Edge Shading Flags¶

SketchUp stores soft/smooth display intent on Edge entities, not directly on faces. The observed modern TLV path is:

entities (0x1388)
  edges (0x138A)
    edge record (0x0BB8)
      entity base (0x07D0)
        entity flags (0x07D3)

0x07D3 is a compact integer bitfield. The bits relevant to edge display are:

Bit	Name	Meaning
`0x01`	hidden	Edge is hidden from normal display
`0x02`	casts shadows	Generic drawing-element shadow state
`0x04`	receives shadows	Generic drawing-element shadow state
`0x08`	soft	Edge is softened
`0x10`	smooth	Adjacent faces may share smoothed normals across the edge

The normal visible drawing-element baseline is 0x06. Smoothed cylindrical geometry, for example, can use 0x1E on longitudinal edges (0x08 | 0x10 plus the baseline) and 0x06 on hard circular boundaries.

The public Edge.flags field intentionally uses a normalized three-bit layout: 0x01 hidden, 0x02 soft, and 0x04 smooth. Parsing and writing translate between that public representation and the modern wire bits above.

Curved-Surface Behavior¶

Both edge classes carry the public smooth bit (0x04). The visual behavior is still not “smooth every flagged edge”: vertical edges should shade smooth between neighboring side faces, while horizontal ring edges should remain a hard break between side faces and the top/bottom caps.

The importer therefore treats the SKP smooth bit as permission to smooth, then checks adjacent face normals. If the dihedral angle is above the smoothing threshold, the edge remains sharp. With the current 40 degree threshold:

side-to-side cylinder edges (~15 degrees) shade smooth
cap-to-side ring edges (90 degrees) shade flat/sharp

Generated triangulation diagonals do not correspond to SKP edges and should not be treated as source edge-shading instructions.

Importer Mapping¶

Entities.prepare_mesh() carries source edge IDs and flags into PreparedFace. PreparedMesh.to_indexed() preserves that data in IndexedPreparedMesh.face_edge_ids and face_edge_flags.

The Blender importer uses those arrays to:

Find adjacent faces for each source edge.
Check whether the source edge has the smooth bit.
Compare adjacent face normals.
If at least one edge actually needs smoothing, set Blender polygon smoothing for the participating faces and mark only the necessary boundaries as sharp.

This keeps cylindrical sides smooth without rounding cap edges. If no source edge requires smoothing, the Blender importer leaves the mesh in Blender’s default flat-shaded state and does not mark extra sharp edges. The Blender importer exposes this behavior as the smooth_edges import option; disable it to keep all imported faces flat shaded.

When vertices are not merged by the consuming importer, adjacent faces cannot share normals even if the source edge is smooth. The Blender add-on enables vertex merging by default, which allows the imported edge-shading data to take effect.

Blender Export Mapping¶

Export uses Blender’s authored shading state rather than reversing the importer’s angle-based interpretation. For every edge that remains after mesh conversion, the exporter counts adjacent polygons and maps the state as follows:

Blender condition	Public `Edge.flags`	SketchUp result
Exactly two adjacent faces, both `use_smooth`, edge not `use_edge_sharp`	`0x02	0x04`
Either adjacent face has `use_smooth = False`	`0x00`	Hard
Edge has `use_edge_sharp = True`	`0x00`	Hard
Fewer than two adjacent faces	`0x00`	Hard boundary
More than two adjacent faces	`0x00`	Hard non-manifold boundary

Requiring both adjacent faces prevents a smooth polygon from rounding its boundary with a flat polygon. Requiring exactly two faces also prevents boundary and non-manifold edges from becoming invisible soft edges. Hidden state is independent and may be combined with these normalized public flags when present in a hand-authored skppy model.

The exporter deliberately does not apply the importer’s 40 degree threshold. If Blender displays two smooth faces across a non-sharp edge, the corresponding SketchUp edge is soft and smooth regardless of angle. Conversely, an explicit sharp mark always wins. Coplanar merging can remove eligible internal edges before this mapping; retained material, UV, outer, hole, non-manifold, and non-coplanar boundaries receive the flags described above.