Terrarium.
A pocket voxel world floating in the dark, running an ecology. The terrain grows itself from noise when you arrive; after that the forest is on its own. Trees mature, throw seeds into the neighbourhood, crowd each other out and die of old age; birds carry seeds between canopies. You get two hands and two dials, and neither dial is you: the rain, which moves the water table, and the sun, which is the whole life of the star this island is standing under. Tap the grass to plant, tap a tree to fell it, let a century run, and then, if you want to see how it ends, let the light go.
A little world with rules, and a star it does not control.
Drag sideways to spin the island. Tap the grass and a tree grows; tap a tree and it comes down. Everything after that is the forest seeding itself. Two things decide what it can become, and neither of them is you: the rain, and the star it is standing under.
Bench notes
- The rules
- A sapling needs four things: dry ground above the water table, moisture within reach of standing water, enough room to see the sky, and light to spend. Nothing else is checked. Spacing is read off the moisture and the light, so a damp, well-lit island packs trees two cells apart and a thin one scatters them four, and a tree seeds more often the better it is standing. Everything matures in under two years, seeds for the rest of its life, and dies somewhere past twenty. There is no population target anywhere in the code; the forest stops growing because it runs out of sky.
- Two dials, one clock
- The rain and the sun are conditions you set. The years are the only thing that runs. That split is the whole instrument: you can move a condition in a second, and then you have to sit through the consequence, which arrives in tree lifetimes. Move the rain and the pond swells or shrinks, the beach line follows, the moisture field redraws, and the forest migrates over the following decade. Move the sun and the same machinery answers a different input. Neither dial contains a single instruction about trees.
- The star's life is the real one
- A Sun-like star brightens by roughly ten percent every billion years, which is enough to boil a planet's water off long before the star leaves the main sequence at all. That is the middle of this dial, and it is astrophysics rather than a climate model: the heat lifts the water off the island, the moisture field collapses behind it, and the forest dies back to whatever is left of the pond while the ground goes to straw. Then the core runs out of hydrogen, the envelope swells and reddens and scours the place to rock, gets shed, and leaves a white dwarf with no fusion in it. The water freezes. The stars come out, because there is finally no daylight to hide them. Then the dwarf goes cold, and much later the sky itself empties, and the island is a rock you can only just make out.
- The birds do the planting
- Half the flights between canopies are carrying a seed, and it goes in where the bird lands. That is how the forest crosses ground no single tree could throw a seed over, and it is why the bird count matters: birds arrive as the canopy thickens and leave when it thins, so a wood that is spreading spreads faster than one that is holding.
- What to notice
- You cannot place a forest here. You can only place a tree and set the conditions, and the conditions decide what the tree amounts to. The same tap in a flood year and a red giant produces a wood and a dead sapling. Every system you will ever build sits on inputs you do not own, and this is what it looks like when one of them goes: not a failure of the design, which is unchanged and still correct, but the ground it was standing on being taken away.
- The manners
- Every voxel is an instance of one cube, so the island, the pond, the trunks and the canopies are four draw calls, and only the trees still moving get their matrices rewritten. The canvas claims one-finger horizontal drags and pinch, never vertical, so scrolling past it on a phone always works. The controls arrive switched off and turn on when the world is real, because a button that looks live and does nothing is worse than one that admits it is waiting. Plant a tree does what tapping the grass does for anyone who is not using a pointer. With reduced motion on the whole instrument stays, including the decade skip and both dials; what goes away is only the things that ran on their own.