Conceptual overview

There's some high-level questions worth answering before you get into setting up and writing code.

How do you map an emotion to an emoji?

Imagine there were only two emotions, fear and happiness, with values ranging from 0 to 1. This lets you plot any face in a 2D emotional space based on the emotion of the user, like so:

A simple graph. Happiness is on the y axis, fear is on the x axis, and the same sample image of a man's face from the introduction unit is plotted about 1/4 up, 3/4 to the right in white space.

You can use this to figure out the emotional point for each emoji just like you did for the face. After you plot the emojis in the 2D emotional space, you can figure out which emoji is closest to the emotion in the picture.

An updated version of the previous graph is shown with various emoji plotted at different points on the happiness-to-fear scale. Some of these include a smiling emoji with heart eyes higher up the happiness axis and lower on the fear axis, a nervous emoji further on the fear axis and lower on the happiness axis, and a crying emoji low on happiness and high on fear.

This calculation is called the Euclidian distance. You won't just use this in a 2D space, but rather in an 8D emotional space, measuring anger, contempt, disgust, fear, happiness, neutral, sadness, and surprise.


To make like easier we used the npm package called euclidean-distance.

How do you calculate the emotion of a face?

The Face API

Calculating emotion is, surprisingly, one of the most accessible parts of your app. The Azure Cognitive Services Face API takes image input and returns information about it, including whether a face was detected and its location. If requested, it calculates and returns the emotions of the faces as well, like so:

  "anger": 0.572,
  "contempt": 0.025,
  "disgust": 0.242,
  "fear": 0.001,
  "happiness": 0.014,
  "neutral": 0.111,
  "sadness": 0.033,
  "surprise": 0.003

Take for instance this image:

A closeup image of a woman's face. She has brown hair, glasses, and is smiling while looking directly at the viewer.

To process this image, you make a POST request to an API endpoint, like this one:

You provide the image in the body:

  "url": "<path-to-image>"


The API doesn’t return the emotion by default. You need to explicitly specify the query parameter returnFaceAttributes=emotion.

The use of a secret key authenticates the API. You need to send this key with the header.

Ocp-Apim-Subscription-Key: <your-subscription-key>

The API with the query parameters above would return a JSON, like so:

    "faceRectangle": {
      "top": 207,
      "left": 198,
      "width": 229,
      "height": 229
    "faceAttributes": {
      "emotion": {
        "anger": 0.001,
        "contempt": 0.014,
        "disgust": 0,
        "fear": 0,
        "happiness": 0.306,
        "neutral": 0.675,
        "sadness": 0.003,
        "surprise": 0.001

It returns an array of results - one per face detected in the image. Each result contains the size and location of the face as faceRectangle and the emotions represented as faceAttributes as a number from 0 to 1.

The calls to the Face API are wrapped into convenience functions for you in the Mojifier code repo

EmotivePoint class

If you look closely at the EmotivePoint class in shared/emotivePoint.ts, you will notice a few things.

The constructor takes an object containing emotive information as input and stores it as local member variables, like so:

  }) {
    this.anger = anger;
    this.contempt = contempt;
    this.disgust = disgust;
    this.fear = fear;
    this.happiness = happiness;
    this.neutral = neutral;
    this.sadness = sadness;
    this.surprise = surprise;

It also has a function called distance, which we can use to calculate the Euclidean distance between two emotive points, like so:

  distance(other) {
    let myPoint = this.toArray();
    let otherPoint = other.toArray();
    return distance(myPoint, otherPoint);

Using this information, you can create two emotive points and calculate how close they are:

let a = new EmotivePoint({
  /* ... */
let b = new EmotivePoint({
  /* ... */
let distance = a.distance(b);

Face class

Another helper class is the Face class, which combines a few different properties, including the EmotivePoint of a face and the Rect class, which is the rectangle that defines the boundaries of the face.

If you review the Face class constructor in shared/face.ts, you'll see this line of code:

this.moji = this.chooseMoji(this.emotivePoint);
  • emotivePoint is the emotive point of the face itself.
  • chooseMoji returns an appropriate emoji based on the emotivePoint of the face.
  chooseMoji(point) {
    let closestMoji = null;
    let closestDistance = Number.MAX_VALUE;
    for (let moji of MOJIS) {
      let emoPoint = moji.emotiveValues;
      let distance = emoPoint.distance(point);
      if (distance < closestDistance) {
        closestMoji = moji;
        closestDistance = distance;
    return closestMoji;
  • MOJIS is the list of emotive coordinates for all the emojis. These are generated from a set a proxy images approximating the emotions of each emoji. You can generate your own set of proxy images and calibrate the set of emoji emotional coordinates later in this module, as an optional step.
  • The chooseMoji function calculates the distance between this face and the emojis, returning the closest one.