Wallpapers, screensavers and the background host

JETCARP can keep cellular automata running on your screen in four ways:

• Screensaver mode in the app fills the screen (or several) with a scene until you press Esc. It runs while the app is open.
• The background host is a separate program, jetcarp host, that keeps scenes running with no app window: in a window of its own, or as the live desktop background behind your windows. It can start at login.
• The system’s screen saver. On Windows and macOS, JETCARP can be added to the system’s own list of screen savers. GNOME has no such list; JETCARP’s lock-screen mode does the job there.
• Still wallpapers, pictures cut to fit your monitors, are covered in Pictures and animations.

What any of these shows is a screensaver profile, a rotation (a list of items that change over time), a scene file or a Quick Start entry. For a first, step-by-step setup, see Your first live wallpaper.

What works on which system

None of this needs administrator rights: everything JETCARP sets up lives in your own folders and settings, and Uninstalling undoes it.

What has been tested

These features were tested by hand in November 2026 on three machines, on the code of JETCARP’s predecessor CART just before it took its present name. The renamed program has not yet been tested on these machines again.

• Fedora 44, GNOME 50.6 on Wayland, one monitor: the live desktop background, the top-bar menu, and the lock screen in both modes, including unlocking with a real password and stopping the host by force while locked (GNOME’s own background came back and unlocking still worked). Several monitors have only been tried in a headless GNOME session. Since those tests, the part of the extension that keeps the host’s windows out of the overview and the window switcher has been rewritten, and the new version has not yet been tested in a real GNOME session.
• Windows 11 Home (version 26H2) on a laptop with two graphics adapters, with and without a second monitor: the live desktop background under the icons (including after Explorer restarts and after a change of display scaling), the screen saver (install, preview, starting after the idle time, removal), the still picture, sleep, and the pauses on battery and when busy.
• macOS 27.0.1 on an M2 Max MacBook Pro, with and without an external monitor: the live desktop background, the screen saver (preview, starting after the idle time, two screens, sleep while it runs, removal), sleep with the lid closed, the still picture on one screen, and the pauses on battery and when busy.

Not tested yet: starting at a real login on any of the three (the entries are written and removed correctly, but no one has logged in with them), a fresh boot on Windows, Linux desktops other than GNOME, Intel Macs, and any other versions of these systems.

Screensaver mode in the app

View → Screensaver mode (F12), or Start the screensaver in the Screensaver pane, hides every control and runs the screensaver; Esc leaves it. It runs full screen on the app’s monitor, or on the monitors chosen in the Display pane.

View → Screensaver settings… opens the Screensaver pane:

• Show chooses what it runs: One profile, A rotation, The current world, A scene file or A Quick Start entry (such as rule:B3/S23).
• While it runs has the caption (Show what is running (the rule and the grid), its corner, size, whether it includes the generation count, and how long it stays up after each new item; press I while the screensaver runs to show or hide it), Keep the computer awake while it runs, Leave on any key or mouse movement (else Esc only), Hide floating and popped-out windows, Start AI control, and Hide the pointer after a delay.
• Background host controls the host (see below).
• Profiles lists and edits the screensaver profiles (see below).

The app can also start straight into screensaver mode, for example from a desktop shortcut:

jetcarp --screensaver --rotation "Wolfram series" --quit-on-exit
jetcarp --screensaver --profile Life --monitors 0,1 --span --cell-px 3 --caption
jetcarp --screensaver --quickstart rule:B3/S23 --keep-awake

--scene FILE shows a scene, --mirror shows the same view on every monitor instead of one canvas across them (--span, the default), and --quit-on-exit closes the app when the screensaver ends. jetcarp --help lists every option. A running app with control switched on (Tools → AI control… or jetcarp --control) can be steered from a terminal with jetcarp app screensaver on --profile NAME, jetcarp app next, jetcarp app profiles and the other jetcarp app commands.

Screensaver profiles

A profile is a named, complete setup for the screensaver: what to show (a scene kept in the profile, a scene file or a Quick Start entry), and optionally a palette and colouring for every automaton, reseed policies that replace the scene’s own, the cell size in pixels, whether to rebuild the tiling to fill the screen, the speed, the GPU simulation, and how many minutes it gets in a rotation. JETCARP has eleven built in: Life, HighLife, Day & Night, Hexagonal Life, Brian’s Brain, Star Wars, Triangles, Snub square Life, Penrose waves, Cyclic spirals and Hodgepodge.

To make your own, set up a world as you like it and press Save current world as profile, or select a profile and change it in the editor below the list. The easiest way to tune one is to press Try in this window, adjust it with the Reseed pane, the palettes and the Run menu, and then press Update from the current world. Tune for this rule runs small copies of the profile’s world with each of the reseed presets and scores how lively each keeps it. Editing a built-in profile saves your own copy under the same name; Revert to built-in goes back.

Profiles are JSON files, one each, in the screensavers folder of the settings folder (see Folders). Export… saves one to share, Import… adds files, and Reload reads them again after editing them by hand.

The background host

The host runs as its own program, so its scenes keep going when the app closes, and only one runs at a time. It shows one or more screens, each running a rotation either in a window or as the desktop background, on a chosen monitor or across all of them.

