The Publishing Project

Picture deep dive

When I first looked at responsive images I thought that srcset and sizes would be enough to handle most of my responsive images needs. See Concepts and examples of responsive images and How To Use Responsive Images for reference but notice that I barely touched on <picture> at all in either of those articles.

When researching how to leverage new image formats in the context of HEIF and AVIF, I discovered that we needed to leverage the other part of the responsive images specification: the picture element.

The picture element has more applications that srcset and sizes. We will explore these applications by analyzing five use cases (adapted from Eric Portis’ Responsive Images Done Right: A Guide To </picture><picture> And srcset)

  1. Our images need to be able to render crisply at different device-pixel-ratios. This is the device-pixel-ratio (DPR) use case
  2. If we have a responsive layout, our images will need to fit it. call this fluid-image use case
    • Note that we can tackle use cases one and two together
  3. Sometimes we’ll want to adapt our images beyond scaling, cropping, or changing them in different ways for different screen sizes. This the art-direction use case
  4. Different browsers support different image formats like WebP, HEIF, and AVIF, with a JPEG fallback for browsers that don’t support the new formats. We’ll call this the type-switching use case.

The basics

At its most basic the picture element has one or more source children corresponding to different formats and use cases and an img child that will only be used if the browser can’t handle any of the source children.

<picture>
  <source srcset="apartment.webp" 
          type="image/webp">
  <img  src="apartment.png"        
        alt="Apartment Building" 
        loading="lazy">
</picture>

We’ll explore the 5 use cases in the following sections. We’ll use img elements to illustrate DPR and Fluid Images. When we begin user pictures, these cases will be used inside picture elements.

Device Pixel Ratio

The first use case is using responsive images to server the correct images for high-dpi devices (Mac’s retina displays and high-density monitors or devices).

I would normally handle this with a srcset attribute indicating what images to use for different densities.

The src attribute is the default for when none of the conditions match or when the browser doesn’t understand srcset.

<img  src="my-image.jpg"
      alt="My image"
      loading="lazy"
      srcset="my-image.jpg 1x,
              my-image2x.jpg 2x,
              my-image3x.jpg 3x">

Fluid images

There may be times when we want the image we see on screen to change depending on attributes of the page or the image itself.

Using the w attribute

The first option to specify the image to use is to use the w attribute. This will tell the browser what image to use based on the width of the viewport the user is viewing the content through.

The idea in this example is that browsers will take the image closest to the size of the viewport, over or under, and use it when asked to deliver the image.

<img
  srcset="large.jpg  1024w,
          medium.jpg 640w,
          small.jpg  320w"
  src="small.jpg"
  alt="A rad wolf" />

Combining w and sizes

Using w on its own doesn’t tell the browser how much of the viewport to use for the image we choose and under what conditions.

We can refine the image use by also specifying a size a sizes attribute. This will tell the browser how much of the viewport the image will cover.

In the following example, the sizes attributes do the following:

  1. If the viewport width is at least 36em then use 50% of the viewport width (indicated with the 50vw value)
  2. If the viewport is larger than 36em but smaller than 45em then take 1/3 of the viewport width (indicated by the 33.3vw value)
  3. If neither condition are met then use 100% of the viewport width (100vw)
<img
  srcset="large.jpg  1024w,
          medium.jpg 640w,
          small.jpg  320w"
  sizes=" (min-width: 36em) 50vw,
          (min-width: 45em) and (max-width: 60em) 33.3w,
          100vw"
  src="small.jpg"
  alt="A rad wolf" />

It’s important to point out that these are not media queries, they just look like them.

Using the media attribute in source

The media attribute specifies a condition (similar to a media query) that the user agent will evaluate for each <source /> element.

In the example for this section, we have three source children elements, each with a media condition that the browser will evaluate. IF the condition is not true, then the browser will move to the next source element and fall back to the default image if none match.

<picture>
  <source
    srcset="small.jpg"
    media="(max-width: 36em)">
  <source
    srcset="medium.jpg"
    media="(min-width: 45em) and (max-width: 60em)">
  <source
    srcset="large.jpg"
    media="(min-width: 60em)">
  <img  src="my-image.jpg"
        alt="My image"
        loading="lazy">
</picture>

We can further tailor this with DPR values for each source element. We’ve modified the example so now each source element has a srcset attribute indicating a 1x, 2x, and 3x resolution version of each image.

We’ve also added a srcset attribute to the default image. If the browser doesn’t understand the attribute, it will ignore it.

<picture>
  <source
    srcset="small.jpg,
            small2x.jpg 2x,
            small3x.jpg 3x"
    media="(max-width: 639px)">
  <source
    srcset="medium.jpg,
            medium2x.jpg 2x,
            medium3x.jpg 3x"
    media=" (min-width: 640px)
            and (max-width: 1023px)">
  <source
    srcset="large.jpg,
            large2x.jpg 2x,
            large3x.jpg 3x"
    media="min-width: 1024px">
  <img  src="my-image.jpg"
        alt="My image"
        loading="lazy"
        srcset="my-image2x.jpg 2x,
                my-image3x.jpg 3x">
</picture>

Art Direction Use Case

Art direction is, to me, the most complicated use case for responsive images.

Let’s assume that the small form factors, less than 36em, we will use a portrait version of the image with multiple DPI versions of each image.

