Manipulating video using canvas

By combining the capabilities of the video element with acanvas, you can manipulate video data in real time to incorporate a variety of visual effects to the video being displayed. This tutorial demonstrates how to perform chroma-keying (also known as the "green screen effect") using JavaScript code.

The document content

The HTML document used to render this content is shown below.

html

<!doctype html><html lang="en-US">  <head>    <meta charset="UTF-8" />    <title>Video test page</title>    <style>      body {        background: black;        color: #cccccc;      }      #c2 {        background-image: url(media/foo.png);        background-repeat: no-repeat;      }      div {        float: left;        border: 1px solid #444444;        padding: 10px;        margin: 10px;        background: #3b3b3b;      }    </style>  </head>  <body>    <div>      <video               src="media/video.mp4"        controls        crossorigin="anonymous"></video>    </div>    <div>      <canvas width="160" height="96"></canvas>      <canvas width="160" height="96"></canvas>    </div>    <script src="processor.js"></script>  </body></html>

The key bits to take away from this are:

This document establishes twocanvas elements, with the IDsc1 andc2. Canvasc1 is used to display the current frame of the original video, whilec2 is used to display the video after performing the chroma-keying effect;c2 is preloaded with the still image that will be used to replace the green background in the video.
The JavaScript code is imported from a script namedprocessor.js.

The JavaScript code

The JavaScript code inprocessor.js consists of three methods.

Initializing the chroma-key player

ThedoLoad() method is called when the HTML document initially loads. This method's job is to prepare the variables needed by the chroma-key processing code, and to set up an event listener so we can detect when the user starts playing the video.

const processor = {};processor.doLoad = function doLoad() {  const video = document.getElementById("video");  this.video = video;  this.c1 = document.getElementById("c1");  this.ctx1 = this.c1.getContext("2d");  this.c2 = document.getElementById("c2");  this.ctx2 = this.c2.getContext("2d");  video.addEventListener(    "play",    () => {      this.width = video.videoWidth / 2;      this.height = video.videoHeight / 2;      this.timerCallback();    },    false,  );};

This code grabs references to the elements in the HTML document that are of particular interest, namely thevideo element and the twocanvas elements. It also fetches references to the graphics contexts for each of the two canvases. These will be used when we're actually doing the chroma-keying effect.

ThenaddEventListener() is called to begin watching thevideo element so that we obtain notification when the user presses the play button on the video. In response to the user beginning playback, this code fetches the width and height of the video, halving each (we will be halving the size of the video when we perform the chroma-keying effect), then calls thetimerCallback() method to start watching the video and computing the visual effect.

The timer callback

The timer callback is called initially when the video starts playing (when the "play" event occurs), then takes responsibility for establishing itself to be called periodically in order to launch the keying effect for each frame.

processor.timerCallback = function timerCallback() {  if (this.video.paused || this.video.ended) {    return;  }  this.computeFrame();  setTimeout(() => {    this.timerCallback();  }, 0);};

The first thing the callback does is check to see if the video is even playing; if it's not, the callback returns immediately without doing anything.

Then it calls thecomputeFrame() method, which performs the chroma-keying effect on the current video frame.

The last thing the callback does is callsetTimeout() to schedule itself to be called again as soon as possible. In the real world, you would probably schedule this to be done based on knowledge of the video's frame rate.

Manipulating the video frame data

ThecomputeFrame() method, shown below, is responsible for actually fetching a frame of data and performing the chroma-keying effect.

processor.computeFrame = function () {  this.ctx1.drawImage(this.video, 0, 0, this.width, this.height);  const frame = this.ctx1.getImageData(0, 0, this.width, this.height);  const data = frame.data;  for (let i = 0; i < data.length; i += 4) {    const red = data[i + 0];    const green = data[i + 1];    const blue = data[i + 2];    if (green > 100 && red > 100 && blue < 43) {      data[i + 3] = 0;    }  }  this.ctx2.putImageData(frame, 0, 0);};

When this routine is called, the video element is displaying the most recent frame of video data, which looks like this:

A single frame of the video element. There is a person wearing a black t-shirt. The background-color is yellow.

That frame of video is copied into the graphics contextctx1 of the first canvas, specifying as the height and width the values we previously saved to draw the frame at half size. Note that you can pass the video element into the context'sdrawImage() method to draw the current video frame into the context. The result is:

A single frame of the video element. There is a person wearing a black t-shirt. The background-color is yellow. This is a smaller version of the picture above.

Calling thegetImageData() method on the first context fetches a copy of the raw graphics data for the current frame of video. This provides raw 32-bit pixel image data we can then manipulate. We then compute the number of pixels in the image by dividing the total size of the frame's image data by four.

Thefor loop scans through the frame's pixels, pulling out the red, green, and blue values for each pixel, and compares the values against predetermined numbers that are used to detect the green screen that will be replaced with the still background image imported fromfoo.png.

Every pixel in the frame's image data that is found that is within the parameters that are considered to be part of the green screen has its alpha value replaced with a zero, indicating that the pixel is entirely transparent. As a result, the final image has the entire green screen area 100% transparent, so that when it's drawn into the destination context usingctx2.putImageData, the result is an overlay onto the static backdrop.

The resulting image looks like this:

A single frame of the video element shows the same person wearing a black t-shirt as in the photos above. The background is different: it is the Firefox logo.