Start and stop it from the Background host section of the Screensaver pane (Start, Stop, Pause, and ◀ ▶ for the previous and next item), from its menu, or from the command line:

$ jetcarp host start --rotation "Wolfram series"
running, pid 676359, up 0.0s, 90 MB, GPU AMD Radeon 8060S Graphics (RADV STRIX_HALO)
screen 0: Wolfram series · W0 (1 of 256) · generation 4 · 57600 cells
$ jetcarp host next
running, pid 676359, up 3.0s, 90 MB, GPU AMD Radeon 8060S Graphics (RADV STRIX_HALO)
screen 0: Wolfram series · W1 (2 of 256) · generation 198 · 57600 cells
$ jetcarp host stop
stopped

The settings are saved in host.json in the settings folder. The app and the command line both write it and tell a running host to read it again, so a setting changed while the host is stopped applies the next time it starts (saved; the host is not running). Besides pausing on battery and when busy, the host also pauses by itself while its desktop background is completely hidden behind other windows and while the screen is locked and dark. jetcarp host status --json shows what it is holding for, under held.

On Windows and macOS the host has an icon in the notification area or the menu bar, and on GNOME the extension adds one to the top bar. Its menu names what is showing and has Next, Previous, Paused, Show on the desktop, Rotation, Open JETCARP, Start at login and Quit (on GNOME, also Start JETCARP host while it is stopped).

The live desktop background

Tick Show on the desktop in the Background host section, or run jetcarp host wallpaper on. To put different rotations on different monitors, set up one screen per monitor:

jetcarp host wallpaper on --screen 0 --monitor 0
jetcarp host wallpaper on --screen 1 --monitor 1 --rotation "Classics, hourly"

jetcarp host status lists the monitors by number. A screen given no monitor fills every monitor no other screen claims, and jetcarp host wallpaper off turns the screens back into windows.

GNOME needs JETCARP’s GNOME Shell extension, JETCARP live desktop, which shows the host’s windows as the background of the desktop, the overview and the lock screen, and adds the top-bar menu. It supports GNOME Shell 50.

jetcarp host gnome install    # copy it into place and switch it on; log out and back in once
jetcarp host gnome status     # "The GNOME extension is running."
jetcarp host gnome remove

The Background host section shows the same status, with an Install button. Without the extension the host shows a plain window and says why, so nothing is left covering the desktop. The extension only places copies of the host’s picture in GNOME’s background; it takes no part in unlocking, and if the host stops, GNOME’s own background comes back.

On the lock screen (jetcarp host lock, or On the lock screen in the app) the choices are off (GNOME’s own background), live (the default: the scene, blurred behind the clock, with GNOME fading the screen to black as usual) and screensaver (the scene stays sharp and lit until someone reaches for the password box). In screensaver mode the screen and the computer stay awake while locked; --awake MINUTES (default 30, 0 for never) lets the screen go dark after that long.

Windows: the host puts a window of its own under the desktop icons, one per monitor, with no extension needed. macOS: one borderless window per monitor at the desktop’s level, behind the icons, on every Space and ignoring the mouse; the host has no Dock icon while it shows only wallpaper. Other Linux desktops are not supported yet: the host shows a window instead.

A still picture. With jetcarp host still on (Leave a still picture when it stops), the host sets a picture of its scene as the desktop’s own background when it stops, and while the live one cannot be shown, through the system’s own setting. Your own picture is remembered and put back when the still or the live background is turned off, unless you have chosen another picture in the meantime. It is off by default.

The system’s screen saver

jetcarp host saver install adds JETCARP to the system’s list of screen savers and opens the system’s screen saver settings, with JETCARP to choose. --rotation NAME chooses what it runs (otherwise the first screen’s rotation); in the app, the same choice is Screen saver runs, with an Install button. jetcarp host saver status says whether it is installed, and remove takes it out.

• Windows. A copy of JETCARP’s windowless launcher is made at %LOCALAPPDATA%\JETCARP\JETCARP.scr and selected in Screen Saver Settings, as right-clicking a .scr file and choosing Install would. It reads JETCARP’s settings each time it starts, so changes apply at once, and its Settings button opens the JETCARP app. It needs jetcarpw.exe, the windowless launcher, beside jetcarp.exe; a build from source makes it with cargo build --release -p jetcarp --features windows-launcher. Removing it also clears the selection if JETCARP was the chosen screen saver.
• macOS. macOS runs screen savers in a sandbox that cannot read JETCARP’s settings, so installing builds ~/Library/Screen Savers/JETCARP.saver with the rotation and every item’s scene copied inside it, signs it for this machine, and opens System Settings where screen savers are chosen (System Settings, Wallpaper, Screen Saver… on macOS 27). Install it again after changing the rotation. It needs libjetcarp_saver.dylib beside the jetcarp program; a build from source makes it with cargo build --release -p jetcarp-saver.
• GNOME has no list of screen savers. Use the lock screen’s screensaver mode instead: jetcarp host lock screensaver --awake 30.

Start at login

jetcarp host login on (or Start at login, in the app or the host’s menu) makes the host start when you log in; jetcarp host login off stops that. It uses each system’s own per-user mechanism:

The entry runs jetcarp host serve (on Windows through jetcarpw.exe when it is there, so no console window opens) at the path the program has now. If you move the program, turn the setting off and on again. A host already running is left alone.

More: Rotations, Reseeding, Folders and Uninstalling.