SurfaceFootprintReplacer and Polygons with holes

This might be a more generic way.

1. GeometryCoercer (Geometry Type: fme_composite_surface): Transform solid into composite surface.
2. Deaggregator (Split Composite: Yes): Decompose the composite surface into individual simple surfaces.
3. 2DForcer: Transform the surfaces into 2D geometries.
4. GeometryFilter: Keep area features only (since the 2DForcer transforms vertical wall surfaces into lines).
5. Dissolver: Dissolve the areas for each set of features from the same original solid.

Good to hear.

The 1st way can be applied only when all walls are precisely vertical. If a wall is sloped or bulged (including a case where there is a a slight error in x and/or y direction between bottom edge and top edge of the same wall), the AreaBuilder may not create correct footprint.

The 2nd way can be applied to the case where the walls are sloped. It's the advantage.

I expect that the 1st way can be applied to most of CityGML dataset, but consider using the 2nd way if you find a sloping or bulging wall.

Found a simpler way.

1. 2DForcer: Transform a solid into 2D geometry (an aggregate consisting of areas and lines).
2. Deaggregator (Mode: Flatten All Levels): Decompose the aggregate into individual geometries.
3. GeometryFilter: Keep area features.
4. Dissover: Dissolve the areas for each set of features from the same original solid.

Note: The 2DForcer transforms a solid into an aggregate consisting of areas and lines, and the Dissolver performs de-aggregation automatically if the input feature has an aggregate. Therefore the Deaggregator and GeometryFilter may be omitted.