Exercise 19: Webcam Fun

Nitish Dayal, Software & Applications Developer - Contact
Last Commit Date: May 12, 2017

We're given an HTML page with a button labeled 'Take Photo' which calls upon a takePhoto() function on a click event, a collection of input elements of type range with RGB min/max labels, a canvas element, a video element, and an empty div with the class strip. Our goal is to write the necessary JavaScript code to ask permission for the user's webcam, display the feed from that webcam on the page, allow the user to save pictures that are displayed, and allow the user to alter the image using the RGB sliders.

Guide

This challenge will require use of the Navigator Web API (MDN documentation), the CanvasRenderingContext2D Web API (MDN documentation, and the MediaDevices Web API (MDN Documentation). If you're not a fan of reading documentation, here's a simple overview:

The Navigator interface is a representation of the application in which the JavaScript code is running (in most cases, the browser)
The MediaDevices interface provides a way to access the user's connected media hardware, such as a webcam or microphone by using the getUserMedia method. The method takes an object as an argument which specifies the media items the program needs access to; in our case, the webcam.

The scripts.js file already has some constants defined as references: a reference to the video element, the canvas element, the canvas context, to the div element where we'll display any photos the user has taken, and a reference to the audio element which uses this file as it's source.

We begin by using the navigator web API in combination with the mediaDevices web API to request permission from the user to access their webcam; this request returns a promise with the resulting MediaStream object or the rejected promise with an error message. Note: This promise is only resolved IF the user selects an option when prompted for request to the webcam; if the user doesn't select anything, the promise won't resolve but our program won't break. This is the beauty of promises; they allow us to ask for something and will allow the rest of the program to run the promise waits for a response. Promises represent a value which may be available now, in the future, or never (as defined by MDN). If we receive a localMediaStream object as a promise, we'll set the source object of the video element to the localMediaStream object, and call upon the play() method of the video element. If the promise resolves to an error, we'll log out that error message.

Then, we need to define a function that will be used as the event handler for when the 'canplay' event of the video element is triggered (whenever the user either approves or denies access to their webcam). The function will adjust the canvas element's width and height properties to be equivalent to the video element's width and height, and will return the interval ID of a setInterval() method. Within the body of the setInterval() method, we will draw onto the canvas by providing a valid canvas image source such as an image element, a video element, another canvas element, or an image bitmap. In our case, we want to display the user's webcam feed in the canvas, so we'll provide the video element as as argument to the canvas' draw method. To manipulate the image using the input HTML elements, we'll need to get access to the underlying pixel data of the image being displayed on the canvas. We'll declare a let variable and define it as the ImageData object returned from calling the getImageData() method of the canvas context. The ImageData object returned from this method is exactly what we were looking for: the underlying pixel data for the current canvas image. We will eventually pass this variable into a function that will change the values of the pixels depending on the input values.

The takePhoto() function that the button HTML element is calling on 'click' events isn't defined; we'll do that next. The takePhoto() function should play the audio file that we referenced earlier with a const variable, convert the current canvas image to a data URI, create an anchor HTML element (a link) with a hyperlink reference pointing to the data URI which will download the image to the user's local storage when clicked, set the inner HTML of that link to be an image HTML element referencing the data URI as the source, and insert this link inside the empty div element with the class .strip.

The last function we'll want to create is one that will bring functionality to the input elements; as the user moves the sliders around, we want to adjust the RGB values of the canvas image to reflect the values of the sliders to present a sort of 'green screen' effect. The function should have one declarared parameter, pixels. We'll begin by declaring a const as an empty object. We'll create a NodeList of all the HTML input elements contained within the div with class .rgb, iterate through this NodeList, and create a key within the previously defined object corresponding with the given input element's name property, and set it's value as the given input element's value. The pixels argument we're expecting will be the underlying pixel data of the current canvas image. This data contains an array of unsigned 8bit integer values between 0-255, corresponding with the RGB attributes of the image. We'll iterate through these arrays and set the values to 0 where the RGB attribute is within the range of the input slider values. This one is tricky, so let's get to the steps.

