Pictures and animations

JETCARP draws every picture with its own renderer, straight from a world, so a picture shows the tiles in their true shapes at any size. This guide covers still pictures of a world, desktop wallpapers cut to fit your monitors, animations, and contact sheets that put many runs side by side. Most still pictures carry their scene inside the PNG, so a picture can always be traced back to the run that made it (see Scenes).

A picture of the world

In the app, File → Export PNG… (Ctrl+E) saves the whole tiling as a PNG; the menu shows the size it will be, such as Export PNG (2400 × 1600)…. Export size, just below it, sets the scale: fit gives 2400 pixels on the longer side, and a number gives that many pixels per tile (up to 64). Large images are written in strips, so they need little memory. Tile outlines are drawn where tiles are at least 7 pixels across.

On the command line, jetcarp run writes a picture of the state it ends in:

jetcarp run scene.json --gens 500 --png out.png                # 1600 pixels on the longer side
jetcarp run scene.json --gens 500 --png big.png --max-side 8000
jetcarp run scene.json --gens 500 --png exact.png --scale 6    # 6 pixels per world unit

A tile edge is about one world unit, so --scale 6 gives tiles about six pixels across. Very large pictures (a small scale on a huge grid) are streamed in strips too.

In a script, w.save_png(path) draws the world at the screen size given by --screen (1920 × 1080 by default), fitted and centred. Its options set the size ([w, h]), scale, max_side, outlines, a palette for this picture only, and the compression (fast, balanced or high); it returns the path, the size and the number of bytes.

Desktop wallpapers

A wallpaper is a picture at exactly the size of a monitor, or of several monitors side by side.

In the app, File → Export wallpaper… opens a window that writes desktop-sized PNGs of the world while the app keeps running:

• Size: one of your monitors (as detected), All monitors as a span, a preset, or Custom size. For a span, Layout gives each monitor as WIDTHxHEIGHT+X+Y (the way xrandr prints it; Detected fills in yours), and Write chooses One span image, One per monitor (cut so that they line up across the gaps) or Both.
• Fit: Cover (the grid fills the image and is cropped, so no stage colour shows), Contain (the whole grid, letterboxed in the palette’s stage colour), Current view (what the grid view shows now, scaled to the image) or Repeat (a grid with wrapped edges, tiled seamlessly over the image at a chosen tile size).
• Grid: Fresh grid sized for the image builds a new grid of the same tiling, with this world’s automata and seeds, big enough to fill the image at a given number of pixels per tile, and runs it for a given number of generations first (on the GPU if you like). The live world is left alone; Open the fresh grid afterwards replaces it with the new grid if you want to keep it.
• Outlines (Tile outlines when tiles are at least so many pixels across), Palette (as the automata are, or one palette for all), and the Folder (Choose…; by default the wallpapers folder of the output folder). Then Export, and Open folder to see the result.

On the command line, jetcarp wallpaper takes a scene file or a Quick Start id:

$ jetcarp wallpaper life --size HD --gens 200 -o life.png
life.png (1920 x 1080)
$ jetcarp wallpaper life --monitors 2560x1440+0+0,1920x1080+2560+180 --split --gens 100 --palette Glacier
/home/ada/Pictures/JETCARP/wallpapers/Life-on-the-square-grid-1-2560x1440-2026-11-10_18-28-08.png (2560 x 1440)
/home/ada/Pictures/JETCARP/wallpapers/Life-on-the-square-grid-2-1920x1080-2026-11-10_18-28-08.png (1920 x 1080)

jetcarp --json quickstart lists the built-in Quick Start entries with their ids (such as life or langtons-ant); entries of your own are find:… and the atlas’s atlas:…. To show a wallpaper that moves, see Wallpapers, screensavers and the background host.

Animations

jetcarp media render makes a set of media: a PNG frame for every step, an animation, a poster frame and a thumbnail.

