Session mode: Annotation comparison tool

[pgivel]

Summary

In session mode (#6), add a comparison interaction: click one annotation, click another, and the tool visually links them with a comparison result (size match/mismatch, color match/mismatch, distance delta).

Use case

"Are these two buttons the same size?" / "Do these two headings use the same color?" — the user has already placed two annotations and wants to compare them without doing mental arithmetic.

Interaction

1. User activates comparison mode (keyboard shortcut, or a modifier like C while in session mode).
2. Click on an existing annotation → it highlights as "selected for comparison."
3. Click on a second annotation → comparison result is displayed.
4. (Optional: Shift+click to compare without entering a dedicated mode — just click two annotations in sequence.)

Display

• A connecting line is drawn between the two annotations.
• A comparison label at the midpoint of the line shows the result:
  • Size annotations (crosshair, container, rect, shrink-to-fit): Match ✓ if dimensions are equal within a configurable tolerance (e.g. ±1 px), or Δ12×3 px showing the width and height difference.
  • Color annotations (New mode: Color picker #8): Match ✓ if colors are identical, or the hex values side-by-side + ΔE color distance. Optionally show WCAG contrast ratio between the two colors.
  • Point annotations (point-to-point New mode: Point-to-point distance with right-angle triangle #9): compare the measured distances — 143 px vs 145 px — Δ2 px.
  • Guide-based measurements (New feature: Distance-to-guide (perpendicular measurement) #11, New feature: Spacing-along-guide #12): compare spacing values.
• Line color indicates result at a glance:
  • Green — match (within tolerance).
  • Red/orange — mismatch, with the delta shown.
• The comparison persists as its own annotation (can be undone with Ctrl+Z).

Tolerance

• Default tolerance: 0 px (exact match) or 1 px (to account for sub-pixel rounding).
• Could be configurable via scroll wheel while in comparison mode, or a global setting.

Implementation notes

• Comparison is a derived annotation type: {type: "comparison", annotationA: id, annotationB: id, result: {...}}.
• Stored in the annotation model alongside regular annotations.
• QML: rendered as a Canvas line + MeasurementLabel for the comparison text.
• Comparison logic lives in Python (@pyqtSlot on RulerBackend) or JS, depending on complexity:
  • Size comparison: trivial arithmetic.
  • Color comparison (ΔE): best done in Python with a small color-distance formula (CIEDE2000 or simpler CIE76).

Depends on

• Session mode: persistent annotations, measurement export & composite screenshot #6 (Session mode + annotation model)
• Works with annotations from any mode: crosshair, rect, container, shrink-to-fit (New mode: Shrink-to-fit rectangle #7), color (New mode: Color picker #8), point-to-point (New mode: Point-to-point distance with right-angle triangle #9), guides (New mode: Orthogonal guides #10).