Both IFC and SketchUp use the same fundamental concept for efficient geometry storage: define a geometry shape once, then reference it from multiple placements. In SketchUp this is a component instance; in IFC it is an IfcMappedItem. Understanding how these two mechanisms correspond is directly relevant to anyone converting SketchUp models to IFC for BIM workflows - it determines whether the output IFC file is compact and structured, or expanded and redundant.
How SketchUp Components Work
A SketchUp component definition stores geometry (faces, edges, materials) once. A component instance is a lightweight reference to that definition with a transformation matrix - position, rotation, and scale. Placing 50 windows in a SketchUp model using component instances stores the window geometry once and adds 50 small transform records.
Without components (using groups or raw geometry), the same 50 windows would store 50 full copies of the geometry. For complex models with furniture, equipment, structural members, or facade elements repeated across a building, components can reduce the SKP file size by 60–90%.
How IFC IfcMappedItem Works
IFC (Industry Foundation Classes) has an equivalent mechanism: IfcMappedItem. In the IFC schema:
- IfcRepresentationMap stores the geometry definition - the shape described once in its own local coordinate system
- IfcMappedItem references the IfcRepresentationMap and provides a
MappingTarget- the cartesian transformation operator (position, rotation, scale) that places the shape in the building model - Multiple IfcMappedItem instances can reference the same IfcRepresentationMap, storing the geometry once regardless of how many placements exist
IFC also supports nested mapping - an IfcMappedItem can itself be part of a larger mapped geometry definition, enabling hierarchical reuse for complex assemblies like curtain wall panels containing repeated frame profiles.
Comparing SketchUp Components and IFC IfcMappedItem
| Concept | In SketchUp SKP | In IFC |
|---|---|---|
| Geometry definition (stored once) | Component Definition | IfcRepresentationMap |
| Placed reference | Component Instance | IfcMappedItem |
| Transform per placement | Position, rotation, scale matrix | IfcCartesianTransformationOperator3D (MappingTarget) |
| Nested reuse | Nested components | Nested IfcMappedItem within IfcRepresentationMap |
| File size benefit | Geometry stored once regardless of instance count | Same - IfcRepresentationMap referenced many times |
| Edit propagation | Edit definition, all instances update | Change the map, all mapped items reflect the change |
SKP to IFC Conversion: How Autoconverter Maps Components to IfcMappedItem
When Autoconverter converts a SketchUp SKP file to IFC, it maps the SKP component structure to the corresponding IFC instancing structure:
- SKP component definition to IFC IfcRepresentationMap - the geometry is written to IFC once as a shared representation map
- SKP component instance to IFC IfcMappedItem - each placed instance becomes an IfcMappedItem referencing the map, with its transformation (position, rotation) preserved in the MappingTarget
- Nested SKP components to nested IfcMappedItem - component hierarchies are preserved as nested mapped items in the IFC output
The result is an IFC file that is compact - not an expanded flat geometry dump - and schema-compliant with buildingSMART standards for geometry sharing in BIM models.
Practical Impact: File Size and BIM Tool Compatibility
For architectural models with heavily repeated elements - apartment buildings with repeated unit layouts, office buildings with repeated workstation modules, facades with repeated panel assemblies - the difference between flattened and mapped IFC output is significant. A model with 200 identical chair instances might produce:
- Flattened IFC - 200 full geometry definitions; large file; slow to load in Revit, ArchiCAD, or IFC viewers
- IfcMappedItem IFC - 1 geometry definition + 200 mapped references; compact file; fast to load; correctly recognized as repeated elements by BIM tools
IFC viewers and BIM coordination platforms (Solibri, BIMcollab, Navisworks) handle IfcMappedItem correctly - the mapped items are displayed as individual instances of the shared geometry at their correct positions.
Scaling Limitation in IFC IfcMappedItem
The IFC IfcCartesianTransformationOperator3D used as the MappingTarget supports uniform scale. Non-uniform scaling (different scale factors on X, Y, Z) is supported via IfcCartesianTransformationOperator3DnonUniform, but support for this entity varies across IFC viewers and BIM tools. For maximum compatibility, avoid non-uniformly scaled component instances in SketchUp before converting to IFC - uniformly scaled or unscaled instances convert most reliably.
Frequently Asked Questions
Does every IFC viewer support IfcMappedItem?
Yes. IfcMappedItem is a core IFC geometry entity in IFC2x3 and IFC4 - all conformant IFC viewers and BIM tools support it. The mapped items display correctly as individual geometry instances at their respective positions and orientations.
What happens to SKP groups when converting to IFC?
SketchUp groups are not the same as components - they are unique geometry containers with no shared definition. Groups convert to individual IFC geometry representations without the IfcMappedItem optimization. For the most compact IFC output, use SKP components (not groups) for repeated elements before converting.
Is there a free trial?
Yes. The free evaluation version of Autoconverter supports up to 10 file conversions including SKP to IFC. The full licensed version provides unlimited conversions and batch processing.
Summary
SketchUp component instances and IFC IfcMappedItem are the same concept in different formats - geometry defined once, referenced from multiple placements with individual transforms. Autoconverter maps SKP component definitions to IFC IfcRepresentationMap and SKP instances to IfcMappedItem, producing compact, schema-compliant IFC output that preserves the reuse structure of the original SketchUp model. Use SKP components (not groups) for repeated elements to get the most efficient IFC output.
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