When you import a mesh into SketchUp - whether from OBJ, FBX, DAE, or any other format - the result is often a fully faceted model with every polygon edge rendered as a hard visible line. This happens because SketchUp's SKP format does not store per-vertex normal vectors the way FBX, OBJ, and DAE do. Instead, SketchUp uses a different mechanism: smooth and soft edge flags. Understanding how this system works explains both why imported models look faceted and how to fix it - either inside SketchUp or automatically during conversion with Autoconverter.
How SketchUp Stores Smoothing: Edge Flags vs Vertex Normals
Most 3D formats store smooth shading as per-vertex normals - a normal vector at each vertex that is an average of the surrounding face normals. The renderer interpolates shading between face boundaries using these averaged normals, producing smooth appearance even on faceted geometry.
SketchUp's SKP format works differently. Instead of per-vertex normals, SKP stores two Boolean flags per edge:
- Smooth flag - controls shading: when set, SketchUp interpolates shading across the edge, blending the face normals on both sides and producing a smooth appearance between adjacent faces
- Soft flag - controls visibility: when set, the edge is hidden in SketchUp's normal display mode; typically set together with the smooth flag for curved surfaces
The practical result is the same as per-vertex normals for rendering - edges flagged smooth display with blended shading - but the data is stored differently. A cylinder in SketchUp looks smooth because the edges along the curved surface are flagged soft and smooth, not because vertex normals are stored.
How to Smooth Edges in SketchUp Manually
SketchUp's built-in Soften/Smooth Edges tool applies the smooth and soft flags to selected edges:
- Select the geometry you want to smooth (triple-click to select all connected faces and edges)
- Right-click and choose Soften/Smooth Edges from the context menu, or go to Window > Soften Edges to open the Soften Edges panel
- Adjust the Angle between normals slider - edges where the angle between adjacent face normals is less than this value will be smoothed; edges with larger angles remain hard
- Enable Smooth normals to apply the smooth flag; enable Soften coplanar to also hide edges between coplanar faces
The angle slider works the same way as the maximum angle threshold in Autoconverter's normal smoothing - lower values preserve more hard edges, higher values smooth more aggressively. A value of 40°–60° works well for most organic and product models; lower values (15°–30°) work better for mechanical parts where sharp edges are intentional.
Why Imported Models Look Faceted in SketchUp
When you import an OBJ, FBX, or DAE file into SketchUp directly, the smooth shading from the source file is lost because SketchUp's importer does not convert the source file's vertex normal data to SKP smooth/soft edge flags. All edges arrive as hard edges, and the model looks fully faceted regardless of how smooth it appeared in the source tool.
There are two ways to restore smooth shading after import:
- Manually in SketchUp - select all geometry and use Soften/Smooth Edges with an appropriate angle threshold. This works but requires manual intervention after every import.
- Automatically during conversion - use Autoconverter to convert the source file to SKP. Autoconverter reads the vertex normal data from the source format and writes the appropriate smooth/soft edge flags to the SKP output file, producing a model that displays correctly in SketchUp without any manual edge softening step.
How Autoconverter Handles Normal-to-SKP Smooth Edge Conversion
When Autoconverter converts OBJ, FBX, DAE, X3D, 3DM, or other formats to SKP, it reads the vertex normal data from the source file and determines which edges should be flagged smooth in the output SKP:
- For edges where the source vertex normals indicate smooth shading (averaged normals spanning the edge), Autoconverter sets the smooth and soft flags in the SKP output
- For edges where the source normals indicate a hard edge (separate normals on each side of the edge), Autoconverter leaves the edge as a standard hard edge
- If the source file has no vertex normals (e.g., STL), Autoconverter applies the configurable angle threshold smoothing - the same angle-based averaging described in the Soften Edges tool - to compute which SKP edges to flag smooth
The result is a SKP file where curved surfaces display smooth and hard geometric edges remain visible - matching the intended appearance of the source model without requiring manual edge softening in SketchUp after import.
Formats That Carry Smooth Normal Data for SKP Conversion
| Source Format | Vertex Normals | SKP Output |
|---|---|---|
| FBX | ✅ Yes | Smooth edges from FBX normals; correct shading in SKP |
| OBJ | ✅ Yes (vn lines in file) | Smooth edges from OBJ vertex normals |
| DAE (Collada) | ✅ Yes | Smooth edges from DAE normal data |
| X3D | ✅ Yes | Smooth edges from X3D normal vectors |
| 3DM (Rhino) | ✅ Yes (mesh normals) | Smooth edges from Rhino mesh normals |
| GLTF/GLB | ✅ Yes | Smooth edges from GLTF normal accessor data |
| USD/USDZ | ✅ Yes | Smooth edges from USD primvar normals |
| STEP/IGES | ❌ No (NURBS, not mesh) | Smooth edges computed from angle-threshold tessellation |
| STL | ❌ No (face normals only) | Smooth edges computed from angle-threshold averaging |
Frequently Asked Questions
Why does my FBX look smooth in Maya but faceted when I import to SketchUp?
SketchUp's native FBX importer does not convert FBX vertex normals to SKP smooth/soft edge flags - all edges arrive as hard. Convert the FBX to SKP using Autoconverter instead, which reads the FBX normals and writes the correct smooth edge flags to the SKP output.
What angle should I use for Soften/Smooth Edges in SketchUp?
For most architectural and product models, 40°–60° is a practical starting point. Increase the angle if curved surfaces still look faceted; decrease it if sharp geometric corners (right angles, bevels) are being incorrectly smoothed. For mechanical parts with intentional hard edges, keep the angle below 30°.
Does smoothing edges in SketchUp affect the geometry?
No. Soften/Smooth Edges only changes the display flags on edges - it does not move, add, or remove vertices or faces. The underlying polygon mesh is identical before and after smoothing; only the shading and edge visibility change.
Is there a free trial?
Yes. The free evaluation version of Autoconverter supports up to 10 file conversions including conversion to SKP with smooth edge preservation. The full licensed version provides unlimited conversions.
Summary
SketchUp stores smooth shading using per-edge smooth and soft flags rather than per-vertex normals. When models are imported to SketchUp without converting these flags, they appear faceted. Fix this manually with SketchUp's Soften/Smooth Edges tool using an appropriate angle threshold, or automatically by converting to SKP with Autoconverter, which maps vertex normal data from OBJ, FBX, DAE, GLTF, and other formats to the correct SKP smooth/soft edge flags - preserving the intended smooth shading without any manual step after import.
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