Rotations

A rotation is what the screensaver or the background host shows over time: a list of items, each a screensaver profile, a scene file, a Quick Start entry or a whole series of rules, played in order, shuffled, or one a day. Each item has the conditions that end it, such as a number of minutes, the world settling, or a time of day, and can be limited to certain days and hours. A rotation can also have a short script that decides when to move on. This guide covers the built-in rotations, using one, and making your own.

The built-in rotations

Use a rotation

• Screensaver mode in the app: in the Screensaver pane, set Show to A rotation and choose it under Rotation, or start the app with jetcarp --screensaver --rotation "Wolfram series".
• The background host: choose it under Rotation in the pane’s Background host section, or run jetcarp host rotation "Classics, hourly" (add --screen N for one screen of several). jetcarp host start --rotation NAME runs one for that run only.
• The system’s screen saver: jetcarp host saver install --rotation NAME, or Screen saver runs in the app. On macOS the rotation is copied into the screen saver, so install it again after changing the rotation (see Wallpapers).

While a rotation runs, the ◀ and ▶ buttons in the pane, Next and Previous in the host’s menu, jetcarp host next and previous, and jetcarp app next for the app’s screensaver skip between items. A rotation that names an item that no longer exists (a deleted profile, say) leaves it out; one that cannot be found at all shows Life instead.

Make a rotation in the app

In the Screensaver pane, set Show to A rotation. Under the rotation’s name are Edit, New, Duplicate and, for one of your own, Delete (for an edited built-in rotation, Revert to built-in). The editor has:

• Name, and Order: In order, Shuffled (a new random order each time through) or One a day (one item per calendar day, the next one tomorrow; it restarts if it ends during the day). Default sets the minutes for items with no ending of their own; untick it and such items run until something else moves on.
• Settings for every item: a cell size, a speed, a palette, whether periodic tilings are rebuilt to fill the screen, and No reseeding. These apply to every item that is not a stored profile.
• The items. Add profile, Add scene…, Add series (one item per rule, for example every Wolfram rule) or a Quick Start entry’s handle typed into the box (such as rule:B3/S23, langtons-ant, an atlas entry’s atlas:… or one of your own find:…; press Enter). Each item can be moved and removed, and has:
  • Ends (any of these), from Add ending: After minutes, After generations, When settled (the same test the reseed policies use, with an optional limit in generations), On low activity (under a share of cells changing, for a number of generations), When finished (a rows rule reaches the bottom, or nothing can change any more), and At a time of day. With no ending, an item takes its profile’s minutes, else the rotation’s default.
  • Hold: seconds to keep showing the finished world before moving on.
  • Only at certain times: the days of the week, and hours Between two times (which may cross midnight). Outside them the item is skipped.
  • Settings for this item, as above, for this item alone.
  • For a series: the Rules (a series name such as W or HexW, a template with {n}, or a list), the Tiling, From and To, and the Seed.
• Script: Add a script (see below).

Save keeps it, Save and start runs the screensaver on it at once, and Close discards the edits. Editing a built-in rotation saves your own copy under the same name.

Rotation files

Each rotation of your own is a JSON file in the rotations folder of the settings folder (see Folders), so rotations can be written by hand, kept and shared. The app, the host and the Windows screen saver read the same files; the macOS screen saver gets a copy when it is installed. This one shows Brian’s Brain until it settles, then Langton’s ant for ten minutes, and from 10 pm to 6 am also the 256 elementary rules:

{
  "name": "Evenings",
  "description": "Brian's Brain until it settles, then a Langton's ant for ten minutes; the Wolfram rules at night.",
  "order": "in_order",
  "default_minutes": 5.0,
  "items": [
    {"source": {"kind": "profile", "name": "Brian's Brain"},
     "end": [{"kind": "settled", "latest": 20000}], "hold_s": 3.0},
    {"source": {"kind": "quick_start", "id": "langtons-ant"},
     "end": [{"kind": "minutes", "minutes": 10.0}],
     "look": {"cell_px": 4.0, "speed": 240.0}},
    {"source": {"kind": "series", "rules": "W", "from": 0, "to": 255},
     "end": [{"kind": "done"}], "hold_s": 3.0,
     "schedule": {"from": "22:00", "to": "06:00"},
     "look": {"cell_px": 4.0, "palette": "Ink"}}
  ],
  "script": "if s.population == 0 { \"next\" }"
}

An item has a source, which is one of {"kind": "profile", "name": …}, {"kind": "scene", "path": …}, {"kind": "quick_start", "id": …} and {"kind": "series", "rules": …, "from": …, "to": …, "family": …, "boundary": …, "seed": …} (a series becomes one item per rule, on a 200 × 140 grid of the family, square and dead edges by default). It may also have a label, a list of end conditions, a hold_s in seconds, its own look, and a schedule with days (mon to sun) and from and to times. The end conditions are {"kind": "minutes", "minutes": …}, {"kind": "generations", "gens": …}, {"kind": "settled", "latest": …} (latest optional), {"kind": "low_activity", "below": …, "for_gens": …}, {"kind": "done"} and {"kind": "clock", "at": "07:30"}.

A rotation’s script

A rotation’s script runs about once a second while the rotation plays. It sees a map s describing what is showing now, and a map state in which it can keep anything it likes between runs:

Its answer decides what happens: nothing (or "continue") carries on, "next" and "previous" move on, "reseed" reseeds the world now, a number goes to that item, and #{ item: "label" } goes to the item with that label. The script runs with tight limits on how much work and memory it may use, and its print output is discarded. New scripts start from this one:

// Runs about once a second. `s` describes what is showing, `state` keeps
// anything you like between runs. Answer "next", "previous", "reseed",
// an item number, or #{item: "label"}. Anything else just continues.
if s.generation > 20000 { "next" }

This is a different, much smaller language environment from jetcarp script: none of the scripting API’s functions are available in it.

More: Wallpapers, screensavers and the background host and Reseeding.