canvas组件下，有一段示例代码，如下：

<!-- canvas.wxml -->
<canvas type="2d" id="myCanvas"></canvas>
// canvas.js
Page({
  onReady() {
    const query = wx.createSelectorQuery()
    query.select('#myCanvas')
      .fields({ node: true, size: true })
      .exec((res) => {
        const canvas = res[0].node
        const ctx = canvas.getContext('2d')

        const dpr = wx.getSystemInfoSync().pixelRatio
        canvas.width = res[0].width * dpr
        canvas.height = res[0].height * dpr
        ctx.scale(dpr, dpr)

        ctx.fillRect(0, 0, 100, 100)
      })
  }
})

不太理解长宽为什么要乘以dpr，并且画布缩放同样的倍数。我把这些去掉后预览看起来和上面一样：

canvas.width = res[0].width
canvas.height = res[0].height
// ctx.scale(dpr, dpr)