DEV Community 👩‍💻👨‍💻

Cover image for Measure Distance Between Two Points In An Image Using JavsScript

Posted on

Measure Distance Between Two Points In An Image Using JavsScript

It's not about complex astrophysics but we all love the sky, Right? Also, we like images of stars, nebulae, galaxies, etc.

So that's why I came up with this idea: what if we can measure the distance (only for fun) between two stars that appear in an image?

Measure distance between stars appear in an image

Well, It might not be about the stars in an image but could be anything such as two cities on the map, whichever we can measure the distance between them with this simple beginner-friendly project.

In my opinion, it is a good idea to use this project to practice basic knowledge of HTML, CSS and, JavaScript. Most importantly, you don't want to use HTML canvas here.

Okay, what does this project do in the first place?

There is an image of the starry night sky on the page, so the user can measure the distance between two or more stars that appear on the image by clicking on them.

So let's break down this project into smaller parts.

1. HTML:

Create div elements for,

  • Image container: <div id="container">
  • Result container to display calculated distance: <div id="results">

  <h1>Measure Distance</h1>

  <div id="container"></div>


  <div id="results">

    <div id="current-result"></div>

    <div id="total-result"></div>


  <input id="reset" type="button" value="Reset" onclick="document.location.reload()">


Enter fullscreen mode Exit fullscreen mode

2. CSS:

  • Set our starry night sky image as background-image of the image container.

#container {
 width: 500px;
 height: 400px;
 border: 2px solid BurlyWood;

 background-image: url(; 
 background-position: center; 
 background-repeat: no-repeat; 
 background-size: cover; 

Enter fullscreen mode Exit fullscreen mode
  • Create and style two class selectors .points and .lines which we will create later in the JavaScript.
  • Make the position: absolute; for the both .points and .lines.

.points {
 position: absolute;
 border: 2px solid red;
 border-radius: 50%;
 opacity: 1;

.lines {
 position: absolute;
 border: 1px solid white;
 height: 0;
 opacity: 1;

Enter fullscreen mode Exit fullscreen mode
  • Add additional styling as your taste.

3. JavaScript:

(1) create variables

  • select #container and #results using DOM.
  • create an object with two arrays to store X and Y coordinates of the user's click points.
  • create two arrays to store created div elements points and lines.

let container = document.querySelector("#container");

let results = document.querySelector("#results");
let currentResult = document.querySelector("#current-result");
let totalResult = document.querySelector("#total-result");

let coordinates = {
  coordX: [],
  coordY: []

let points = [];
let lines = [];

Enter fullscreen mode Exit fullscreen mode

(2) addEventListener() to the container

  • store X and Y coordinates in the coordinates object.
  • call the functions createPoints() then createLines() within the callback function.

container.addEventListener("click", (e) => {

  createPoints(e.x, e.y);

  let prevX = coordinates.coordX[coordinates.coordX.length - 2];

  let prevY = coordinates.coordY[coordinates.coordY.length - 2];

  if (coordinates.coordX.length > 1) {
    createLines(prevX, prevY, e.x, e.y);

Enter fullscreen mode Exit fullscreen mode

(3) createPoints() function

  • Create a for-loop that runs from 0 to the number of clicked points.
  • Create a div in each iteration inside the loop.
  • Set its className as points.
  • Set left and top coordinates of the div (e.x and e.y)
  • Append it to the #container

function createPoints(posX, posY) {
  for (let i = 0; i < coordinates.coordX.length; i++) {
    points[i] = document.createElement("div");
    points[i].className = "points";
    points[i].style.left = `${coordinates.coordX[i]}px`;
    points[i] = `${coordinates.coordY[i]}px`;


Enter fullscreen mode Exit fullscreen mode

(4) createLines() function

Okay, We reach the more important part of this project. Because here we have to use some math! I don't suppose to be your math teacher, but these are the steps we're going to follow.

  • Get the distance between the two points.

  let distance = Math.sqrt(((x2-x1) * (x2-x1)) + ((y2-y1) * (y2-y1)));

Enter fullscreen mode Exit fullscreen mode
  • Find the middle point of the two points.

// X and Y coordinates of the middle point

    let midX = (x1+x2)/2;
    let midY = (y1+y2)/2;

Enter fullscreen mode Exit fullscreen mode
  • Draw a horizontal line of that distance across the midpoint.

 lines[i].style.width = `${distance}px`;
 lines[i].style.left = `${(midX - (distance/2))}px`;
 lines[i] = `${midY}px`;

Enter fullscreen mode Exit fullscreen mode
  • Calculate the angle to rotate it around the middle point to fit it into the actual two points.

/* get the inclination in radian unit and then convert it into degree */

 let inclinationInRadian = Math.atan2(y1-y2, x1-x2);
 let inclinationInDegree = (inclinationInRadian * 180)/ Math.PI;

Enter fullscreen mode Exit fullscreen mode
  • Rotate the line.

  lines[i].style.transform = 'rotate('+inclinationInDegree +'deg)';

Enter fullscreen mode Exit fullscreen mode

Here is the complete createLines() function.

function createLines(x1, y1, x2, y2) {
  let distance = Math.sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));

  let midX = (x1 + x2) / 2;
  let midY = (y1 + y2) / 2;

  let inclinationInRadian = Math.atan2(y1 - y2, x1 - x2);
  let inclinationInDegree = (inclinationInRadian * 180) / Math.PI;

  for (let i = 0; i < coordinates.coordX.length; i++) {
    lines[i] = document.createElement("div");
    lines[i].className = "lines";

    lines[i].style.width = `${distance}px`;
    lines[i].style.left = `${midX - distance / 2}px`;
    lines[i] = `${midY}px`;
    lines[i].style.transform = "rotate(" + inclinationInDegree + "deg)";


  currentResult.innerHTML = `<strong>Current Result:-</strong> <br>`;

  totalResult.innerHTML = `<strong>Total Result:-</strong> <br>`;


Enter fullscreen mode Exit fullscreen mode

(5) getDistance() function

Now we have the distance in pixels, so we should convert it to the centimeter.

  • 1 pixels = 0.0264583333cm

  • Distance in centimeter = distance in pixels × 0.0264583333

function getDistance(distance) {
  let pixelToCm = distance * 0.0264583333;
  pixelToCm = Number(pixelToCm.toFixed(2));

  totalDistance += pixelToCm;
  totalDistance = Number(totalDistance.toFixed(2));

  currentPath += `Line ${++lineCounter}:- ${pixelToCm}cm<br>`;

  totalPath += `${totalDistance}cm<br>`;

  currentResult.innerHTML += currentPath;

  totalResult.innerHTML += totalPath;

  results.scrollTop = results.scrollHeight;

Enter fullscreen mode Exit fullscreen mode

Below is the live demo @CodePen.

This project described above only can be used for practicing purposes, so it might not cover all the cases which come with real web projects such as, responsiveness, browser compatibility.


Congrats! we have finished with the project and, now only remains is to measure the distance between your favorite stars!

So count stars, measure distance and have fun.

PS: if you enjoy this article, you can support me at ko-fi. I always appreciate your support and, it encourages me to keep going.

Happy Coding!


Top comments (0)


Thank you.

Thanks for visiting DEV, we’ve worked really hard to cultivate this great community and would love to have you join us. If you’d like to create an account, you can sign up here.