history

Gait trajectory image-coordinate preview

id: LK-20260716-gait-image-coordinates
date: 2026-07-16
sequence: 67
type: fix
compatibility: compatible
component: `labkit_GaitAnalysis_app` | `1.1.0 -> 1.1.1`
scope: trajectory preview

Context

Marker coordinates use the image convention with an upper-left origin and Y increasing downward. Gait Analysis plotted those values on MATLAB's default Cartesian axes, so the trajectory preview appeared vertically flipped relative to the source video.

Decision and rationale

Preserve the imported coordinate values and scientific calculations. Reverse only the trajectory preview's Y axis so its visual orientation matches Video Marker and Image Marker. Time-series angle and step plots explicitly retain the conventional upward Y direction.

Changes

• Render trajectory previews with a reversed Y axis.
• Restore the normal Y direction when switching to Angles or Steps.
• Document the preview coordinate convention.

User and data impact

Tracked motion now appears in the same orientation as the source image. Stored coordinates, calculations, tables, and exports are unchanged.

Compatibility and migration

The change is display-only and requires no project or data migration.

Validation

The hidden Gait Analysis workflow test checks both trajectory and time-series axis directions while exercising source load and analysis.

Evidence

• Gait Analysis documents the image-coordinate preview convention.

Known limitations and follow-up

The preview assumes imported marker coordinates follow the documented image coordinate convention. Generic coordinate files with a Cartesian convention must be converted by their producer before import.