Imager.js is an alternative solution to the issue of how to handle responsive image loading, created by developers at BBC News.

There are many responsive image solutions in the wild: srcset, src-n, PictureFill and so on. They are either verbose or hard to debug (and to maintain/integrate). Some of them don't deal well with pixel density and suffer from double asset payload (meaning you end up downloading assets unnecessarily).

We wanted something simple, which works and which is fast as well as network friendly (only download what you need, when you need it).

Imager implements the BBC Responsive News technique which incorporates:

• loading any image once
• loading the most suitable sized image

Imager runs through the following workflow:

1. lookup placeholder elements
2. replace placeholders with transparent images
3. update src attribute for each image and assign the best quality/size ratio URL

Finally, it will lazy load images to speed up page load time even further.

Imager is tested against the following mobile and desktop browsers:

• Chrome stable
• Firefox stable
• IE 8, 9, 10 and 11
• Safari 5, 6 and 7
• Mobile Safari 4, 5, 6 and 7
• Android 2, 3 and 4

<div style="width: 240px">
    <div class="delayed-image-load" data-src="http://placehold.it/{width}" data-alt="alternative text"></div>
</div>

<script>
    new Imager({ availableWidths: [200, 260, 320, 600] });
</script>

This will result in the following HTML output:

<div style="width: 240px">
    <img src="http://placehold.it/260" data-src="http://placehold.it/{width}" alt="alternative text" class="image-replace">
</div>

<script>
    new Imager({ availableWidths: [200, 260, 320, 600] });
</script>

260 has been elected as the best available width (as it is the closest upper size relative to 240 pixels).

Let's say we have generated 4 sizes of images (200, 260, 320 and 600) in 3 different pixel ratio flavours (1, 1.3 and 2):

<div style="width: 240px">
    <div class="delayed-image-load" data-src="http://example.com/assets/{width}/imgr{pixel_ratio}.png" data-alt="alternative text"></div>
</div>

<script>
    new Imager({ availableWidths: [200, 260, 320, 600], availablePixelRatios: [1, 1.3, 2] });
</script>

The img[src] will be computed as following (according to the reported window.devicePixelRatio value by the device):

• http://example.com/assets/260/imgr.png if no pixel ratio is detected, or advertised as 1
• http://example.com/assets/260/imgr-2x.png if pixel ratio is advertised as 2 (or any value greater than 2)
• http://example.com/assets/260/imgr-1.3x.png if pixel ratio is advertised as 1.3

Head to this device pixel density test resource to learn more about the available pixel ratio for your device.

Imager has the ability to replace {width} with a non-numeric value if you provide the widthInterpolator option, which is a function that returns the string to be injected into the image URL for a given width. This feature allows you to use a human readable name or integrate with third-party image providers.

<div style="width: 240px">
    <div class="delayed-image-load" data-src="http://example.com/assets/imgr-{width}.png" data-alt="alternative text"></div>
</div>

<script>
    new Imager({
        availableWidths: [200, 260, 320, 600],
        widthInterpolator: function(width, pixelRatio) {
          return width + 'x' + (width / 2);
        }
    });
</script>

The img[src] will be computed as http://example.com/assets/imgr-260x130.png instead of http://example.com/assets/imgr-260.png.

Alternatively you can define availableWidths as an Object where the key is the width, and the value is the string to be injected into the image URL.

<div style="width: 240px">
    <div class="delayed-image-load" data-src="http://example.com/assets/imgr-{width}.png" data-alt="alternative text"></div>
</div>

<script>
    new Imager({
        availableWidths: {
            200: 'square',
            260: 'small',
            320: 'medium',
            600: 'large'
        }
    });
</script>

The img[src] will be computed as http://example.com/assets/imgr-small.png instead of http://example.com/assets/imgr-260.png.

You might want to generate HiDPI responsive images. But what if you also include images from another provider which serves a totally different set of sizes, without pixel ratio?

Here is an example to serve your own images alongside Flickr images.

<div style="width: 240px">
    <div class="delayed-image-load"        data-src="http://placehold.it/{width}" data-alt="alternative text 1"></div>
    <div class="delayed-flickr-image-load" data-src="//farm5.staticflickr.com/4148/4990539658_a38ed4ec6e_{width}.jpg" data-alt="alternative text 2"></div>
</div>

<script>
    var imgrPlaceholder = new Imager('.delayed-image-load', {
        availableWidths: [200, 260, 320, 600]
    });

    var imgrFlickr = new Imager('.delayed-flickr-image-load', {
        availableWidths: {
            150: 't_d',
            500: 'd',
            640: 'z_d'
        }
    });
</script>

This will result in the following HTML output:

<div style="width: 240px">
    <img src="http://placehold.it/260" data-src="http://placehold.it/{width}" alt="alternative text 1" class="image-replace">
    <img src="//farm5.staticflickr.com/4148/4990539658_a38ed4ec6e_d.jpg" data-src="//farm5.staticflickr.com/4148/4990539658_a38ed4ec6e_{width}.jpg" alt="alternative text 2" class="image-replace">
</div>

The HTML API helps you control how Imager works from the content point of view.

