Synchronous / Asynchronous Image Decode using〈img decoding〉Attribute

How Browser Renders an Image in Page

• First the image is downloaded from the server.
• Then image data is read (decoded).
• Using the decoded data, image is rendered on page.

Controlling Image Decode through the decoding Attribute

The <img> tag can include a decoding attribute through which we can specify a hint to the browser whether decoding of the image should be done synchronously or asynchronously.

The decoding attribute can have 3 values :

• sync : Decode the image synchronously in the main thread. Page rendering and image decoding happens one after the other (depending upon the position of image in the page).

  <!-- synchronously decode the image -->
  <img src="img/test.jpg" decoding="sync" >
  

• async : Decode the image asynchronously. Rendering of page and decoding of image is done in parallel. This obviously makes the page to render faster.

  <!-- asynchronously decode the image -->
  <img src="img/test.jpg" decoding="async" >
  

• auto : This is the default value. It is left to the browser to decide whether to decode the image synchronously or asynchronously.

  <!-- let the browser handle decoding method -->
  <img src="img/test.jpg" decoding="auto" >
  

Improve Page Performance with decoding Attribute

Decoding is an expensive operation. Until the image is decoded, the page remains in a non-responsive "frozen" state.

To improve rendering time of the page, decoding="async" can be specified to decode the image in parallel.

<!-- asynchronously decode the image -->
<img src="img/test.jpg" decoding="async" >

Asynchronous Image Loading and Decoding with Javascript ?

The decode() method can be used to load and decode an image using Javascript. See Pre-Loading and Pre-Decoding Images with Javascript for Better Performance for more.