问如何从组件获取帧<video></video>

EN

var videoId = 'video';
    var scaleFactor = 0.25;
    var snapshots = [];

    /**
     * Captures a image frame from the provided video element.
     *
     * @param {Video} video HTML5 video element from where the image frame will be captured.
     * @param {Number} scaleFactor Factor to scale the canvas element that will be return. This is an optional parameter.
     *
     * @return {Canvas}
     */
    function capture(video, scaleFactor) {
        if (scaleFactor == null) {
            scaleFactor = 1;
        }
        var w = video.videoWidth * scaleFactor;
        var h = video.videoHeight * scaleFactor;
        var canvas = document.createElement('canvas');
        canvas.width = w;
        canvas.height = h;
        var ctx = canvas.getContext('2d');
        ctx.drawImage(video, 0, 0, w, h);
        return canvas;
    }

    /**
     * Invokes the <code>capture</code> function and attaches the canvas element to the DOM.
     */
    function shoot() {
        var video = document.getElementById(videoId);
        var output = document.getElementById('output');
        var canvas = capture(video, scaleFactor);
        canvas.onclick = function() {
            window.open(this.toDataURL(image/jpg));
        };
        snapshots.unshift(canvas);
        output.innerHTML = '';
        for (var i = 0; i < 4; i++) {
            output.appendChild(snapshots[i]);
        }
    }

(function() {
  var captureit = document.getElementById('cit');
  captureit.click();
})();