How to Use JavaScript or Jquery to Read a Pixel of an Image When User Clicks It

How can I use JavaScript or jQuery to read a pixel of an image when user clicks it?

If you can draw the image in a canvas element then you can use the getImageData method to return an array containing RGBA values.

var img = new Image();
img.src = 'image.jpg';
var context = document.getElementById('canvas').getContext('2d');
context.drawImage(img, 0, 0);
data = context.getImageData(x, y, 1, 1).data;

How to check if a specific pixel of an image is transparent?

Building on Jeff's answer, your first step would be to create a canvas representation of your PNG. The following creates an off-screen canvas that is the same width and height as your image and has the image drawn on it.

var img = document.getElementById('my-image');
var canvas = document.createElement('canvas');
canvas.width = img.width;
canvas.height = img.height;
canvas.getContext('2d').drawImage(img, 0, 0, img.width, img.height);

After that, when a user clicks, use event.offsetX and event.offsetY to get the position. This can then be used to acquire the pixel:

var pixelData = canvas.getContext('2d').getImageData(event.offsetX, event.offsetY, 1, 1).data;

Because you are only grabbing one pixel, pixelData is a four entry array containing the pixel's R, G, B, and A values. For alpha, anything less than 255 represents some level of transparency with 0 being fully transparent.

Here is a jsFiddle example: http://jsfiddle.net/thirtydot/9SEMf/869/ I used jQuery for convenience in all of this, but it is by no means required.

Note: getImageData falls under the browser's same-origin policy to prevent data leaks, meaning this technique will fail if you dirty the canvas with an image from another domain or (I believe, but some browsers may have solved this) SVG from any domain. This protects against cases where a site serves up a custom image asset for a logged in user and an attacker wants to read the image to get information. You can solve the problem by either serving the image from the same server or implementing Cross-origin resource sharing.

Javascript image manipulation? Pixel by pixel

HTML5 Canvas element is the way to go if you don't have browser limitations.

Javascript mouse click coordinates for image

You can actually use HTML for this. The image tag has an attribute known as ismap.

What this attribute does is specify that an image is part of a server-side image map. When clicking on such map, the click coordinates are sent to the server as a url query string.

Images must be nested under links for this to work. Here is an example

<a href="http://www.google.com">
    <img src="myimage.png" alt="My Image" ismap">
</a>

If you can't use image maps for this, here is a javascript/jquery solution

First, you need to include the jQuery library at the bottom of your body tag.

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>  

$(document).ready(function() {    $("img").on("click", function(event) {        var x = event.pageX - this.offsetLeft;        var y = event.pageY - this.offsetTop;        alert("X Coordinate: " + x + " Y Coordinate: " + y);    });});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<img src="http://vignette1.wikia.nocookie.net/seraphina/images/b/b2/Dragonseraphina.jpg/revision/latest?cb=20160103194957" height="200" width="200" alt="dragon">

---

Related Topics