BIM to Geo conversion using voxels

Contributors: Arda Baysal · Daman Dogra · Tejas Ziegler

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How to build

mkdir build && cd build
cmake .. -DCMAKE_BUILD_TYPE=Release
make -j4

or more modern:

cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build

Requires: CMake ≥ 3.16, CGAL, C++17 compiler.

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How to run

Step 1 — Convert IFC → OBJ (done once, outside the C++ code)

Before running the C++ pipeline, convert the source IFC to OBJ. This requires IfcConvert from the IfcOpenShell library.

How to run: If you are using the provided Conda environment (ifco_env), you can call IfcConvert directly. If you are using a standalone executable, ensure the .exe is in your project root and use ./IfcConvert instead.

Run the following command from the repository root: for wellness_center_sama.ifc:

IfcConvert data/input/Wellness_center_Sama.ifc data/intermediate/Wellness_center_Sama.obj `
     --include entities IfcWall IfcSlab IfcRoof IfcWindow IfcDoor `
     --exclude entities IfcSite IfcOpeningElement IfcFurnishingElement IfcBuildingElementProxy IfcFlowTerminal IfcStair IfcRailing `
     --use-element-names `
     --reorient-shells

or macOS-friendly:

IfcConvert data/input/Duplex.ifc data/intermediate/Duplex.obj \
     --include entities IfcWall IfcSlab IfcRoof IfcWindow IfcDoor \
     --exclude entities IfcSite IfcOpeningElement IfcFurnishingElement IfcBuildingElementProxy IfcFlowTerminal IfcStair IfcRailing \
     --use-element-names \
     --reorient-shells

The resulting OBJ file(s) must be placed in data/intermediate/.

Step 2 — Run the voxelisation pipeline

This is the default configuration that reproduces the submitted CityJSON and the best result in the report (voxel size 0.25 m, with min_width = 0.3 m hard-coded into src/marker.cpp). Note: a 0.25 m grid is the slow, fine-resolution case.

./build/bim2geo data/intermediate/Wellness_center_Sama.obj 0.25 data/output/Wellness_center_Sama_0.25.city.json

Arguments:

Position	Description	Example
1	Path to input OBJ	data/intermediate/mybuilding.obj
2	Voxel size in metres	0.5
3	Path to output CityJSON	data/output/mybuilding.city.json

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Reproducing the experiment results (Section: Experiments and results)

The voxel-size sweep figures in the report are generated by two helper scripts. Both scripts use paths relative to the repository root, so run every command from inside GEO1004_HW3/.

Prerequisites

• The bim2geo binary is built in cmake-build-debug/ (see the build instructions above). The sweep does not compile — it calls the existing binary, so rebuild after any source change.
• The intermediate meshes exist in data/intermediate/*.obj (produced by the IfcConvert step).
• Python 3 with matplotlib is installed (pip install matplotlib).
• The folder data/output/ exists (the binary writes results there).

Steps

1. Build the binary (or press Run in the IDE), so cmake-build-debug/bim2geo is up to date with src/marker.cpp.
2. Run the sweep — this voxelises every OBJ in data/intermediate/ at voxel sizes 1.0 → 0.1 (step 0.05) and writes one CityJSON per (building, size):

bash data/voxel_size_sweep

3. Plot the room counts (one panel per building):

python data/room_analysis.py

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Reproducing each figure

The two filters are compile-time constants in src/marker.cpp (min_volume, in m³, and min_width, in m). To regenerate a specific figure, set the values below, rebuild, clear old results (rm data/output/*.city.json, so the plot is not contaminated by a previous configuration), then rerun steps 2–3:

Figure (label)	min_volume	min_width
vanilla (no filtering)	0.0	0.0*
min_volume (6 m³ / 4 m³)	6.0 / 4.0	0.0*
min_width = 0.3	0.0	0.3

*To make min_width = 0.0 a true "off" switch, the one-line floor if (k < 1) k = 1; in room_marker() must be removed (otherwise one erosion pass always runs regardless of min_width).

Filename convention (do not change)

room_analysis.py parses <building>_<voxelsize>.city.json by splitting on the last underscore, so building names may contain underscores (e.g. Wellness_center_Sama_0.40.city.json) but the voxel size must be the final _<number> before .city.json. Files that don't match are skipped.

Repository layout

geo1004-hw3/
├── CMakeLists.txt
├── README.md
├── data/
│   ├── input/          ← place source IFC file here
│   ├── intermediate/   ← OBJ file(s) from IfcConvert go here
│   └── output/         ← generated CityJSON written here & experiment scripts
├── include/
│   ├── obj_reader.h        (Person A)
│   ├── voxel_grid.h        (Person A)
│   ├── voxeliser.h         (Person B)
│   ├── marker.h            (Person B)
│   ├── surface_extractor.h (Person C)
│   └── cityjson_writer.h   (Person C)
├── src/
│   ├── main.cpp
│   ├── obj_reader.cpp      (Person A)
│   ├── voxel_grid.cpp      (Person A)
│   ├── voxeliser.cpp       (Person B)
│   ├── marker.cpp          (Person B)
│   ├── surface_extractor.cpp (Person C)
│   └── cityjson_writer.cpp   (Person C)
└── report/
    └── report.pdf

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