Use density descriptors
Explore This Demo
- To preview the site, mouse over the editor, press the App button, then the Show button.
- Reload the page using different devices to see the browser load different images.
You can use the device emulator for this. If you're looking for specific display densities, here are some devices to try:
|1x density||Blackberry Playbook, many external monitors|
|2x density||iPad or IPhone 5/6|
|3x density||Galaxy S5 or iPhone X|
index.htmlfor the code that makes this work.
How does it work?
The concept of density descriptors may be unfamiliar to most folks. To better understand them, it helps to have a bit of background on how the browser works with pixels.
What are pixels
Let's start at the very beginning by defining what a pixel is. This sounds simple, but "pixel" can actually have many meanings:
- Device pixel (a.k.a. "physical pixel")
- The smallest dot of color that can be displayed on a device.
- Logical pixel
- Information that specifies the color at a particular location on a grid. This type of pixel has no inherent physical size.
- CSS pixel
- The CSS spec defines a pixel as a unit of physical measurement. 1 pixel = 1/96th of an inch.
Pixel density (also referred to as "screen density" or "display density") measures the density of device pixels in a given physical area. This is commonly measured using pixels per inch (ppi).
For many years, 96 ppi was a very common display density (hence CSS defining a pixel as 1/96th of an inch). Starting in the 1980s it was the default resolution of Windows. In addition, it was the resolution of CRT monitors.
This began to change as LED monitors became common in the 2000s. In particular, Apple made a big splash in 2010 when it introduced Retina displays. These displays had a minimum resolution of 192 ppi, which was twice the resolution of "regular" displays (192 ppi/96 ppi = 2).
With the introduction of newer display technology, "device pixels" began to vary
in physical size and shape
and were no longer the same size as "CSS pixels". The need to define the
relationship between the size of "device pixels" and "CSS pixels" is what led to
the introduction of the
devicePixelRatio (sometimes called the "CSS Pixel
devicePixelRatio defines the relationship between device pixels and CSS pixels
for a particular device. A 192 ppi device has a
devicePixelRatio of 2 (192
ppi/96 ppi = 2) because "2 of its display pixels are the size of 1 CSS pixel".
These days most devices have a device-pixel-ratio between 1.0 and 4.0.
This ratio doesn't have to be a whole number.
2.5 are all
device-pixel-ratios of common devices.
Determine the pixel density of a device by typing
window.devicePixelRatioin the console.
View this table to see the pixel ratios of common devices. Most are between 1.0 and 4.0.
So how do you actually apply this information?
Size images based on device-pixel-ratios
In order for images to look their very best on high resolution screens, it's
necessary to provide different image versions for different
|Device Pixel Ratio||Indicates that:||On this device, an <img> tag with a CSS width of 250 pixels, will look best when the source image is...|
|1||1 device pixel = 1 CSS pixel||250 pixels wide|
|2||2 device pixels = 1 CSS pixel||500 pixels wide|
|3||3 device pixels = 1 CSS pixel||750 pixels wide|
Things to note:
- The pixel dimensions listed in image editors, file directories, and other places are a measurement of logical pixels.
- For higher resolution screens and larger displays you'll need images with larger dimensions. Merely enlarging smaller images defeats the purpose of serving multiple image versions. The browser would have done this anyway if a high resolution image was not provided.
Tools like sharp make it easy to create multiple sizes of an image. This is covered in more detail here.
Use Density Descriptors to serve multiple
Density descriptors, in conjunction with the "srcset " attribute, can be used to serve different images to different devicePixelRatios.
- Take a look at the
index.htmlfile and note the
This example put into words:
3xare all density descriptors that tell the browser the pixel density that an image is intended for. This saves the browser from needing to download an image to determine this information.
- The browser can choose between three images:
flower-1x.jpg(intended for browsers with a
flower-2x.jpg(intended for browsers with a
2.0pixel density), and
flower-3x.jpg(intended for browsers with a
flower.jpgis the fallback image for browsers that do not support
How to use this:
- Use a devicePixelRatio and the
xunit to write density descriptors. For example, the density descriptor for many Retina screens (
window.devicePixelRatio = 2) would be written as
- If a density descriptor isn't provided, it is assumed to be
- Including density descriptors in filenames is a common convention (and will help you keep track of files) but is not necessary. Images can have any filename.
- There is no need to include a
sizesattribute is only used with width descriptors.
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