docs: function dataURLtoBlob(dataUrl) {
      let pos = dataUrl.indexOf(',', 0)
      let arr = [ dataUrl.slice(0, pos), dataUrl.slice(pos+1) ]
      let mime = arr[0].match(/:(.*?);/)[1],
          bstr = atob(arr[1]),
          n = bstr.length,
          u8arr = new Uint8Array(n)
      while(n--) {
          u8arr[n] = bstr.charCodeAt(n)
      }
      return new Blob([ u8arr ], { type:mime })
  }