js 转换image data 成黑白色。

We can even make it pure black and white by using the following calculations:

• black = 0 - 382
• white = 383 - 765
  And it will result in the following loop:

for (i = 0; i < imgData.data.length; i += 4) {
    let count = imgData.data[i] + imgData.data[i + 1] + imgData.data[i + 2];
    let colour = 0;
    if (count > 383) colour = 255;

    imgData.data[i] = colour;
    imgData.data[i + 1] = colour;
    imgData.data[i + 2] = colour;
    imgData.data[i + 3] = 255;
}