docs: function unpackPNG(w, h, c, str) {
    var canvas = document.createElement("canvas")
    canvas.width = h
    canvas.height = w
    var img = new Image()
    img.src = "data:image/png;base64," + str
    var context = canvas.getContext("2d")
    context.drawImage(img, 0, 0)
    var pixels = context.getImageData(0, 0, h, w)
    return ndarray(pixels.data, [w, h, c], [4, 4*w, 1], 0)
  }