图片转灰度

function toGray(alpha) {
  var imageData = canvas2dContext.getImageData(0, 0, width, height).data
  var r, g, b, v
  // 转灰度公式: R*0.299 + G*0.587 + B*0.114
  for (var i = 0; i < imageData.length - 4; i += 4) {
    r = imageData[i]
    g = imageData[i + 1]
    b = imageData[i + 2]
    v = (r * 0.299 + g * 0.587 + b * 0.2973)
    imageData[i] = v // red
    imageData[i + 1] = v // green
    imageData[i + 2] = v // blue
    imageData[i + 3] = imageData[i + 3] * alpha // Alpha
  }
}