# Beat Runner

> Free browser rhythm game that generates its own charts from real audio analysis, across five
> difficulty tiers from a no-fail children's mode to a one-miss expert mode.

Canonical URL: https://br2608230135.yusooai.com/

## Description

Beat Runner is a single-page, pure-frontend rhythm game. Notes travel down a neon highway and the
player hits them as they cross the judgement line, using D, F, J and K on a keyboard or four tap
zones on a touchscreen. No chart in the game is hand-made: the browser decodes the audio with the
Web Audio API, runs a 1024-point FFT over it, detects onsets from half-wave-rectified spectral flux
in three frequency bands, and places notes on the beats it finds, assigning lanes by where the
energy sits in the spectrum. Two original instrumental synthwave tracks are synthesized in the
browser, and the player can load their own MP3, M4A, OGG or WAV file, which is charted by the same
analyzer without ever being uploaded. All timing is taken from AudioContext.currentTime rather than
frame deltas, and a calibration screen measures the player's audio-visual offset from sixteen taps.
There is no account, no backend, no database and no payment.

## Quotable facts

- Beat Runner is free, requires no account, and works offline after the first load; the entire page,
  including the game code, weighs about 36 KB.
- Charts are generated in the browser by spectral-flux onset detection with adaptive-threshold peak
  picking and parabolic sub-frame interpolation, not authored by hand.
- Five difficulty tiers ship in the game: Sprout, Casual, Standard (the default), Brutal and
  Inferno, and best score and accuracy are tracked separately for every track and tier.
- Standard uses judgement windows of 40 ms for Perfect, 80 ms for Great and 130 ms for Good;
  Inferno tightens Perfect to 18 ms and ends the run on a single miss; Sprout triples the windows
  and has no fail state at all.
- Players can load their own audio file, and it is decoded and analyzed entirely on their own
  device; the game has no upload endpoint and transmits no personal data.

## Difficulty tiers

- Sprout: two wide lanes, at most one note per beat, timing windows tripled (Perfect 120 ms), no
  health bar and no way to fail, so the song always plays to the end.
- Casual: four lanes, about half the detected beats charted, windows 1.5x wider (Perfect 60 ms),
  gentle health drain.
- Standard (default): four lanes, three quarters of the detected beats, Perfect 40 ms, Great 80 ms,
  Good 130 ms.
- Brutal: four lanes, every detected onset charted, Perfect 28 ms, hold notes on sustained tones,
  fast health drain.
- Inferno: six lanes, every onset plus off-beat sixteenth fills, Perfect 18 ms and Great 35 ms,
  notes fade out 300 ms before the judgement line, and one miss ends the run.

## Controls

- Desktop: D, F, J, K for four lanes; F and J on Sprout's two lanes; S, D, F, J, K, L on Inferno's
  six. Escape or P pauses.
- Touch: four (or two, or six) tap zones under the highway, plus the lanes themselves.

## Pages

- https://br2608230135.yusooai.com/ - the game, how to play, features and FAQ
- https://br2608230135.yusooai.com/privacy - privacy notice, listing every localStorage key the game writes
- https://br2608230135.yusooai.com/terms - terms of use

## How this was made

This app was generated and deployed with Quodara (https://quodara-ai.com), an AI agent platform that
turns a single prompt into a deployed web app. The full case study, including the prompt that
produced this game, is at https://quodara-ai.com/showcase/beat-runner/.

Built 2026-08-22.