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AUTOMATIC RECONSTRUCTION OF CONCEPT MODELS BY USING A DIGITAL PHOTOGRAMMETRIC 
MEASUREMENT SYSTEM 
Peter Krzystek, Frank Petran 
Inpho GmbH 
Smaragdweg 1, D - 70174 Stuttgart 
e-mail: peter@inpho.de 
Heinrich Schewe 
Technet GmbH, Kônigstr. 53, D - 78628 Rottweil 
e-mail:100321.1056@compuserve.com 
KEY WORDS: Automatic surface reconstruction, still video cameras, bundle adjustment, reverse engineering, integra- 
tion into CAD systems. 
ABSTRACT: 
The automatic reconstruction and CAD modelling of car concept models plays an important role in modern car design 
strategies. Because of today's specific accuracy requirements digital still video cameras can successfully be applied to 
the automatic measurement of concept models by means of digital image processing techniques. The paper describes 
a digital photogrammetric measurement system that incorporates interactive measurements, area-based and feature- 
based matching methods, hierarchical image pyramid techniques, and robust surface reconstruction. The surface is 
described with a dense 3D point cloud or by profiles, respectively. The images to be processed can either be digitized 
from film by scanners, or they can be provided by digital cameras. Furthermore, the system concept incorporates im- 
age orientation and camera calibration based on a new close-range bundle adjustment program and the utilization of 
multiple digital images. Practical results are given in the conclusion of the paper. A DCS 200 Kodak still video camera 
was calibrated with an accuracy better than 1 micron using 15 digital images. Furthermore, the paper reports about the 
measurement results of a 20 96 concept model and an interior clay design model with respect to accuracy and measur- 
ing speed. 
1. INTRODUCTION 
CAD-based design of free-form shapes usually starts with digitizing a physical model which has been manually created 
by conceptual designers. Laser tracker, holography, structured light methods and photogrammetry are currently ap- 
plied as digitizing techniques and they are competing for introduction into the industrial workflow environments. Since 
the practical introduction of the InduSURF system (Sorgatz,1988) based on the software package SURF 
(Schewe, 1987) photogrammetry proved its successful applicability to digitize clay models in car design. 
Let us shortly recall the simple but paramount reasons for its practical acceptance. The car design model is occupied 
only for a short time by the photogrammetric system during the taking of the photographs. Thus, the whole design 
process is interrupted only for a short period. Also, the photographs individually document each phase of the design 
and they can be used later for post-measurement of details. The achievable absolute accuracy of the system is in the 
order of + 0.1 mm which is still accepted in the whole design process chain. Nevertheless, the system showed up 
some disadvantages mainly caused by its specific hardware configuration (see Schewe, 1987). Without stretching the 
related details too much, the system showed mechanical effects (temperature drift, backlash) and it was restricted to 
the use of stereo image pairs. The measuring speed of approx. 1 point/sec could not be accelerated because of the 
mechanical limitations of the analytical plotter C100 and the controlling computer HP 1000. However, the system is 
still successfully used and stands for one of the first automatic photogrammetric measurement systems being applied 
in industrial environments. 
It is quite evident that the mentioned disadvantages could easily be overcome by digital photogrammetry. The utiliza- 
tion of digital images together with powerful workstations gives a photogrammetric measurement system at least the 
prospect to be more accurate and faster. Especially, accuracy is gained since no mechanic effects have to be consid- 
ered, multiple ray intersections of object points from multiple images becomes possible, and correlation techniques 
can be applied for maximum measurement precision. Furthermore, the individual process steps can be automated or 
at least semi-automated and hence human interaction can be reduced to a minimum. After an interactive preparation 
step several measurement jobs can be set up as a complete batch job and processed during night time. 
Today, digital images can be provided either by scanners or digital cameras. There is a wide field of photogrammetric 
and non-photogrammetric scanners which are currently investigated with regard to precision and practical applicability 
(Baltsavias, 1994). Also, numerous digital cameras are presently available which differ not only in price, but also in 
technical solutions. Basically, both ways of digitization appear feasible to provide digital input for close range photo- 
grammetry. They have to be selected according to the accuracy and speed specifications which are required for an 
operational integration of the measurement system into an industrial workflow environment. 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop “From Pixels to Sequences”, Zurich, March 22-24 1995 
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