Auto-Partition.

Auto-Partition is what gives Magic Fill something to flood into. Three settings control it:

Crease threshold — the dihedral angle above which an edge becomes a partition boundary. Low (~30°) = many small regions split on every gentle ridge. High (~80°) = only hard, unmistakable edges separate regions.

Min region (faces) — drops boundaries that would create regions smaller than N faces. Filters out clutter from tiny noisy crease segments while keeping major structural lines.

Surface change — finds soft part-boundaries the crease angle alone steps right over: a horn meeting a head, an arm meeting a torso, anywhere the surface bends gradually instead of folding to a hard edge. Turn it up to split those seams without shattering smooth areas. Magic Fill's Catch soft edges option runs the same detection, so a tap stops at these seams too.

Adjusting any of the three re-computes the partition live; yellow lines reflow.

• The most-impactful setting for how Magic Fill feels.
• Drop the threshold for chunky CAD models with subtle bevels — you want bevels to count as boundaries.
• Raise the threshold for organic models with noisy normals — you don't want every dimple to make a region.
• Raise Min region when the model has lots of tiny crease loops that aren't useful as paint regions.
• Turn up Surface change for figures and creatures where parts blend in instead of meeting at a hard edge — it catches the seam a horn makes against a head.

1. Setup tab → Auto-Partition section.
2. Drag Crease threshold. Yellow lines proliferate at low values, collapse to structural edges at high values.
3. Raise Min region from 20 to 60 to wink out tiny crease segments.
4. Turn up Surface change if a soft seam — a horn meeting a head — isn't getting its own region.
5. Test with Magic Fill. Tap a part, then nudge the sliders — the flood re-snaps to the new seams.
6. Press B to hide yellow boundaries for a clean reference shot.

• Cranking threshold to 80° and wondering why Magic Fill floods too far? That's by design at 80° — only hard edges count, so there's little to stop the flood.
• Hand-drawn boundaries don't get clobbered by a re-partition — drawn boundaries layer on top of auto-partition.
• Magic Fill has its own spread too — scroll to set it (it starts tight at 5°), separate from these sliders. Same idea, different dial.