MegaTag is a **Limelight-specific** robot localization system that uses AprilTags + IMU (gyro) data to determine the robot's position on the field. It's not a Limelight-only concept — similar approaches exist with PhotonVision + WPILib pose estimators — but "MegaTag" specifically refers to Limelight's implementation.
**Key point:** Team 2890 uses **PhotonVision**, not Limelight. MegaTag documentation is useful for understanding the **concept** of vision-based odometry fusion, but the implementation uses PhotonVision's pose estimator + WPILib's `SwerveDrivePoseEstimator`.
**MegaTag 2** fuses robot orientation data from the IMU with vision pose. This means even if vision is slightly noisy or a tag is partially visible, the gyro steadies the reading.
## How It Works (Conceptual)
```
AprilTag camera → Robot pose (x, y, rotation)
↓
Gyro heading → Corrections / steadying
↓
Pose Estimator → Combines:
- Wheel odometry (always drifting)
- Vision pose (accurate but intermittent)
↓
Final robot position (fused)
```
## For Team 2890 (PhotonVision)
Team 2890 runs PhotonVision on Raspberry Pi, AprilTags on the field. The equivalent fusion happens in WPILib:
```java
// SwerveDrivePoseEstimator fuses:
// 1. Wheel odometry (always running, drifts)
// 2. Vision measurements (accurate when tags visible)
SwerveDrivePoseEstimator estimator = new SwerveDrivePoseEstimator(
kinematics,
gyro.getRotation2d(), // IMU heading
modulePositions, // wheel encoders
new Pose2d(x, y, rotation) // initial estimate
);
// Add vision measurement when tag visible:
estimator.addVisionMeasurement(
photonPoseEstimator.getEstimatedPosition(),
Timer.getFPGATimestamp()
);
```
## The Key Concept for Students
**Robot localization = knowing where you are on the field.**
AprilTags give you a reference point. The gyro gives you orientation. Fusing them gives you accurate, stable position tracking even when:
- Only one tag is visible
- Tags are partially obscured
- Robot is moving fast
This is critical for:
- Autonomous paths that need to return to same spot
2.**Vision pose** — accurate when AprilTags visible
3.**Pose estimator** — fuses both for best of both worlds
4.**Field constants** — AprilTag positions, field dimensions must be correct
The MegaTag concept (vision + gyro fusion) is the same regardless of whether you use Limelight or PhotonVision. Understanding it helps students debug pose estimation issues.
