遍历 HTML5 画布 getImageData 时获取 X 和 Y 像素坐标

I am iterating over some image data pulled from a canvas like so:

var imageData = this.context.getImageData(0, 0, this.el.width, this.el.height);
var data = imageData.data;

for (var i = data.length; i >= 0; i -= 4) {
    if (data[i + 3] > 0) {
        data[i] = this.colour.R;
        data[i + 1] = this.colour.G;
        data[i + 2] = this.colour.B;
    }
}

如何计算我当前所在的 X 和 Y 像素坐标?