Available placeholders are:

• {width}: best available image width (numeric value)
• {pixel_ratio}: device pixel ratio (either blank or -1.3x, -2x, -3x etc.)

So the following HTML...

<div data-src="http://placehold.it/{width}"></div>

...is converted to...

<img src="http://placehold.it/260" data-src="http://placehold.it/{width}" class="image-replace">

data-width is the enforced size of the image placeholder; where the actual image will eventually be loaded.

This can be especially useful if you don't want to depend on the image container width.

So the following HTML...

<div style="width:600px">
    <div data-src="http://placehold.it/{width}" data-width="300" data-alt="alternative text"></div>
</div>

...is converted to...

<div style="width:600px">
    <img src="http://placehold.it/300" data-src="http://placehold.it/{width}" width="300" alt="alternative text" class="image-replace">
</div>

These two data-* attributes are copied from the responsive placeholder to the response img element.nnot process images or who have image loading disabled. It is converted to the alt attribute of the `img element.

So the following HTML...

<div data-src="http://placehold.it/{width}" data-alt="alternative text"></div>
<div data-src="http://placehold.it/{width}" data-class="london calling"></div>

...is converted to...

<img src="http://placehold.it/260" data-src="http://placehold.it/{width}" alt="alternative text" class="image-replace">
<img src="http://placehold.it/260" data-src="http://placehold.it/{width}" alt="" class="london calling image-replace">

You can create one or several concurrent configurations of Imager within the same page. Its configuration options are described below.

Advanced JavaScript API documentation lies in the docs/ folder.

This option is intended to reflect the available widths of each responsive image. These values will be used as replacements for the {width} variable in data-src placeholders.

The following examples demonstrate the results of passing through different object types for the availableWidths option...

Array: the widths are represented as numeric values

new Imager({
    availableWidths: [240, 320, 640]
});

Object: the widths associate a string value for their numeric counterpart

new Imager({
    availableWidths: {
        240: 'small',
        320: 'medium',
        640: 'large'
    }
});

Function: must return a value for the provided width argument

// will return a double sized image width as a numeric value
new Imager({
    availableWidths: function (image) {
        return image.clientWidth * 2;
    }
});

An Array which indicates what are the available pixel ratios available for your responsive images.

These values will be used as replacements for the {pixel_ratio} variable in data-src placeholders.

new Imager({ availablePixelRatios: [1, 2] });

Default value: [1, 2]

A String which indicates what the className value will be added on the newly created responsive image.

new Imager({ className: 'image-replace' });

Default value: image-replace

An Integer value (in milliseconds) to indicate when Imager will check if a scroll has ended. If a scroll has stopped after this delay and the lazyload option is true, Imager will update the src attribute of the relevant images.

Default value: 250

new Imager({ scrollDelay: 250 });

Notice: set the scrollDelay value to 0 at your own risk; unless you know what you're doing, setting the value to zero will make the user experience totally janky! (and that would be an odd thing to do as you have chosen to use Imager to improve the user experience)

A Boolean value. If set to true, Imager will update the src attribute of the relevant images.

Default value: true

new Imager({ onResize: true });

An experimental Boolean value. If set to true, Imager will update the src attribute only of visible (and nearly visible) images.

Default value: false

new Imager({ lazyload: true });

A Number of extra pixels below the fold taken in account by the lazyloading mechanism.

Default value: 0

new Imager({ lazyload: true, lazyloadOffset: 300 });

A callback Function. Runs after Imager updates the src attribute of all relevant images.

Its first and unique argument is an Array of HTMLImageElement, the ones processed by Imager.

new Imager({
    onImagesReplaced: function(images) {
        console.log('the src of all relevant images has been updated');
    }
});

Additional and fully working examples lie in the demos folder.

• BBC News
• The Guardian
• x-imager Web Component

This is an experiment in offering developers an interim solution to responsive images based on the ImageEnhancer concept researched and developed by the team at BBC News.

At present, support for srcset and PictureFill are not widespread and the polyfills for these solutions also come with a number of drawbacks.

Mark McDonnell (@integralist) documented the process and rewrote the original code so it could be evolved and improved with the help of the open-source community.

The goal of this project is to automate the process with the help of the Grunt JavaScript task runner (potentially via grunt-responsive-images for image generation based on a source directory).

Much of this work can be repurposed to work with a more standards-based approach once support improves in modern browsers.

For the purposes of maintaining a distinguishment between the ImageEnhancer concept built by BBC News and this project, we're calling it Imager.js

• Mark McDonnell
• Tom Maslen
• Thomas Parisot
• Addy Osmani

Copyright 2014 British Broadcasting Corporation

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.