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Using HTML5 <video> with <canvas>

Onur Şuyalçınkaya

Onur Şuyalçınkaya

• 3 mins read

You’ve already learned a lot about the <video> element in my previous article “What I’ve Learned From Working With HTML5 Video Over A Month”, but did you know that the <video> and <canvas> elements would be a great choice to use together? In fact, the two elements are absolutely wondrous when you combine them! I’m going to show you a handy demo using these two elements, which I hope will prompt cool future projects from you. Firstly, place the <video> and <canvas> elements in your HTML. Give your dimensions(width, height) to <canvas> according to your video. And of course, hide your video with the hidden attribute or “display: none” styling. Because

<canvas> will do the trick for you ;) Let’s go through one by one.

<canvas width="1280" height="720"></canvas>
<video src="foo.mp4" preload="auto" autoplay playsinline webkit-playsinline muted hidden></video>

We’re hiding the video element because we just want <canvas> to show the video, not <video> element itself.

Let’s declare the variables that will be used.

const canvas = document.querySelector('canvas')
const video = document.querySelector('video')

These our <canvas> and <video> elements that will be used later.

const fps = 60

fps means frame per second. It’s important according to the video’s frame. You may want to use it as 24, or 30, or 60. It will be used for the intervals millisecond value.

const width = 1280
const height = 720

width and height are the dimensions in pixels of the video. They will be given to the canvas element. If you do not specify them, <canvas> will take 300 for width and 150 for height as default which may break your view.

const canvasInterval = null

I will use canvasInterval variable to set and clear an interval to make <canvas> follow and unfollow the <video> element.

Here is the magical part, with the whole code.

const canvas = document.querySelector('canvas');
const video = document.querySelector('video');
const fps = 60;
const width = 1280;
const height = 720;
const canvasInterval = null;

drawImage() {
  canvas.getContext('2d', { alpha: false }).drawImage(video, 0, 0, width, height);
}

canvasInterval = window.setInterval(() => {
  drawImage(video);
}, 1000 / fps);

video.onpause = function() {
  clearInterval(canvasInterval);
};

video.onended = function() {
  clearInterval(canvasInterval);
};

video.onplay = function() {
  clearInterval(canvasInterval);
  canvasInterval = window.setInterval(() => {
    drawImage(video);
  }, 1000 / fps);
};

drawImage() method draws the video onto the canvas. video.onpause(), video.onended(), and video.onplay() methods are related to the performance issue. When the video paused or ended, we do not need to draw <video> onto the <canvas>, because you know, the frame is not changing after paused or ended. And when the <video> starts playing/resuming, <canvas> should start/resume drawing the <video> onto <canvas> as well.

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