$ jetcarp media render --scene docs/scenes/automata/turmite-llrr.json --gens 120 --every 2 --no-frames --out llrr
webp   llrr/anim.webp
poster llrr/poster.png
thumb  llrr/thumb.png

• What to render. --scene FILE renders any scene from its first generation; without --out, the files go to media/<file name> in the output folder. --quickstart ID with a built-in Quick Start entry’s id or number (as jetcarp quickstart lists them) renders that entry’s scene the same way, into media/<id> without --out. The other targets come from the pattern library: --pattern ID (a catalogued pattern, shown alone), --quickstart HANDLE (one of the library’s own Quick Start entries: find:ID under My finds, atlas:…, or a pack’s qs-…), --ant ID (an ant run) and --missing (everything in the library whose media are missing or out of date, limited with --class, --only-patterns, --only-quickstart and --limit). Their media are kept in the library’s media folder, each file once under the SHA-256 of its bytes (see Library format), and rendered again only when something has changed (--force renders anyway). jetcarp media list KIND ID lists a record’s files with their paths, jetcarp media export KIND ID --to DIR copies them into a folder as frames/0000.png…, anim.gif, poster.png and thumb.png, and jetcarp media gc deletes the files no record uses any more (--dry-run lists them first).
• How. A pattern alone is shown on an empty field for --periods full periods (default 1), so the animation loops seamlessly; a spaceship’s window moves with it. From the seed, the whole grid runs for --gens generations (default 200; row rules one frame per row), with a frame every --every generations. --mode overrides the choice.
• Format. With --format auto (the default), the animation is a GIF when each frame is at most 320 × 320 pixels, the whole animation at most 8 million pixels and 256 colours, and an animated lossless WebP otherwise; --format gif and --format webp choose one. --no-frames skips the per-step PNGs.
• Look. Media have a house style: a fixed set of colours for each kind of field, cells coloured by state and never by age (so loops close), outlines where tiles are at least 8 pixels across, whole pixels per cell, about 320 pixels across for a pattern alone and 512 from the seed, and no text. --palette NAME uses a palette instead. Media files do not carry the scene inside them.

In a script, w.media(#{ gens: 200, every: 2 }) renders a world from its current state into a new media folder, and media(#{ pattern: id }) renders library media, with the same options as the command. The animations in this book are made from their scenes by a separate tool, docs/tools/render-animations.sh, which uses Python to crop and label the frames.

Contact sheets

A contact sheet lays many small pictures out in a grid, each labelled, to compare rules at a glance.

• Elementary and hexagonal row rules. File → Export contact sheet… draws the world’s first row rule and the rules after it (64 of them, or all 16 binary hexagon rules) in its palette, and Contact sheet… on an automaton’s card in the CAs pane does the same for that automaton’s rule. From the command line, jetcarp sheet --kind w --from 0 --count 64 --png sheet.png does the same for any range, with --cols, --thumb (pixels per thumbnail), --width (cells across each), --random-row DENSITY (start from a random top row instead of one cell) and --palette. The kinds are w (rules 0 to 255), hexw (0 to 15), hexw3 (0 to 19,682), hext (0 to 255), hext3 and hext3sum (see Hexagonal row rules).
• Sweep folders. jetcarp sheet --folder FOLDER --runs "#12,#40,n=588" --png picks.png lays out chosen runs of a sweep, each under its label (--labels) and rule.
• Quick Start. jetcarp quickstart --sheet sheet.png draws every built-in entry, numbered as jetcarp quickstart lists them, after --gens generations (default 300); --filter TEXT keeps the entries whose name, rule, family or tiling contains the text.
• Isohedral types. jetcarp isohedral --png sheet.png draws all 81 types (see Drawing and editing tiles).

The exploratory search makes contact sheets of its own for reviewing what it found (see Search methods).

Other pictures

• jetcarp hashlife … --png draws a pattern far in the future: with JETCARP’s renderer up to --max-cells cells (16 million by default), and as a density map beyond that (see HashLife and very long runs).
• jetcarp objects scene.json --png objects.png colours each object of a world by its class: still life, oscillator, spaceship or other.
• jetcarp library render ID --png draws a catalogued pattern’s phases side by side (see The pattern library).
• jetcarp pile draws a large sandpile (see Large sandpiles).

The still figures in this book are made with jetcarp run --png and the contact-sheet commands, from the scene named under each, by docs/tools/render-figures.sh.