wip: explore face-centric vector network region model

[softmarshmallow]

Summary

Exploratory PoC investigating a face-centric model for VectorNetworkRegion where fill_rule: FillRule is replaced by fill_mask: u64 — each loop owns one face, a bitmask marks which faces are filled, and fill rules become a codec at the import/export boundary.

• fill_mask_poc.rs — isolated test file with the proposed model, nesting tree computation, fill-rule encode/decode codec, and Skia rendering
• Rendered comparison PNGs — visual proof across 6 concentric-ring scenarios and 6 complex scenarios

Key findings

Scenario	fill_rule (current)	fill_mask (proposed)	faces (ideal)
Concentric rings (simple)	✓ (some patterns need multi-region workaround)	✓ all patterns	✓
Open + closed paths	✓	✓	✓
Shared edges / T-junctions	✓	✓	✓
Self-intersecting (Z, pentagram)	⚠ ambiguous	✗ (1 loop = 1 face)	✓ (needs planarization)
Overlapping circles	✗ double-paints	✗ nesting tree wrong	✓ (needs planarization)

Rendered outputs

fill_mask model (proposed) — concentric rings:

Current model — same scenarios (3 WRONG):

Current model — workaround with multi-region decomposition:

Complex scenarios — stress testing model boundaries:

Design direction

The exploration points toward a face-centric model where:

1. fill_rule is removed from the core struct (becomes SVG/Figma import/export codec)
2. Faces are first-class entities alongside vertices and segments
3. Auto-planarization on draw handles self-intersections
4. Paint bucket maps directly to face.fills = paint (no decomposition puzzle)

Test plan

• cargo test -p cg --test fill_mask_poc — 10 tests pass (7 unit + 3 visual render)

🤖 Generated with Claude Code

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