Using images for every resolution

Published

A couple of days ago I needed to add an icon next to a text into a website I've been working on.

<img src="really-big-icon.png" alt="" />

It was really easy and everything looks nice.

But then, a coworker mentioned that I was delivering a really big file for this small icon. Indeed, by the styles the icon size should be 25px height, and the original icon was 150px. So I went ahead, resize the icon an updated the URL

<img src="icon-25.png" alt="" />

but this time, the icon looked plain bad on my screen. —What's wrong? I wonder myself.

High-density screens #

After a bit of search, I realized I was using a high-density screen. In particular, I've been using a somewhat recently MacBook Pro that has a Retina display, which provides a really crispy image all the time —but not now.

The solution I found implies using the srcset attribute with a list of higher resolution images separated by commas, and let the browser choose what is best for the current screen. In my case, it looked like this

<img
src="icon-25.png"
srcset="icon-25.png 1x, icon-50.png 2x"
alt=""
/>

But I still didn't understand why should I provide an image with the double of the pixels. Pixels are pixels. Right?

"CSS px" #

So I kept reading and I found an interesting article [1] where it states, when we talk about pixels in CSS, we refer to a special unit.

When we talk about CSS px, we are not talking about screen pixels.

CSS px are defined to be small but visible, and in contrast to inches or cm, its size depends from where the user should see the screen. So a CSS 1px in a phone will be smaller than a CSS 1px in a TV because the phone is a few centimeters from our face, but the TV can be a couple of meters away. Anyway, we should be able to see them in both places.

CSS provides this unit, CSS px, that allow us to work for all these different devices without having to worry on how many screen pixels per inch my screen has. All that complexity is hidden behind CSS px. And, of course, in the other CSS units that are px based like em and rem.

In my MacBook Pro screen, the relationship between screen pixels and CSS px is 2x. In order for images to look crisp, I need to provide a resource with the double of the size.

But how do I knew the relationship between screen pixels and CSS px? Thankfully the browser exposes a property called devicePixelRatio. I opened DevTools and I typed

window.devicePixelRatio // my result was "2"

This result depends on the screen the browser is being used. If you happen to have another connected screen, this result may vary. Older desktop screens usually return 1. The newest phones are returning 3.

How many images should I provide? #

This depends on the users of your website. If you need to support mobile device users with high-end phones, you should provide 3x, 2x and 1x versions of your assets.

Is "srcset" supported by browsers? #

This is important. At the time of writing this article, support for srcset attribute is pretty good [2] —a good thing to notice is if we keep the src attribute we will deliver an image to older browsers too.

For this case, it makes sense to deliver the 1x URL in src.

In my example, the code for my icon looks like this

<img
src="icon-25.png"
srcset="icon-25.png 1x, icon-50px 2x, icon-75.png 3x"
alt=""
/>

Conclusion #

Summarizing,

  • Know about your users and which devices they are using
  • Define which resolutions you should provide: 1x, 2x and/or 3x
  • Provides the assets they need by using srcset attribute
  • Deliver the experience for unsupported browsers by using src attribute
  • If it makes sense, remember to add an alternative text using alt attribute

That's all for now. Have a good day!


  1. Bert Boss wrote an article for the W3C explaining the different units available in CSS, a bit of history, and which should we use depending on each situation https://www.w3.org/Style/Examples/007/units.en.html ↩︎

  2. srcset worldwide support is over 90%. Anyway, you should understand who your users are, what devices are using, and provide what makes sense for your case https://caniuse.com/#search=srcset ↩︎

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