3D Triangulation¶

LUC3D reconstructs 3D poses from 2D annotations across multiple camera views using two methods: DLT (Direct Linear Transform — fast) and Bundle Adjustment (iterative — most accurate). Set the default in Help ▸ Settings ▸ Default Triangulation; the toolbar dropdowns can also pick a method per action. For the math behind each, see How triangulation works.

Prerequisites¶

Before triangulating, you need:

Camera calibration loaded (TOML or JSON with intrinsic and extrinsic parameters)
2D annotations in at least 2 camera views
Instance groups linking the same individual across views

Triangulating a Frame¶

Single Frame¶

Press T or go to Edit → Triangulate Current Frame.

LUC3D will:

For each instance group in the current frame, gather 2D keypoints from all linked camera views
Compute the 3D position of each keypoint using DLT
Store the 3D point cloud on the InstanceGroup
Generate reprojected instances (the 3D points projected back to each camera's 2D view)

Multiple Frames¶

Go to Edit → Triangulate Multiple Frames to batch-triangulate a range of frames. A dialog lets you specify the start and end frames.

Batch Triangulation¶

For full-session processing:

Edit → Group by Track & Triangulate All — Auto-groups by track assignment, then triangulates every frame
Edit → Group by Identity & Triangulate All — Auto-groups by identity, then triangulates every frame

Reprojection Error¶

After triangulation, LUC3D computes the reprojection error — the distance (in pixels) between where a keypoint was annotated and where the triangulated 3D point projects back onto that camera view.

Reading the Error¶

In the Instances tab of the info panel:

Overall RMS error is shown per instance group
Per-camera errors show which views have the worst fit
Per-node breakdown highlights which keypoints are problematic

Error Color Coding¶

Error Range	Color	Meaning
< 2 px	Green	Excellent — annotation is consistent across views
2–5 px	Yellow	Acceptable — minor inconsistencies
> 5 px	Red	Poor — check for annotation errors or calibration issues

Toggling Error Display¶

Press E to toggle error visualization on the video views
Press R to toggle reprojected instance overlays
Use the Visibility tab for fine-grained control

Robust triangulation¶

An optional reprojection error threshold (set in the Tracking Wizard) automatically drops per-view keypoint observations whose reprojection error exceeds the threshold and re-triangulates from the remaining views. It is off by default. Use it to make triangulation resilient to the occasional badly mislabeled point without hand-nulling each one. See Robust triangulation.

3D Viewport¶

Press \ or go to View → Toggle 3D Viewport to open the 3D visualization.

Import existing 3D points

You can load precomputed 3D points via File ▸ Load 3D Points (H5) and view them without any videos loaded (skeleton + points only). With no video, playback is driven by a frame stepper, and the timeline still works. Cameras that have no linked video are flagged in the 3D viewer.

Viewport Controls¶

Left-click + drag — Orbit the camera
Scroll wheel — Zoom in/out
Right-click + drag — Pan

Viewport Features¶

Camera frustums — Wireframe pyramids showing each camera's position and viewing direction
3D skeleton — Triangulated keypoints connected by skeleton edges
Color coding — Follows the current track/identity color mode

Viewport Buttons¶

Button	Action
Show Camera View	Snap the 3D camera to match a specific camera's perspective
Show Initial View	Reset to the default orbiting viewpoint
Set Env	Freeze the current frame's 3D skeleton as a persistent reference overlay
Clear Env	Remove the environment reference overlay

Troubleshooting¶

High reprojection error

If error is consistently high across all keypoints:

Check that your calibration file matches the video resolution
Verify that camera names are correctly mapped to videos
Re-examine the calibration quality (try recalibrating)

Some keypoints have high error

If only specific keypoints show high error:

Check those keypoints in each camera view — they may be mispositioned
Mark occluded keypoints as occluded (right-click) rather than guessing their position
Keypoints visible in only 1 view cannot be triangulated

Note

Triangulation requires a keypoint to be visible in at least 2 camera views. Keypoints marked as occluded or visible in only one view are excluded from 3D reconstruction.