Steps:

Declare a const variable getVideo and define it as an arrow function. This function will use the Navigator and mediaDevices web APIs to ask permission to access the user's webcam and, upon success, will set the source object of the video element as the localMediaStream.

  const getVideo = () => {
    navigator.mediaDevices.getUserMedia({ video: true, audio: false })
      .then(localMediaStream => {
        video.srcObject = localMediaStream;
        video.play();
      })
      .catch(err => {
        console.error(`OH NO!!!`, err);
      })
  };

Declare a const variable paintToCanvas and define it as an arrow function.

  const paintToCanvas = () => { /* ... */};

In the body of this function, declare two const variables and define them as the video element's width and height, and update the canvas width and height to reflect those values.

  const paintToCanavas = () => {
    const width = video.videoWidth;
    const height = video.videoHeight;
    canvas.width = width;
    canvas.height = height; 

     /* More to do... */
   };

Still in the body of the function, return the interval ID of a setInterval() function call, and within the body of that function call, draw the current video element on to the canvas.

  let paintToCanavas = () => {
    const width = video.videoWidth;
    const height = video.videoHeight;
    canvas.width = width;
    canvas.height = height; 

    return setInterval(() => {
      ctx.drawImage(video, 0, 0, width, height);
    }, 16);
  };

Declare a const variable takePhoto and define it as an arrow function that will play the audio element on the HTML page, create a link which targets a data URI representation of a still image taken from the canvas, and places that link into the front of the empty div element.

  const takePhoto = () => {
    // Play the audio element on the HTML page
    snap.currentTime = 0;
    snap.play();

    const data = canvas.toDataURL('image/jpeg');
    const link = document.createElement('a');
    link.href = data;
    link.setAttribute('download', 'handsome');
    link.innerHTML = `<img src="${data}" alt="Handsome Man" />`;
    strip.insertBefore(link, strip.firsChild);
  };

Declare a const variable greenScreen and define it as an arrow function that will accept an argument pixels.

  const greenScreen = (pixels) => { /*...*/ };

In the function body, declare a const variables levels and define it as an empty object. Iterate through a NodeList of all input elements within the div with class rgb and set a key in the levels object as a given input's name property, and set the value to be the given input's value.
```
  /* In function body */

  const levels = {};

  document.querySelectorAll('.rgb input').forEach((input) => {
    levels[input.name] = input.value;
  });
```

Update the values of the pixels in the image so that we remove all RGB values that are within the range defined by the user, and return the given argument.

  /* In function body */
  for (i = 0; i < pixels.data.length; i += 4) {
    red = pixels.data[i + 0];
    green = pixels.data[i + 1];
    blue = pixels.data[i + 2];
    alpha = pixels.data[i + 3];

    if (red >= levels.rmin
      && green >= levels.gmin
      && blue >= levels.bmin
      && red <= levels.rmax
      && green <= levels.gmax
      && blue <= levels.bmax) 
    {
      pixels.data[i + 3] = 0;
    }
  }
  return pixels;

Update the getVideo function so that the setInterval method within the function body creates a variable defined as the underlying pixel data, passes that variable into the greenScreen function, and places the returned pixel data into the canvas context.

/* In paintToCanvas function body */

return setInterval(() => {
  ctx.drawImage(video, 0, 0, width, height);
  
  let pixels = ctx.getImageData(0, 0, width, height);

  pixels = greenScreen(pixels);
  
  ctx.putImageData(pixels, 0, 0);
}, 16);

Call upon the getVideo function we previously defined.
Attach an event listener to the video HTML element that will call the paintToCanvas function on the 'canplay' event, and attach an event listener to the button HTML element that will call the takePhoto function on a 'click' event.
```
  video.addEventListener('canplay', paintToCanavas);
  document.querySelector('.takePhoto').addEventListener('click', takePhoto);
```

Wrap it all up in an IIFE, take a step back, and enjoy your glorious new photo-and-greenscreen application!

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