Lap Timer (计圈计时)

Android lap timer for Mini 4WD / slot tracks. Uses CameraX ImageAnalysis to detect START-line crossings, then records lap times. Optional crossing stills come from an analysis-frame ring buffer (not ImageCapture).

Current release: v4.0.0 · versionCode 26 · minSdk 29

Download a prebuilt debug APK from the Releases page (LapTimer-*-debug.apk), or build from source below.

中文说明 · Changelog · Contributing · Security

Features

• Brightness-band crossing detection (default); optional HSV color multi-car mode
• First crossing starts the clock; later crossings record lap times
• Pre-start digital zoom (up to 4× soft cap) and torch; locked after Start
• Optional crossing photos via analysis-frame ring + short post-roll
• False-trigger guard (settings): narrower band, reject uniform shade, min lap gap
• Session history, CSV export, ZH/EN UI

Requirements

Item	Value
Android Studio	Hedgehog / Iguana+ recommended (JDK 17)
JDK	17
minSdk	29
compileSdk / targetSdk	34
Device	Back camera required

Build

./gradlew :app:assembleDebug
# Windows: gradlew.bat :app:assembleDebug

APK output:

app/build/outputs/apk/debug/app-debug.apk

Install:

adb install -r -t -g app/build/outputs/apk/debug/app-debug.apk

Unit tests:

./gradlew :app:testDebugUnitTest

Dependency resolution may use Aliyun Maven mirrors configured in settings.gradle.kts (helpful in CN networks). You can switch back to Google/Maven Central if preferred.

Usage (short)

1. Grant camera permission; pick or create a track.
2. On the pre-start screen, adjust zoom / torch if needed.
3. Tap Start → place phone on the track → first crossing starts timing.
4. Subsequent crossings add laps; Finish saves the session.

Detection tip: keep the sample band on the lane surface; raise zoom so the car fills more of the band. Use torch + shade for stable lighting if ambient shadows cause false triggers.

Architecture (high level)

CameraX Preview + ImageAnalysis
        │
        ▼
 AnalysisFacade ──► BrightnessCrossingDetector / ColorCrossingDetector
        │                    │
        │                    └── Fire → TimingSession / MultiCarTimingBoard
        │
        └── BurstPhotoPipeline (ring buffer → async JPEG → attach to lap)

Entry activity: MainActivityV3.

Project layout

app/src/main/java/com/timming/laptimer/
  analysis/     detectors + frame ring photo pipeline
  camera/       CameraX preview, zoom/torch, JPEG helpers
  timing/       pure lap timing boards
  data/         Room + DataStore
  ui/v3/        current Compose screens

Contributing

See CONTRIBUTING.md. Please open an issue before large design changes.

License

MIT © 2026 kynasln

Disclaimer

Hobby / DIY timing aid. Not a certified race timing system. Results depend on mounting, lighting, and settings.