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CLUMP_ANALYSIS.md

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XPD Clump Analysis - box.xpd

Discovery

The clump membership for XGen splines is encoded in the clump guide UV coordinates at indices 28-29 of the primitive data array.

Clump Distribution

box.xpd contains 5 clumps with 144 total primitives:

Clump ID Guide UV (u, v) Primitive Count
1 (0.470866, 0.606274) 44 primitives
2 (0.539555, 0.440540) 31 primitives
3 (0.711953, 0.840411) 25 primitives
4 (0.482326, 0.429606) 23 primitives
5 (0.906047, 0.784779) 21 primitives

Total: 144 primitives

Data Format Correction

Previous (Incorrect) Labeling:

Index 25-26: guideUV        -> [0.0, 2.0] (all identical)
Index 27-29: surfaceNormal  -> [?, ?, ?]

Corrected Labeling:

Index 25-26: guideUV               -> [0.0, 2.0] (guide curve UV reference)
Index 27:    clumpType/unknown     -> 0.0 or 3.0 (purpose TBD)
Index 28-29: clumpGuideUV          -> Unique UV per clump (CLUMP IDENTIFIER)

Example Data

Primitive from Clump 1:

{
  "index_27": 3.000000,
  "clumpGuideUV": [0.470866, 0.606274]  // Clump 1
}

Primitive from Clump 3:

{
  "index_27": 0.000000,
  "clumpGuideUV": [0.711953, 0.840411]  // Clump 3
}

Usage in Alembic Export

For proper clump-based hair rendering, we should:

  1. Add clump ID as primitive attribute: Use the clump guide UV as a unique identifier
  2. Group primitives by clump: Create separate curve groups or add clump ID metadata
  3. Preserve clump UV data: Store as a V2f user property on curves

Next Steps

  • Update xpd_reader_detailed.cc to correctly label clumpGuideUV
  • Update xpd_reader_abc.cc to export clump IDs as Alembic arbitrary geometry parameters
  • Update DATA_FORMAT_COMPARISON.md with corrected field names
  • Add clump visualization/grouping in exported Alembic files