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calibration frame
fig.2: recording configuration
In order to assign the bipupil-line, the patients were
requested to focus on a distant point during recor-
ding. Emphasizing anatomical landmarks, such as
infraorbital borders or median line, makes it easier to
compare prae- and post-therapeutical conditions
(fig.3).
We used 4 CCD-Cameras with a 2/3" sensor
(768*484 pixel = 8.91*6.58 mm?) and 16 mm
lenses (fig.4). The images can be controlled imme-
diately on the screen (on-line) and can be repeated
right away if necessary. Amongst other things the
quality of the digital image (sufficient contrast and
brightness) has to be taken into account. One has to
ensure that the surface is measured comprehensively
without covering up important details, excluding sha-
dows and overlapping (fig.5). The pictures can be
stored on harddisk in conventional image formates
(e.g. TIFF, BMP). A capacity of about 1,8 MByte for
4 images is needed. In pictures with various points of
view the natural structure of the human skin is
insufficient to identify obvious points on the surface.
That's why a projection of regular patterns is
needed. We tested different numbers, forms and
sizes of lines, cross-lines and point patterns. It is also
necessary to illuminate the whole face in consistent
distribution avoidung extended shadowy areas.
3.2 Digital Evaluation System
Depending on the recording configuration the fol-
lowing procedures take place:
The relative positions of the cameras to one another
and their absolute position are unknown. Therefore
the external orientation must be determined by means
of a bundle adjustement, such as CAP (Combined Ad-
justement Program), with the help of a calibration
field (Hinsken, 1989). The internal orientation can be
determined within a simultanous calibration.
If the internal and external orientations are already
known by advanced calibration, the object coordi-
nates can be assigned with a spatial forward inter-
section. Measurement of image coordinates (fig.6) is
carried out with a specific module of the image pro-
cessing program PHAUST (Photogrammetrische
Auswerte Station, Woytowicz, 1993).
First the measurement images are correlated. The
semi-automatic point measurement is performed by
elliptical and center-of-gravity operators to determine
the image coordinates. The next step is the
calculation of object coordinates either by bundle
adjustment or spatial intersection.
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fig.3: anatomical reference points
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
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