html5 访问像素值 (拾色器)

<div class="wrap">
    <canvas id="myCanvas" width="500" height="500"></canvas>
</div>

function draw(id){
        var canvas = document.getElementById(id);
        if(canvas == null){
            return false;
        }
        var context = canvas.getContext("2d");
        var img = new Image();
        img.src = 'heng.jpg';
        img.onload = function(){
            context.drawImage(img,0,0,500,500);
        };

        canvas.onclick = function(e){
            var canvasX = Math.floor(e.pageX - canvas.offsetLeft);// 向下舍
            var canvasY = Math.floor(e.pageY - canvas.offsetTop);

            var imageData = context.getImageData(canvasX, canvasY, 1,1);// 获取点击的那一点的坐标
            var pixel = imageData.data;
            var pixelColor = "rgba("+ pixel[0] + ", " + pixel[1] +", "+ pixel[2] +", " + pixel[3] +")";

            console.log(pixelColor);
            document.body.style.background = pixelColor;
        };
    }

    draw("myCanvas");

　　其中有点不明白，alpha的值不是0-1,为什么我访问pixe[3]得到的是255呢？