如何将video base64更改为画布图像？

Question

如何将video base64更改为画布图像缩略图？

   var video = $("#video_demo");
   var canvas = $('#thecanvas');

    draw(video, canvas);

    function draw(video, canvas) {
        var context = canvas.getContext('2d');
        context.drawImage(video, 0, 0, canvas.width, canvas.height);
        var dataURL = canvas.toDataURL();
        console.log(dataURL);
    }