The lake

Each frame, the height of the water steps forward on the GPU: a damped wave equation on half-float textures that swap every step, at a fixed 60 Hz whatever the display runs at.

Sampling eight neighbours rather than four keeps a ring round as it spreads, and the shore stops it dead.

Painting the terrain takes the better part of a second on a laptop and several on a phone. A worker does it on an OffscreenCanvas, and the layers come back as bitmaps whose memory is handed over rather than copied.

Sit down to fish and the lake shows a QR code and six letters. The phone that scans them becomes the rod: the way it points steers the hook, a bite tugs in your hand, and lifting it sharply brings the fish out. The page is the game; the phone only sends where it points and when it was lifted, sixty times a second.

Pairing is the part that can fail, so there are two servers that can do it and they share nothing: our own Cloudflare Worker, one hibernating Durable Object per room, and PeerJS's free public one. The page opens the room on both. The phone tries ours, and brings PeerJS in as well if ours is slow, unreachable or has no page in that room. Whichever answers carries the game, and if it drops the other takes over.

Once either has introduced them the messages go straight between the devices over a WebRTC data channel, usually a few milliseconds apart, with no server in the path: the aim on an unordered channel that never resends, since a late sample is worthless, and everything else in order. Where a network will not allow a direct path the relay keeps carrying them itself, a little slower but never stuck. A socket can die without closing, so both sides send a heartbeat Cloudflare answers without waking the room, and one phone holds a room at a time.

A phone does not report where it points, and the one angle that looks like it (beta) stops meaning anything once the phone is rolled. So the aim is read from the whole orientation: the three angles are turned into the direction of the phone's top edge in the world, or of its back for a phone held up like a screen, and the yaw and pitch of that direction become how far out and how far to the side the hook goes. Rolling the phone then does not move the aim, and which way it is held is decided once, when you point at the screen to set the centre.

A lift is read from the same direction rising fast, rather than from the rotation rates, because Safari and Chrome do not name those axes the same way. And while the phone is swinging the aim holds where it was a moment before, so the lift itself does not drag the hook off the fish it was lifted for. What is left is smoothed with a one euro filter: steady when the hand is still, quick when it moves.

None of it is required. With no sensors to read, refused, not on https, or simply absent, the lake on the phone becomes a trackpad: the float follows your finger and a tap lifts.