将视频帧渲染到画布丢失了一些像素颜色信息

Question

大家好，

我将一帧视频渲染到画布上，当我这样做时，我注意到了这一点。我在渲染之前和之后检查RGB值，并注意到值不同。

<video id="video" width="400" height="300" controls>
    <source src="save.mp4" type="video/mp4">
  </video>
  <button onclick="draw()">Draw</button>
  <canvas id="canvas"></canvas>
  <script>

    function draw() {
      const video = document.getElementById('video'),
        canvas = document.getElementById('canvas'),
        canvasContext = canvas.getContext('2d');
        canvas.width = video.offsetWidth;
        canvas.height = video.offsetHeight;

        canvasContext.drawImage(video, 0, 0, video.offsetWidth, video.offsetHeight);
    }

  </script>