How to set the opacity of a Circle using FabricJS?

In this tutorial, we are going to learn how to set the opacity of Circle using FabricJS. Circle is one of the various shapes provided by FabricJS. In order to create a circle, we will create an instance of fabric.Circle class and add it to the canvas. We can customize a circle object by adding a fill colour to it, eliminate its borders or even make changes in its dimensions. Similarly, we can also change its opacity by using the opacity property.

Syntax

new fabric.Circle({ opacity: Number }: Object)

Parameters

options (optional) − This parameter is an Object which provides additional customizations to our circle. Using this parameter colour, cursor, border width and a lot of other properties can be changed related to the object of which opacity is a property

Options Keys

opacity − This property accepts a Number that allows us to control the opacity of an object. The default value of opacity property is 1.

Example 1

Default appearance of a circle object

Let's see a code to see how our circle object looks like with the default value of opacity property.

<!DOCTYPE html>
<html>
   <head>
      <!-- Adding the Fabric JS Library-->
      <script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/510/fabric.min.js"></script>
   </head>

   <body>
      <h2>Setting the opacity of Circle using FabricJS</h2>
      <p>Here we haven't used the <b>opacity</b> property, but by default, it is set to 1. This is the default appearance. </p>
      <canvas id="canvas"></canvas>

      <script>
         // Initiate a canvas instance
         var canvas = new fabric.Canvas("canvas");
         var circle = new fabric.Circle({
            left: 115,
            top: 50,
            radius: 50,
            fill: "#ff1493"
         });

         // Adding it to the canvas
         canvas.add(circle);
         canvas.setWidth(document.body.scrollWidth);
         canvas.setHeight(250);
      </script>
   </body>
</html>

Example 2

Passing the opacity property as key

In this example, we will see how assigning a value to the opacity property changes the opacity of the circle object in our canvas. Here we have used 0.3 as opacity which makes our circle object look translucent instead of fully opaque.

<!DOCTYPE html>
<html>
   <head>
      <!-- Adding the Fabric JS Library-->
      <script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/510/fabric.min.js"></script>
   </head>

   <body>
      <h2>Setting the opacity of Circle using FabricJS</h2>
      <p>Here we have set the <b>opacity</b> at 0.3, which is why the circle appears dull. </p>
      <canvas id="canvas"></canvas>
   
      <script>
         // Initiate a canvas instance
         var canvas = new fabric.Canvas("canvas");
         var circle = new fabric.Circle({
            left: 115,
            top: 50,
            radius: 50,
            fill: "#ff1493",
            opacity: 0.3
         });

         // Adding it to the canvas
         canvas.add(circle);
         canvas.setWidth(document.body.scrollWidth);
         canvas.setHeight(250);
      </script>
   </body>
</html>