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Getting started with NgOptimizedImage

Note: the NgOptimizedImage directive is currently in the "Developer Preview" mode. The Angular team will stabilize the APIs based on the feedback and will make an announcement once the APIs are fully stable.

The NgOptimizedImage directive makes it easy to adopt performance best practices for loading images.

The NgOptimizedImage directive ensures that the loading of the Largest Contentful Paint image is prioritized by:

  • Automatically setting the fetchpriority attribute on the <img> tag
  • Lazy loading other images by default
  • Asserting that there is a corresponding preconnect link tag in the document head

In addition to optimizing the loading of the LCP image, NgOptimizedImage enforces a number of image best practices:

  • Using image URLs to apply image optimizations
  • Requires that width and height are set
  • Warns if width or height have been set incorrectly
  • Warns if the image will be visually distorted when rendered

Prerequisites

You will need to import the directive into your application. In addition, you will need to set up an image loader. These steps are explained in the Setting up NgOptimizedImage tutorial.

Usage in a template

Overview

To activate the NgOptimizedImage directive, replace your image's src attribute with ngSrc.

<img ngSrc="cat.jpg" width="400" height="200">

The built-in third-party loaders prepend a shared base URL to src. If you're using one of these loaders (or any other loader that does this), make sure to omit the shared base URL path from src to prevent unnecessary duplication.

You must also set the width and height attributes. This is done to prevent image-related layout shifts. The width and height attributes should reflect the intrinsic size of the image. During development, the NgOptimizedImage warns if it detects that the width and height attributes have been set incorrectly.

Marking images as priority

Always mark the LCP image on your page as priority to prioritize its loading.

<img ngSrc="cat.jpg" width="400" height="200" priority>

Marking an image as priority applies the following optimizations:

  • Sets fetchpriority=high (read more about priority hints here)
  • Sets loading=eager (read more about native lazy loading here)

Angular displays a warning during development if the LCP element is an image that does not have the priority attribute. A page’s LCP element can vary based on a number of factors - such as the dimensions of a user's screen. A page may have multiple images that should be marked priority. See CSS for Web Vitals for more details.

Adding resource hints

You can add a preconnect resource hint for your image origin to ensure that the LCP image loads as quickly as possible. Always put resource hints in the <head> of the document.

<link rel="preconnect" href="https://my.cdn.origin" />

By default, if you use a loader for a third-party image service, the NgOptimizedImage directive will warn during development if it detects that there is no preconnect resource hint for the origin that serves the LCP image.

To disable these warnings, add {ensurePreconnect: false} to the arguments passed to the provider factory for your chosen image service:

providers: [ provideImgixLoader('https://my.base.url', {ensurePreconnect: false}) ],

Adjusting image styling

Depending on the image's styling, adding width and height attributes may cause the image to render differently. NgOptimizedImage warns you if your image styling renders the image at a distorted aspect ratio.

You can typically fix this by adding height: auto or width: auto to your image styles. For more information, see the web.dev article on the <img> tag.

Handling srcset attributes

If your <img> tag defines a srcset attribute, replace it with ngSrcset.

<img ngSrc="hero.jpg" ngSrcset="100w, 200w, 300w">

If the ngSrcset attribute is present, NgOptimizedImage generates and sets the srcset attribute using the configured image loader. Do not include image file names in ngSrcset - the directive infers this information from ngSrc. The directive supports both width descriptors (e.g. 100w) and density descriptors (e.g. 1x) are supported.

You can also use ngSrcset with the standard image sizes attribute.

<img ngSrc="hero.jpg" ngSrcset="100w, 200w, 300w" sizes="50vw">

Disabling image lazy loading

By default, NgOptimizedImage sets loading=lazy for all images that are not marked priority. You can disable this behavior for non-priority images by setting the loading attribute. This attribute accepts values: eager, auto, and lazy. See the documentation for the standard image loading attribute for details.

<img ngSrc="cat.jpg" width="400" height="200" loading="eager">

@reviewed 2022-08-25