HOTARU-AR World

Leave glowing AR messages, treasure chests and lanterns at real places. Anyone nearby finds them on the map, holds up their camera, and opens them right where they were left.

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Inspiration

Hotaru is Japanese for firefly. On summer nights in Japan, fireflies turn an ordinary riverbank into a place you remember. I wanted the same thing for the streets we walk every day: small lights that only exist in one spot, left there by someone, waiting for the next person who passes by.

Most messaging apps send words to a person. HOTARU sends words to a place. An anniversary note where two people first met, a tip at a favorite café, the last clue of a treasure hunt: the place itself becomes part of the message.

What it does

  • Place it. Write a message and place a glowing box where you stand. Fine-tune the spot on an illustrated map, and choose who can open it: set a password, make it open only once, or hide it from the map so only people who know where to look will find it.
  • Find it. The Map tab shows every light within 500 m and how many meters are left before you can open it.
  • Open it in AR. Hold up your camera and the lights appear in the street, floating above the ground. Tap to open. Everything you find is kept in My Boxes.
  • Play with it. Lay a trail of bread markers and put a treasure chest at the finish to build your own hunt. Line a street with paper lanterns and turn it into a festival night. Collect rare coins other people have left around town.
  • Private worlds. Tap the app icon on the Home screen five times and enter a number. Anything placed there is visible only to people who enter the same number, so a family or a class can run its own hunt.

HOTARU is free: finding, opening and collecting are always free, and the free plan can place 5 boxes and 5 bread markers every day. HOTARU Premium removes the limits and unlocks treasure chests, lanterns and coins.

How I built it

  • iOS app: Swift, SwiftUI, ARKit and RealityKit. Every marker is generated in code (glow, float, spin, and the opening animation) and its label is baked into a texture once and cached, so dozens of lights stay smooth.
  • Placing things in the real world: GPS is only accurate to a few meters, so the app treats GPS as "are you close enough" and places each light by its real distance and bearing from the AR session's origin. Once a light has been seen by the camera, its horizontal position never moves again, even when GPS jumps.
  • Backend: Supabase (Postgres with PostGIS) with anonymous sign-in. Nearby lights come from a single spatial query; passwords are stored as hashes, separate from the message.
  • Monetization: RevenueCat with StoreKit 2 for the Premium subscription, plus a RevenueCat webhook into a Supabase Edge Function so the server knows who is Premium and enforces the placement limits, not just the UI.
  • Languages: Japanese, English and Traditional Chinese, switchable inside the app.

Challenges

  • Keeping AR honest with GPS. Early builds let markers drift and jump whenever the GPS reading changed. I rebuilt the placement around one fixed session origin and a rule that nothing moves once it has been seen, and recorded field traces on real streets to replay and check the math.
  • App Review. The first version was rejected several times: text contrast, missing subscription details on the paywall, and a front-camera feature a reviewer's iPad could not find. Each rejection made the app clearer. I removed what was confusing instead of explaining it.
  • Client-only limits are not limits. A purchase first unlocked the UI while the server still refused treasure chests. Wiring RevenueCat's webhook into the database fixed it and made Premium work across devices.

What I learned

Location-based AR is mostly about trust: the light has to be where you expect it, every time. Small details (a glow that breathes, a sound when a lantern is lit, a map that looks like the objects you place) matter more than feature count.

What's next

Events and local guides: shops, schools and towns placing lights for visitors, and shared hunts that more people can join at once.

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