automatically adapted to the surface curvature. Thus the
system provides surface data which are ideally suited to get a
high quality of the surface description in the subsequent CAD
design process. The accuracy of the photogrammetric system
depends on the image scale and it is in the order of 0.2-0.3 mm
under the typical configuration. The typical rate at which
points are measured is of the order of 70 points per second with
current hardware.
The system is integrated in a modern CAD system (ICEM
Surf) which takes full advantage of the surface measurements
of the photogrammetric system. Since the measurement can be
done on the same workstation where the CAD-system is
operated, the traditional work at the CAD workstation changes.
It is now not only the construction of some object on the
screen of a computer. Instead the measuring system can be
operated by the same person on the same workstation, thus
enabling a dialogue between the constructive part of the
CAD-system and the measurement system. Also, the result of
the design work can be superimposed with digital images to get
realistic feedback as much as possible.
The construction of some areas of the design may require a
varying point density. Whereas a slightly curved large area
may require only a few points to be measured, it may be
necessary to measure a few hundred points where surfaces
become very complex. The traditional approach of splitting the
construction of the object on the CAD-station and the
measuring of the model leads to the problem, that in general
much more points are measured than would actually be
required. With most measurement techniques applied today
remeasuring and refinement of the measurements is not
possible once the model has been taken out of the measuring
machine, because either the model has been modified already
or the measuring machine is occupied by some other model.
Traditional coordinate measuring machines are designed to
operate on a rather small range of object sizes, a machine built
for a large object, like a full scale model of a motor vehicle,
can hardly be used for small parts, like a radio blind on the
instrument panel, and vica versa.
Another advantage of digital photogrammetry is the complete
documentation of the development phase which allows
measurement refinements and densifications even during the
construction phase, because digital images are availabe online.
Furthermore, it becomes possible to measure objects of greatly
varying size with the same equipment. This makes the method
suitable for bulky objects like the full scale model of a car as
well as small detail studies like a rear view mirror. Since the
equipment is light and easy to transport it is not necessary
anymore to carry the prototype to the measuring machine,
risking to damage it, instead the camera is carried to the
object.
The paper also reports on the application of the integrated
system to a design model. Surface data, which are recorded by
the photogrammetric system, are used in the CAD system to
mathematically approximate the measured surface by Bezier
polygon patches. Rapid prototyping can be achived by using
the surface facets digitized by the photogrammetric system and
the global modelling part of the CAD system. Thus, the
digitized prototype can be "virtually" changed like clay solely
in the computer. The system approach is highly effective and
appears as a tumkey system for CAD based car design and fast
reverse engineering.
476
2 The measurement system
The description of the measurement emphasizes three distinct
elements:
= The Hardware used during the recording phase.
= The Software used during the measurement
phase.
* The Calibration of the cameras to determine
their interior orientations and lense distortion
characteristics.
2.1 The Hardware Setup
The hardware used consists of two Kodak DCS460 digital
cameras, sometimes also referred to as still video cameras, a
modified slide projector, a stereo base and a tripod.
The Kodak DCS460 cameras are equipped with either standard
24 mm or 28 mm Nicon lenses with fixed focusing ring to
avoid a change in the interior oientation parameters of the
camera.
Currently a special 24 mm lense which will be available later
this year is under developement. This lens is specially
designed for high image quality at close range and feature
reduced lense distortion and distinct focusing positions to give
higher flexibility in image scale. Thus it becomes possible to
record the gross of an object efficiently from a moderate
distance, and allows for close up detail with higher accuracy
when it is required in certain detail areas.
Figure 1 - DCS460 camera pair in front of a car model
The camera used is a standard Nicon housing with all features
of its 35 mm film counterpart. The camera back developed by
Kodak replaces the film storage and transport system. It houses
a 3072 x 2048 Pixel CCD black and white or colour sensor.
The colour variant of the CCD sensor is less suitable for
mensuration purposes because the individual pixels are masked
in a regular pattern with red, green and blue filters. Thus, the
green channel of the image for instance has interpolated green
values in places where either red or blue masked pixels are
situated, the same is true for the remaining channels. Although
75 96 of the pixels are masked in green, because this is the
channel the eye is most susseptible to. To measure on either of
the channels alone would always result in in a reduction of the
physical resolution of the sensor. The consideration of all
colour channels simultaneously during the matching process
would lead to a decrease of the measuring speed by a factor of
three and only a slight improvement of geometrical accuracy in
comparison to evaluating only the green channel.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
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