Mercator vs Mollweide: orientation mismatch (azimuth or vertical flip)

[mbreiser]

Summary

There are orientation issues in the 2D projection views:

1. Mollweide pole alignment is still WRONG

The pole marker and pattern convergence in the Mollweide view are visually obviously incorrect. Despite the v0.9.38 sign fix attempt, the alignment does not look right.

2. Eye FOV overlay is also wrong in Mollweide

The eye FOV boundary overlay is now incorrect in the Mollweide view (it was correct before the sign changes).

3. Mercator vs Mollweide orientation mismatch

The Mercator and Mollweide 2D projection views show a discrepancy in pattern orientation relative to each other. For symmetric patterns (rotation, translation), it's unclear whether the mismatch is:

1. A vertical flip (north-at-top vs south-at-top)
2. A 180° azimuth offset (longitude zero shifted)

Both possibilities would look identical for symmetric patterns.

Suggested approach

Simplest fix may be to vertically flip the Mollweide and Mercator rendered images (i.e., flip the canvas or negate y in the display transform) rather than changing pattern code or moving vectors. This would preserve:

• Generator output (correct, matches MATLAB)
• 3D viewer vectors (confirmed correct)
• Eye FOV CSV data (pre-computed from MATLAB)

The flip should happen at the rendering/display level only.

Context

• The 3D viewer pole vector alignment was confirmed correct by the user (El=+15° → above equator)
• Multiple attempts to fix the latitude convention in projection-viewer.js have made things worse
• The eye FOV overlay was working before the sign changes and is now broken
• The underlying issue may be simpler than previously thought — just a display-level vertical flip

Investigation needed

• Test a simple canvas/axis flip in Mollweide and Mercator rendering
• Verify eye FOV overlay returns to correct position after flip
• Verify pole markers align with pattern convergence after flip
• Use asymmetric test pattern to confirm orientation matches 3D viewer

Priority

Medium — does not affect pattern generation or .pat file output, but incorrect visualization is misleading for users verifying patterns.