Proceedings 18 th International Symposium CIPA 2001 
Potsdam (Germany), September 18 - 21, 2001 
IMAGE-BASED 3D ACQUISITION TOOL 
FOR ARCHITECTURAL CONSERVATION 
Joris Schouteden, Marc Pollefeys, Maarten Vergauwen, Luc van Gool 
Center for Processing of Speech and Images, K.U.Leuven, 
Kasteelpark Arenberg 10, B-3001 Leuven, Belgium 
Tel. +32 16 321064; Fax +32 16 321723 
Marc.Pollefeys@esat.kuleuven.ac.be 
KEY WORDS: Photogrammetry, Architecture, Heritage Conservation, Image-based 3D Reconstruction, Dense Stereo 
ABSTRACT 
In this paper an image-based 3D acquisition tool is presented. The tool has been developed with architectural conservation in mind. 
The proposed tool is developed as part of a project that aims at developing a technology that enables the operator to build up an 
accurate three dimensional model - without too much repetitive work - starting from photos of the objects and measured reference co 
ordinates. This model can in a later phase be used as a core for a multimedia database, to represent designs for interventions, or for 
distribution to the wider public. The presented 3D acquisition tool is largely automated. A limited amount of interaction is used to 
indicate the reference points in the images to allow global localization in world coordinates. These points are also used to simplify 
the matching process so that wide baselines can be coped with. Depending on the number of reference points the camera needs to be 
pre-calibrated or not. From this point on the approach automatically finds additional matches and refines the calibration. A globally 
consistent estimate can be obtained through bundle adjustment. The next step consists of dense depth estimation based on stereo 
matching. From the computed information a dense textured 3D surface reconstruction of the recorded structure is computed. The 
tool also allows extracting 2D slices or pointing measurements. The results can be used both for measurements and for visualization 
purposes. 
1. INTRODUCTION 
At this moment many architects involved in conservation still work in the traditional way. They use instrument based (theodolite, 
total station, photogrammetry) or even hand-measured (tapes, plumb-bobs, levels...) survey methods. As many architects are shifting 
towards computer-aided design for new buildings, they also try to apply these programs to renovation or conservation projects. 
However, the number of tools available to accomplish the task of 'getting the existing building in the CAD program' is limited, and 
mainly directed to 'translate' traditional methods to CAD (automatic import of full station co-ordinates, error-adjustment of 
triangulation...). Based on a limited number of actually measured points, 2D plans and sections or a 3D model can be constructed. 
This typically results in a very 'simplified' representation of the building, which is absolutely not in line with the high requirements 
for conservation purposes. 
The presented 3D acquisition tool is largely automated. A limited amount of interaction is used to indicate the reference points in the 
images to allow global localization in world coordinates. These points are also used to simplify the matching process so that wide 
baselines can be coped with. Depending on the number of reference points the camera needs to be pre-calibrated or not. From this 
point on the approach automatically finds additional matches and refines the calibration. A globally consistent estimate can be 
obtained through bundle adjustment. The next step consists of dense depth estimation based on stereo matching. A complete model 
is obtained by combining the results of different disparity maps into a 3D reconstruction. The texture is extracted from the original 
photographs. From the computed information a dense textured 3D surface reconstruction of the recorded structure is computed. The 
tool also allows extracting 2D slices or pointing measurements. The results can be used both for measurements and for visualization 
purposes. 
The proposed tool is developed as part of a project that aims at developing a technology that enables the operator to build up an 
accurate three dimensional model - without too much repetitive work - starting from photos of the objects and measured reference co 
ordinates. This model can in a later phase be used as a core for a multimedia database, to represent designs for interventions, or for 
distribution to the wider public (Nuyts 2001). 
2. CALIBRATION 
The first module allows to calibrate the camera and to compute the pose in world coordinates for the different views. For this purpose 
the tool assumes the knowledge of 3d reference points measured by surveying methods as well as their projection in the imagery. As 
a part of this tool an interface is provided for identifying the 3d reference points in the images. A snapshot of this interface can be 
seen in Fig. 1. The reference points have been indicated in both images.