This is a somewhat contrived example but it shows how we can leverage responsive images to provide rich user experiences in different form factors while keeping our content performant.

<picture>
  <source
    srcset="portrait.jpg,
            portrait2x.jpg 2x,
            portrait3x.jpg 3x"
    media="(max-width: 36em)">
  <source
    srcset="cropped.jpg,
            cropped2x.jpg 2x,
            cropped3x.jpg 3x"
    media=" (min-width: 45em)
            and (max-width: 60em)">
  <source
    srcset="landscape.jpg,
            landscape2x.jpg 2x,
            landscape3x.jpg 3x"
    media="min-width: 60em">
  <img  src="my-image.jpg"
        alt="My image"
        loading="lazy"
        srcset="my-image2x.jpg 2x,
                my-image3x.jpg 3x">
</picture>

We can add ad many sources, each with its combination of DPI (x), width (w), media, and srcset attributes. As many as we need to cover the art directing needs.

It’s up to us as designers and developers not to let it get out of hand.

Different Formats for Different browsers

The final, and to me most useful, use case for responsive images is serving different file formats for browsers that support them.

It used to be the case that WebP was the only image format that wasn’t universally supported. But that’s not the case anymore. According to caniuse.com Safari 14 (to be released with the new version of iOS and macOS) will support the format leaving IE and Opera Mini as the only browsers that don’t support it.

However, there are two new formats to consider as we future proof our content: HEIF and AVIF. These are next-generation image formats based on video codecs (HEVC for HEIF and AV1 for AVIF) that produce smaller file sizes of equivalent quality.

HEIF is not currently supported by any browser but, considering that the format is supported in both macOS and iOS, it shouldn’t take much for Apple to add support to Safari in the near term.

AVIF is fully supported in Chrome (85 and later) and partially supported in Firefox (behind the image.avif.enabled flag).

So what does mean for our responsive images using picture?

It means a few things:

  1. We need to be careful with the order of the formats. If the browser supports picture and source, it will use the first one it can read
  2. We need to add the type attribute with the mime type of the image format to help the browser figure out if it can use it or not
  3. We still need to provide a format for older versions of modern browsers. This is still a problem in companies where IT will restrict browser upgrades

At the very minimum our picture element will look like this:

<picture>
  <source
    srcset="wolf-medium.heif"
    type="image/heif">
    <source
    srcset="wolf-medium.avif"
    type="image/avif">
  <source
    srcset="wolf-medium.webp"
    type="image/webp">
  <img  src="wolf-medium.jpg"
        alt="Grey Wolf in the snow"
        loading="lazy">
</picture>

When the browser sees this it will run through the following questions

  1. Do I support HEIF?
    • If I do then display the HEIF image
    • If not then go to step 2
  2. Do I support AVIF?
    • If I do then display the AVIF image
    • If not then go to step 3
  3. Do I support WebP?
    • If I do then display the WebP image
    • If not then go to step 4
  4. Display the default JPG image

Using different browsers as an example:

Safari 13.1 will fall through to the default in step 4 as it doesn’t support any of the formats we’re using.

Safari 14.0 will stop in step 3 and serve WebP.

Chrome 85 will stop in step 2 and serve the AVIF image. However, Chrome 84 will stop in step 3 and sever WebP instead.

Firefox 80 with the image.avif.enabled flag enabled will stop in step 2. Without the flag, it will stop in step 3

We can also add other attributes and change the naming scheme to accommodate DPI or Fluid images. We can also leverage the formats with sizes and keep the layout control without losing the benefits of the advanced formats.

The example below adds 2x and 3x HDPI versions to all formats.

<picture>
  <source
    srcset="wolf-medium.heif,
            wolf-medium2x.heif 2x,
            wolf-medium3x.heif 3x"
    type="image/heif">
    <source
    srcset="wolf-medium.avif,
            wolf-medium2x.avif 2x,
            wolf-medium3x.avif 3x"
    type="image/avif">
  <source
    srcset="wolf-medium.webp ,
            wolf-medium2x.webp 2x,
            wolf-medium3x.webp 3x"
    type="image/webp">
  <img  src="wolf-medium.jpg"
        alt="Grey Wolf in the snow"
        loading="lazy"
        srcset="wolf-medium2x.jpg 2x,
                wolf-medium3x.jpg 3x">
</picture>

We can further tailor this with DPR values for each source element. We’ve modified the example so now each source element has a srcset attribute indicating a 1x, 2x, and 3x resolution version of each image.

We’ve also added a srcset attribute to the default image. If the browser doesn’t understand the attribute, it will ignore it.

<picture>
  <source
    srcset="small.jpg,
            small2x.jpg 2x,
            small3x.jpg 3x"
    media="(max-width: 36em)">
  <source
    srcset="medium.jpg,
            medium2x.jpg 2x,
            medium3x.jpg 3x"
    media=" (min-width: 45em)
            and (max-width: 60em)">
  <source
    srcset="large.jpg,
            large2x.jpg 2x,
            large3x.jpg 3x"
    media="min-width: 60em">
  <img  src="my-image.jpg"
        alt="My image"
        loading="lazy"
        srcset="my-image2x.jpg 2x,
                my-image3x.jpg 3x">
</picture>

To conclude, picture-based responsive images can be as simple or complex as we want to make them. We need to keep the KISS Principle firmly in mind as we work in developing these images particularly because HEIF and AVIF tooling is just getting started.