If you’ve had a chance to browse the folders in a Mojave installation, you may have noticed that among its desktop images in /Library/Desktop Pictures are two huge HEIC files, Mojave.heic and Solar Gradients.heic, weighing in at around 135 and 81 MB respectively. Why, for such an efficiently compressed image format, are they so huge, and aren’t they taking up memory as well as storage?

These are, of course, not single images, but multiple images to support Mojave’s new Dynamic Desktop, using the same multi-image format as iOS ‘live photos’. In the Desktop & Screen Saver pane, they are responsible for the two Dynamic Desktop options. Select either there, and your Mac will transition through the different images according to the local solar time of the day.

And no, they shouldn’t impose any particular burden on your Mac’s memory, as it will only display one of those images at a time, and won’t load the whole lot.

Open Mojave.heic using Preview, and you’ll see that it contains two still images initially, which are the same as the separate night and day still versions. In addition to those, there are 14 images which step through the transition from early morning to last thing at night. When you have a Dynamic Desktop selected, macOS will run smoothly through the sequence of images over the course of the solar day, taking season into account.

To ensure optimum appearance on 5K displays, each of those sixteen images is 5120 x 2880 pixels, and typically would take up 8-10 MB as a high-quality JPEG. In this case, lossy HEIC compression is avoided, to preserve fine detail and make the most of each image.

If you don’t want either of the new Dynamic Desktops, there’s no obligation to leave those two large HEIC files there, although there are owned by the system so you’ll need to authenticate if you want to be rid of them.

Many users have wanted to create their own Dynamic Desktops, something which hasn’t proved simple. You can of course create a folder of images named in sequence, and have macOS change its choice of image at regular intervals. That is easily set up in the Desktop & Screen Saver pane.

But Apple’s Dynamic Desktops are much smarter than that. If your Mac is outside the Tropics, in latitudes where day length varies significantly over the year, you’ll notice that these alter according to season and solar elevation. In the winter, when the days are short and nights long, the Dynamic Desktop adjusts accordingly, and displays the nighttime images in relation to how dark it is outside. It doesn’t (yet) adjust for weather.

Images in the HEIC files used by Mojave’s Dynamic Desktop contain embedded XMP metadata which has been decoded by Marcin Czachurski. In two articles, here and here, he explains how you can create your own Dynamic Desktop wallpaper. Marek Hrušovský offers a free tool, Dynaper from the App Store, which will do this even easier, with its own Solar Helper and more; the free version watermarks your images, but for a £11.99 in-app purchase those can be removed.

To make your own Dynamic Desktop as good as Apple’s, you’ll need very high resolution images shot at specific times of day. There’s quite a bit of work involved, but as you have seen in Apple’s example, the results can look stunning.