Question

我将网络摄像头Feed流式传输到video元素，然后将其绘制到canvas元素上。然后我在画布上运行一个人脸检测算法，在脸上画一个正方形。

问题是要执行此操作，我需要调用context.getImgData()方法。这导致画布闪烁＆＃34;闪烁＆＃34;调用方法时它瞬间变成全黑，然后恢复正常。看起来很糟糕。我已经关注了其他一些使用网络摄像头进行画布的人的例子，他们也使用这种方法，所以我真的不知道如何绕过它。

我在JSfiddle下面有一些示例代码。我在mac OS上使用firefox。有两个按钮。第一个将流从video元素绘制到canvas，第二个只运行getImgData()。你可以清楚地看到我所描述的问题。有什么想法吗？

<body>
    <div>
        <button onclick="paintCanvas()">paint</button>
        <button onclick="testFunc()">test</button>
    </div>
    <div>
        <canvas id="canvas" width="500" height="375"></canvas>
        <video autoplay loop="false" src="media/vid.mp4" type="video/mp4" id="videoElement">
    </div>
</body>
<script>
    var video = document.querySelector("#videoElement");
    var myCanvas = document.getElementById('canvas');
    var myContext = myCanvas.getContext('2d');

    navigator.mediaDevices.getUserMedia({ video: true })
        .then((stream) => {
            video.srcObject = stream;
            // vid2.srcObject = stream;
            // document.getElementById('controls').innerHTML = "Switch back to video for player controls";
            // document.getElementById('timing').innerHTML = '';
        })
        .catch(function (err) {
            media = 'video';
            console.log(err.name);
        });
    let ch, cw;
    function paintCanvas(e) {
        console.log('painting canvas')
        const v = document.getElementById('videoElement');
        const canvas = document.getElementById('canvas');
        const context = canvas.getContext('2d');
        cw = Math.floor(canvas.clientWidth);
        ch = Math.floor(canvas.clientHeight);
        canvas.width = cw;
        canvas.height = ch;

        draw(v, context, cw, ch);
    }
    function draw(v, c, w, h) {
        // console.log('drawing')
        videoRunning = true;
        if (v.paused || v.ended) return false;
        c.drawImage(v, 0, 0, w, h);
        myVar = setTimeout(draw, 60, v, c, w, h);
    }
    function testFunc() {       
        // setInterval(function () {
            console.log(video.videoWidth, video.videoHeight)
            x = myContext.getImageData(0, 0, video.videoWidth, video.videoHeight);
            console.log(x);
        // }, 200);
    }
</script>

Answer 1

好的想通了 - 如果你想使用getImageData()方法，必须在与绘图相同的函数中调用它。

function draw(v, c, w, h) {
    // console.log('drawing')
    videoRunning = true;
    if (v.paused || v.ended) return false;
    c.drawImage(v, 0, 0, w, h);
    myVar = setTimeout(draw, 60, v, c, w, h);
    x = myContext.getImageData(0, 0, video.videoWidth, video.videoHeight)

}

调用context.getImgData（）会导致画布闪烁＆＃34;闪烁＆＃34;

1 个答案: