实例 - 在线编辑器

          <canvas id="myCanvas" width="300" height="150" style="border:1px solid #d3d3d3;">
您的浏览器不支持 HTML5 canvas 标签。
</canvas>
<script>
  var c = document.getElementById("myCanvas");
  var ctx = c.getContext("2d");
  ctx.fillStyle = "red";
  ctx.fillRect(10, 10, 50, 50);
  var imgData = ctx.getImageData(30, 30, 50, 50);
  red = imgData.data[0];
  green = imgData.data[1];
  blue = imgData.data[2];
  alpha = imgData.data[3];
  alert(red + " " + green + " " + blue + " " + alpha);
</script>
        