GENERATION OF OBJECT REPRESENTATIONS OF 3D OBJECTS 
IN CAD/CAM BY DIGITAL PHOTOGRAMMETRY 
      
  
  
Rongxing Li, Assistant Researcher 
Pacific Mapping Center, Department of Civil Engineering 
University of Hawaii, U.S.A. 
ISPRS Commission V 
ABSTRACT 
3D object representations in CAD/CAM are usually created by geometric modelers. However, in 
cases where geometric information such as dimensions and shapes of objects are not available, 
measurements of physically existing objects become necessary. This paper presents a method of 
generating object representations of 3D objects by digital photogrammetry. Geometric 
parameters of primitives of 3D object representations such as Boundary Representation (B-rep) 
and Constructive Solid Geometry (CSG), and digital surface models are determined by digital 
image matching techniques. An algorithm for reconstruction of surfaces with discontinuities is 
developed. Interfaces between digital photogrammetric data and object representations are 
realized. Applications of this method could be found in fields of design and manufacturing in 
mechanical engineering, automobile industry, robot technology and others. 
KEY WORDS: 3D object representation, Digital image matching, Photogrammetric interface, 
Spatial data base. 
1. INTRODUCTION 
In Computer Aided Design (CAD) and Computer Aided 
Manufacturing (CAM) systems objects are described by object 
representations, by means of which the tasks associated with 
all geometric aspects of design and manufacturing of products 
can be handled. Usually, such an object representation of a 3D 
object can be generated by a geometric modeler effectively 
and comfortably if a design concept and necessary related data 
are known. But in some cases, where geometric information 
such as dimensions and shapes of objects are not available, 
measurement of physically existing objects become necessary. 
A typical example of the above is prototyping. After the first 
prototype is produced, some tests have to be accomplished to 
acquire technical data. According to the test results, the shape 
of the prototype may be modified in order to reach a better 
design. The modified model is tested and improved again. 
This procedure repeats itself until the optimal design is 
achieved. If the tests above concern only geometric 
characteristics of the product, they can be done by computer 
simulation. In this case, modification of the prototype means 
just modifying its object representation. In other cases, 
non-geometric tests take place and a computer simulation may 
not be possible. In this circumstance, the model has to be 
modified physically. Thus the modified model will be 
measured so that a corresponding object representation can be 
derived. 
Photogrammetry is a technique which acquires 3D geometric 
data of objects without direct physical contacts. Digital 
photogrammetric measuring procedure is becoming very 
efficient and automated with recent developments of software 
and hardware technology and progresses of computer vision. 
With help of digital image matching, geometric elements of 
primitives of 3D object presentations such as points, edges, 
curves and surfaces can be derived from stereo images. An 
algorithm for generation of digital surface models with surface 
discontinuities by area and feature based image matching is 
developed. An interface between the digital image matching 
system and CAD/CAM systems makes it possible to convert 
the data acquired by digital photogrammetry to 3D object 
representations. Therefore, design and manufacturing 
procedures can be automated and processing time can be 
reduced by applying digital photogrammetry. 
2. BACKGROUND 
2.1 Previous Work 
Measuring machines are often utilized to measure 3D 
coordinates of objects in mechanical engineering 
(VW-GEDAS, 1989, Albertz, 1989). A surface point 
measured is touched by a contact point of the measuring 
machine. The position of the contact point can be determined 
by its 3D translation according to the origin of the coordinate 
system of the machine. Thus, the object can be measured point 
after point. Among others two drawbacks of this method are: 
a) physical contacts between objects and the measuring 
machine may limit its applications where such contacts are 
impossible; b) the long operating time makes it not suitable for 
measuring mass points, e.g. for digital surface models. 
Close-range photogrammetry derives object positions in the 
object space by analyzing parallaxes of identical points in 
stereo images. This enables an optical measurement of objects 
without contacting objects. Digital photogrammetry,