document.createElement("canvas");
      canvas.width = 10;
      canvas.height = 256;
      let ctx = canvas.getContext("2d");

      // 创建线性渐变色
      let linearGradient = ctx.createLinearGradient(0, 0, 0, canvas.height);
      const max = legends[0].v,
        min = legends[legends.length - 1].v - 10
      this.colorStops = {}
      legends.forEach(legend => {
        const k = (legend.v - min) / (max - min)
        linearGradient.addColorStop(k, legend.c);
      })

      // 绘制渐变色条
      ctx.fillStyle = linearGradient;
      ctx.fillRect(0, 0, canvas.width, canvas.height);

      // 读取像素数据
      this.imageData = ctx.getImageData(0, 0, 1, canvas.height).data;
      this.canvas = canvas;
    }

    colorPicker(position) {
      return this.imageData.slice(position * 4, position * 4 + 3);
    }
  }

  const palette = new Palette(legends)

  const img = new Image()
  img.src = './tem.jpg'
  let imgData = null
  img.onload = function () {
    // 离屏
    const w = img.width, h = img.height
    const _canvas = document.createElement('canvas');
    _canvas.width = w
    _canvas.height = h
    const _ctx = _canvas.getContext('2d');
    _ctx.drawImage(img, 0, 0)
    imgData = _ctx.getImageData(0, 0, w, h)
    ctx.putImageData(imgData, 0, 0)

    const data = imgData.data
    let val = []
    for (let i = 0; i < data.length; i+=4) {
      let pos = data[i];
      val.push(pos)
    }
    val = val.sort((a, b) => a - b)
    const min = val[0],
      max = val[val.length - 1]

    for (let i = 0; i < data.length; i+=4) {
      let pos = data[i];
      pos = Math.floor(((pos - min) / (max - min)) * 256)
      let color = palette.colorPicker(pos);
      data[i] = color[0];
      data[i + 1] = color[1];
      data[i + 2] = color[2];
      data[i + 3] = 255;
    }
    // 展示img
    ctx.putImageData(imgData, w + 10, 0)

    // 绘制图例
    const _w = palette.canvas.width, _h = palette.canvas.height / 2
    const x = 25, y = canvas.height - 20 - _h
    ctx.fillStyle = 'white'
    ctx.shadowBlur = 10
    ctx.shadowColor = '#ccc'
    ctx.shadowOffsetX = 2
    ctx.shadowOffsetY = 2
    ctx.textAlign = 'left'
    ctx.fillRect(x - 10, y - 10, _w + 40, _h + 20)
    ctx.drawImage(palette.canvas, x, y, _w, _h)
    // 绘制文字
    ctx.fillStyle = '#000'
    ctx.textBaseline = 'top'
    ctx.fillText(legends[legends.length - 1].v, x + _w + 6, y)
    ctx.textBaseline = 'bottom'
    ctx.fillText(legends[0].v, x + _w + 6, y + _h)
  }

结合到openlayers中：

  const legends = [{"v":50,"c":"rgb(43, 0, 1)"},{"v":40,"c":"rgb(107, 21, 39)"},{"v":30,"c":"rgba(190, 48, 102, 0.92)"},{"v":25,"c":"rgba(229, 109, 83, 0.92)"},{"v":20,"c":"rgba(234, 164, 62, 0.92)"},{"v":15,"c":"rgba(235, 215, 53, 0.92)"},{"v":10,"c":"rgba(190, 228, 61, 0.92)"},{"v":5,"c":"rgba(89, 208, 73, 0.92)"},{"v":0,"c":"rgba(75, 182, 152, 0.92)"},{"v":-5,"c":"rgba(62, 121, 198, 0.92)"},{"v":-10,"c":"rgba(85, 78, 177, 0.92)"},{"v":-15,"c":"rgba(36, 24, 106, 0.92)"},{"v":-20,"c":"rgba(145, 9, 145, 0.92)"},{"v":-30,"c":"rgba(255, 170, 255, 0.92)"},{"v":-40,"c":"rgba(238, 238, 238, 0.92)"}]

  class Palette {
    constructor(legends) {
      this._init(legends)
    }

    _init(legends) {
      // 创建canvas
      let canvas = document.createElement("canvas");
      canvas.width = 10;
      canvas.height = 256;
      let ctx = canvas.getContext("2d");

      // 创建线性渐变色
      let linearGradient = ctx.createLinearGradient(0, 0, 0, canvas.height);
      const max = legends[0].v,
        min = legends[legends.length - 1].v - 10
      this.colorStops = {}
      legends.forEach(legend => {
        const k = (legend.v - min) / (max - min)
        linearGradient.addColorStop(k, legend.c);
      })

      // 绘制渐变色条
      ctx.fillStyle = linearGradient;
      ctx.fillRect(0, 0, canvas.width, canvas.height);

      // 读取像素数据
      this.imageData = ctx.getImageData(0, 0, 1, canvas.height).data;
      this.canvas = canvas;
    }

    colorPicker(position) {
      return this.imageData.slice(position * 4, position * 4 + 3);
    }
  }

  const palette = new Palette(legends)

  const img = new Image()
  img.src = './data/tem.jpg'
  img.onload = function () {
    // 离屏
    const w = img.width, h = img.height
    const _canvas = document.createElement('canvas');
    _canvas.width = w
    _canvas.height = h
    const _ctx = _canvas.getContext('2d');
    _ctx.drawImage(img, 0, 0)
    const imgData = _ctx.getImageData(0, 0, w, h)
    const data = imgData.data
    let val = []
    for (let i = 0; i < data.length; i+=4) {
      let pos = data[i];
      val.push(pos)
    }
    val = val.sort((a, b) => a - b)
    const min = val[0],
      max = val[val.length - 1]

    for (let i = 0; i < data.length; i+=4) {
      let pos = data[i];
      pos = Math.floor(((pos - min) / (max - min)) * 256)
      let color = palette.colorPicker(pos);
      data[i] = color[0];
      data[i + 1] = color[1];
      data[i + 2] = color[2];
      data[i + 3] = 150;
    }
    // 展示img
    _ctx.putImageData(imgData, 0, 0)
    const imgLayer = new ol.layer.Image({
      source: new ol.source.ImageStatic({
        url: _canvas.toDataURL(),
        imageExtent: [-180, -90, 180, 90]
      })
    })
    map.addLayer(imgLayer);
}