最近做一个项目，需要绘制热力图，但是发觉现成的库封装都很不灵活
heatmap.js
百度Echarts
等都有封装

参考百度echarts的热力图插件，我整理了一下绘制热力图的思路

数据构成

[[x, y, cnt], [x, y, cnt]]
// x y 表示这个点在canvas上的位置

对canvas的简单封装

function Canvas(w, h) {
  this.canvas = document.createElement('canvas')
  this.canvas.width = w
  this.canvas.height = h
}
Canvas.prototype.getContext = function() {
  return this.canvas.getContext('2d') 
}
Canvas.prototype.getCanvas = function(){
  return this.canvas
}

创建canvas的每个点的渐变色

function creatHeadtMapCircle(size) {
  const shadowBlur = size / 2
  const r = size + shadowBlur
  const offsetDistance = 1000
  const circle =  new Canvas(r * 2, r * 2)
  let ctx = circle.getContext()
  ctx.shadowBlur = shadowBlur
  ctx.shadowColor = 'black'
  ctx.shadowOffsetX = ctx.shadowOffsetY = offsetDistance
  ctx.beginPath()
  ctx.arc( r - offsetDistance, r2 - offsetDistance, size, 0, Mathi.PI * 2 , true)
  ctx.closePath()
  ctx.fill()
  return circle
}

绘制单个点的结果

image.png

绘制多个点，根据权重值，可以得到如下的图形， 不同的权重 设置不同的透明度

image.png

创一个颜色映射

class ColorRange {
  constructor (options) {
  this.gradient = options.gradient || {
      0.25: 'rgba(0, 0, 255, 1)',
      0.55: 'rgba(0, 255, 0, 1)',
      0.85: 'rgba(255, 255, 0, 1)',
      1.0: 'rgba(255, 0, 0, 1)'
    }
    this.max = options.max || 30
    this.min = options.min || 0
    this.initPalette()
  }
  initPalette () {
    const gradient = this.gradient
    const canvas = new Canvas(256, 1)
    const ctx = this.ctx = canvas.getContext()
    const lineGradient = ctx.createLinearGradient(0, 0, 256, 1)
    for (let key in gradient) {
      lineGradient.addColorStop(parseFloat(key), gradient[key])
    }
    ctx.fillStyle = lineGradient
    ctx.fillRect(0, 0, 256, 1)
  }
   getImageData (v ) {
    const imageData = this.ctx,getImageData(0, 0, 256, 1).data
    if (v === undefined) return imageData
    if (v > this.max)  v = this.max
    if (v < this.min )  v = this.min
    var index = Math.floor((v - min) / (max - min) * 255 ) * 4
    return [imageData[index], imageData[index + 1], imageData[index + 2], imageData[index + 3]]  
  }
}

颜色映射

根据不同的绘图结果把不同的颜色透明度，映射到不同的颜色上，得到色彩图

let colored = sctx.getImageData(0, 0, canvas.width, canvas.height)
let gradient = (new ColorRange()).getImageData()
const opacity = 0.8
for(let i = 0, len = colored.data.length, j; i < len; i += 4) {
  j = colored.data[i] * 4
  if (colored.data[i] / 256 > opacity) colored.data[i] = 256 * opacity
  colored.data[i - 3] = gradient[j]
  colored.data[i - 2] = gradient[j + 1]
  colored.data[i - 1] = gradient[j + 2]
}
ctx.putImageData(colored, 0, 0)

image.png

最后整理成一个快速好用的库，提供给大家