CityJSON Geometry Processing for volume and area computation

A C++ application for processing 3D building models stored in the CityJSON format. The project operates and outputs an LoD2.2 building geometries from the 3DBAG dataset and performs geometry simplification, constrained triangulation, volume, and roof area computation before exporting a valid CityJSON file.

Contributors

• Julia Fossa Marques
• Belina Aileen Santoso

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invalid 3D geom, non-watertight...

Features

• Extracts and preserves only LoD2.2 building geometries and merging of BuildingPart children objects into their parent Building.
• Triangulates polygonal surfaces while preserving semantic information.
• Computes building volumes using signed volume computation.
• Calculates the total roof surface area from semantic RoofSurface objects.
• Exports a valid CityJSON 2.0 model enriched with computed attributes.

Processing Pipeline

1. LoD2.2 Extraction

The application removes all Levels of Detail except LoD2.2. The remaining geometry is transferred from each BuildingPart to its parent Building, simplifying the object hierarchy.

2. Surface Triangulation

Each polygonal surface is triangulated without relying on CGAL's built-in triangulation function.

The implemented workflow:

1. Compute the best-fitting plane for each polygon via PCA.
2. Project polygon vertices into 2D via the plane.
3. Perform a constrained triangulation.
4. Identify interior triangles using the odd-even rule.
5. Extract the interior triangles back to 3D using the plane’s to_3d()

Consistent orientation must be preserved while validity should also be maintained; this is especially important for meshes with holes, in which orientation (CCW) must be opposite of outer envelope orientation (CW)

3. Building Volume

Building volume is computed via the signed volume calculation for 3D mesh as tetrahedron is formed by each surface triangle and a chosen fixed reference point.

The calculated value is stored as:

geo1004_volume

4. Roof Surface Area

All semantic surfaces labelled RoofSurface are identified, and their combined surface area is calculated.

The result is stored as:

geo1004_total_roof_area

Technologies

• C++17
• CGAL
• nlohmann/json
• CMake

Project Structure

├── cpp
│   ├── src
│   │   ├── main.cpp
│   │   └── main.h
│   ├── include
│   │   └── json.hpp
│   ├── validation.md
│   ├── validate_cityjson.py
│   ├── CMakeLists.txt
│   └── README.md
├── data
│   ├── nextbk_2b.city.json
│   └── 9-284-556_out.city.json
├── img
│   ├── a.png
│   ├── b.png
│   ├── c.png
│   └── output.png
└── README.md

Building

Clone repository and go to the C++ project directory:

cd cpp
mkdir build
cd build
cmake ..
make

Usage

Run executable by providing a CityJSON file as input:

./cityjson_processor path/to/input.city.json

The outputted model is written to the same directory as the input file, with an addition _out appended to the filename

Example:

9-284-556.city.json
becomes
9-284-556_out.city.json

Dataset

This project is designed to process building models from the 3DBAG dataset, an open dataset containing detailed 3D models of buildings throughout the Netherlands stored in the CityJSON format.

Output

The generated CityJSON model:

• contains only LoD2.2 geometries and replaces BuildingPart objects with their parent Building. Triangulated surfaces are stored as well.
• includes computed building volume,
• includes total roof surface area,
• follows specification of the CityJSON 2.0 specification.

Acknowledgements

This project was developed in collaboration with Julia Fossa Marques. Her GitHub contributions may not appear in the contribution history because the project was uploaded as a newly created repository by the repository owner. This repository contains an independently uploaded version from a collaborative academic assignment. However, course-specific materials and submission-related information have been removed for privacy while preserving contributor attribution.