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Tracker App — Zeroing at the Camera Nodal Point

Summary

When using the MRMC Tracker App, the phone must be zeroed at the camera's nodal point — not at the robot base or an arbitrary position. If the phone is zeroed at the Flair system origin (which sits under the centre of the robot base), all imported move data will carry a large positional offset and the robot will not reproduce the intended camera path. The fix is to hold the phone at the lens nodal point, with the robot in Home position, and press the house/zero button in the Tracker App before recording. A related issue: the phone cannot be physically placed at the nodal point of the lens (which may be inside the lens barrel), so a small physical offset is inherent — see TRACKER FBX/CSV Offsets Tool for a software method to correct this offset.

Symptoms

  • Imported tracker move results in unexpected large offset from current robot position.
  • Robot moves to a completely different location than expected on import.
  • Move looks correct in the Tracker App replay but is wrong when run on the robot.
  • Move path is correct in shape but positioned relative to the Flair system origin, not the camera.

Community Guidance

[RESOLVED] Zero the Tracker at the Camera Nodal Point

Community — Peter Constan-Tatos, Jeremy Andrews — December 2021

Peter Constan-Tatos: "Ebek where did you zero the tracker? It's best to put bolt into home position and hold the phone at the camera nodal when you zero it (using the house button on the tracker)."

Jeremy Andrews: "So the problem is that the tracker zero (3D origin) is not being set as close as possible to the camera nodal point (as Peter suggested). It's still set at the Flair origin which is under the centre of the robot base."

Correct zeroing procedure:

  1. Move the robot to Home position.
  2. Physically hold the iPhone/iPad as close as possible to the lens nodal point of the camera (front of the lens, or the marked nodal point for prime lenses).
  3. Hold the phone steady and press the house icon (zero/reset) button in the Tracker App.
  4. The app now treats this position as the 3D origin.
  5. Begin recording the move.

confidence_score: 0.88

[INFORMATIONAL] Accuracy of Nodal Zeroing

Community — Simon Wakley — 2021

Simon Wakley: "Zeroing the Tracker accurately is VITAL. If you home it on the rig and it is out by even 1 degree, it will not work right. I would be tempted to level the camera and zero it on the front of the lens."

The phone should be: - Level (matching the camera's horizon) — any tilt error at zeroing will carry through to the entire move. - As close to the optical centre of the lens as possible. - Stationary during the zero press — the app averages a brief window of IMU data.

confidence_score: 0.85

[INFORMATIONAL] Physical iOS Offset from Lens Nodal

Community — Timothy Heys Cerchio, Jeremy Andrews — 2023

Because the iOS device cannot be placed exactly at the lens nodal point (which may be inside the lens barrel), there is always a small physical offset between where the phone is zeroed and the true nodal point.

Timothy Heys Cerchio: "Basically you just need to find the fixed relation between your iPhone Lens and the Nodal Point on the Shooting Camera and then apply that offset in reverse when you transfer to the rig."

For a workflow to mathematically correct this offset using a browser-based visualiser tool, see: TRACKER FBX/CSV Offsets Checker Tool.

confidence_score: 0.83

[INFORMATIONAL] Import Option: Translate to Current → Camera

Community

When importing the Tracker App CSV in Flair, the Translate to Current option with Camera mode attempts to align the imported move origin with the current camera position. This partially compensates for zeroing errors — but only if the robot is in the correct physical position at the time of import. The preferred approach is to zero accurately in the app rather than relying on import-time correction.

confidence_score: 0.80