Image Comparison (A/B)
The A/B comparison tool lets you align and quantitatively compare two images: forward/backward scan pairs, before/after tip changes, or any two frames you want to subtract.
Setting up a comparison
Right-click any preview or pop-out → Compare submenu:
| Action | Effect |
|---|---|
| Set This View as Compare A | Freezes the current view as slot A |
| Set This View as Compare B | Freezes the current view as slot B |
| Compare A with This | Sets A to the stored slot, B to this view, opens dialog |
| Compare B with This | Sets B to the stored slot, A to this view, opens dialog |
| Open A/B Comparison | Opens the dialog with current A and B |
| Swap A and B | Swaps the two slots |
| Clear Compare Selection | Resets both slots |
The comparison dialog opens as a normal tracked popup window.
Compare dialog panels
The dialog renders four panels simultaneously:
- A — image A with the current colormap
- B aligned — image B after applying the current transform
- A − B — difference map (always on
RdBu_r, symmetric) - |A − B| — absolute difference (on
magma)
Metrics shown: overlap area, Pearson r, RMSE, fit slope.
Manual alignment controls
| Control | Effect |
|---|---|
| Rotation | Rotate B relative to A (degrees) |
| Shift X / Shift Y | Translate B in physical units |
| Intensity matching | Scale B intensity to minimise A−B |
| Lock A/B range | Fix both images to the same color scale |
| Stretch | Linear or histogram-equalised display |
Automatic alignment
Auto fit aligns B to A automatically. Two modes:
- Translate — finds the best XY offset
- Rigid — finds the best rotation + translation
Landmark-based alignment
For cases where auto fit is insufficient:
- Enable Pick landmarks.
- Click A1 on image A, then the corresponding point B1 on image B. Continue for A2/B2, etc.
- Choose Translate or Rigid.
- Press Fit from points.
Numbered landmark markers are drawn on both panels. Use Undo point or Clear points to reset.
Profile and overlap analysis
- Profile mode adds a linked crosshair across all four panels with a status readout.
- Overlap scatter panel shows a pixel-wise scatter plot of A vs B with a 1:1 line, Pearson r, and fit slope.
Export
Export saves a 300 DPI 4-panel PNG and an A−B difference CSV.