javascript - 获取画布笔划点

Question

如何从画布笔划中获取黑点？ 这里 - https://jsfiddle.net/dsu3Lmfm/2/ - 画布上的所有点都是黑色的。

var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.beginPath();
ctx.moveTo(20, 20);
ctx.lineTo(20, 100);
ctx.lineTo(70, 100);
ctx.closePath();
ctx.stroke();

var imageData = ctx.getImageData(0, 0, c.width, c.height);
var pix = imageData.data;

// Loop over each pixel and get the black pixels
for (var i = 0, n = pix.length; i < n; i += 4) {
   if (pix[i] === 0 && pix[i+1] === 0 && pix[i+2] === 0) {
       var x = (i / 4) % c.width;
       var y = (i / 4) / c.width;

       console.log("x=" + parseInt(x, 10) + ", y=" + parseInt(y, 10));
   }
}