AprilTag Detector for Android

Real-time AprilTag detection on Android. Point your camera at AprilTags and the app draws each detected tag's outline and decoded ID over the live preview.

This is a modernized fork maintained by Binnacle-Tech, continuing the original johnjwang/apriltag-android demo app from the University of Michigan APRIL Robotics Lab. The original targeted deprecated APIs (Camera1, the support library, PreferenceActivity) and no longer builds on current tooling. This fork brings it onto a current AndroidX + CameraX stack while keeping the native AprilTag detector unchanged.

Features

• Detects multiple tag families: tag36h11 (default), tag36h10, tag25h9, tag25h7, tag16h5
• Live camera preview with detections drawn as an overlay (outline + tag ID)
• Fully on-device detection via the native AprilTag C library — no network needed
• Tunable detector settings (decimation, blur, threads, hamming error)
• Optional on-screen diagnostics (camera/detection FPS and latency)

Requirements

• A recent Android Studio
• JDK 17 (required by Android Gradle Plugin 8.x)
• Android NDK and CMake (install via SDK Manager → SDK Tools) to build the native library
• A device or emulator running Android 5.0 (API 21) or higher

Toolchain in use:

• Gradle 8.7, Android Gradle Plugin 8.5.2
• compileSdk / targetSdk 34, minSdk 21, Java 17
• CameraX 1.4.1
• Native libraries are 16 KB page-size aligned, for Android 15+ / 16 KB-page devices

Build & run

1. Open the project root in Android Studio and let Gradle sync.
2. Confirm the NDK and CMake are installed (SDK Manager → SDK Tools).
3. Choose a device or emulator and Run. Grant the camera permission when prompted.

To test in an emulator, add a tag image to the virtual scene by following Google's ARCore emulator instructions.

How it works

The app is a thin Java/AndroidX shell around the native AprilTag C detector.

CameraController configures CameraX with a Preview use case (rendered in a PreviewView) and an ImageAnalysis use case, both bound to the same 4:3 aspect ratio so their fields of view match. Each analysis frame arrives as YUV_420_888; the luma (Y) plane is copied into a packed grayscale buffer and passed to the native detector (ApriltagNative.apriltag_detect_yuv) over JNI, since only the Y channel is needed for detection.

DetectionThread runs detection on a background thread and draws each result — a filled quad, colored border, and the tag ID — onto a transparent TextureView overlay stacked above the preview. Detection coordinates are mapped from image space to view space using the frame's reported rotation and a fit-center transform, so the overlay lines up with what the preview shows. The native library libapriltag.so is compiled from the bundled AprilTag C sources under src/main/apriltag/ via CMake and the NDK.

Settings

Open the gear icon in the toolbar. Changes take effect when you return to the camera view; the refresh icon resets everything to defaults.

Setting	What it does	Default
Tag family	Which AprilTag family to detect.	tag36h11
Image decimation	Downsamples the frame before detection. Higher is faster but reduces range and accuracy.	8
Gaussian blur (sigma)	Blurs the frame before detection; can help with noisy input. 0 disables it.	0.0
Number of threads	Detector worker threads.	CPU core count
Max Hamming error	Bit errors tolerated when decoding a tag. Higher detects more tags but increases false positives.	0
Enable Diagnostics	Shows camera/detection FPS and latency on screen.	Off

Attribution & licenses

• App: originally johnjwang/apriltag-android, an AprilTag detector demo for Android from the University of Michigan APRIL Robotics Lab. This fork modernizes the build system and camera pipeline.
• AprilTag library: the C sources under src/main/apriltag/ are from AprilRobotics/apriltag, Copyright (C) 2013–2016 The Regents of the University of Michigan, developed in the APRIL Robotics Lab under the direction of Edwin Olson. Distributed under the BSD 2-Clause license; the original copyright and license notices are retained in the source headers.
• Fonts: Space Grotesk, Inter, and IBM Plex Mono (bundled in res/font/), © their respective authors, licensed under the SIL Open Font License 1.1. Full notices are in licenses/.

Please keep these notices intact in any redistribution.