3D modelling of the built environment

Course Content

The main topics described in the course are:

• voxel models and voxelisation;
• constructive solid models (CSG) and Nef polyhedra;
• (constrained) tetrahedralisation;
• the industry foundation classes (IFC);
• CityGML and CityJSON; and
• applications of 3D models (eg calculation of volumes and solar potential).

Checkout my lab repository

Simple polyhedron processing

For this assignment I implemented a method to simplify a 3D city model by merging neighbouring buildings into blocks. This method involved the following steps:

1. read the triangles in the input 3D city model from an OBJ file and load it into a data structure;
2. for each triangle, compute its (expanded) bounding box;
3. use the bounding boxes to find pairs of neighbouring triangles;
4. assign groups of neighbouring triangles to blocks;
5. in each block, store all the points of its triangles’ vertices;
6. for each block’s points, compute its convex hull and its min/max z values;
7. using each block’s convex hull and min/max, create triangles for its output roof, floor and wall geometries;
8. output the triangles to an OBJ file.

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Enriching the 3DBAG with new attributes

In its current form, the 3DBAG dataset contains for each of the 10M+ buildings in the Netherlands:

1. the 3D geometry at three LoDs: 1.2+1.3+2.2
2. several attributes, eg the construction year, the height of the ground, the type of roof, current status of the building, etc.

For this assignment, we are asked to calculate a few useful attributes (those can be useful for others when processing the 3DBAG files) and to add them to the file. The attributes are namely:

• volume
• orientation
• area+orientation of the "RoofSurface" surfaces
• geometric difference between the LoD1.3 and LoD2.2 (with Hausdorff distance)

The updated model must be delivered in the CityJSON format.

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BIM to Geo conversion using Boolean set operations on Nef polyhedra

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Foorprint

Building footprint

1st floor

First floor

2nd floor

Second floor

IFC model

IFC model

The aim of this assignment is to implement a BIM to Geo conversion for one building. In short, this will involve five steps:

1. converting the BIM model to OBJ using IfcConvert;
2. loading each object in the OBJ file(s) into a CGAL Nef polyhedron;
3. processing the Nef polyhedra into a single big Nef polyhedron representing the space filled by the building’s floors, walls, windows and so on;
4. extracting the geometries representing the building’s exterior surface and individual rooms;
5. writing the geometries to a CityJSON file with correct semantics.

Everything was implemented in C++ with the help of packages from the CGAL library and Niels Lohmann’s JSON for Modern C++ library.