CIPA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
2 DATA PROCESSING
In earlier work (Adler et al., 2001) a scanning technology
was described, which directly acquired the profile line by
projecting a laser-line on the sherd. This laserline was cap
tured by a camera for the inner side and a second cam
era for the outer side. The merged images of these two
cameras contained the profile line. This system had draw
backs concerning its portability, ease of use and automa
tion. Therefore we choose the Eyetronics Shape Snatcher
Technology (Cosmas et al., 2001) (see Figure 3), because
it is portable and can be operated without expert knowl
edge. It consists of a CCD-camera and a flashlight to ac
quire the shape based on the structured light principle. The
image, together with the knowledge about the pattern and
its relative position to the camera, are used to calculate the
coordinates of points belonging to the surface of the ob
ject (Kampel and Sablatnig, 1999). Since the 3D-scanner
can only capture one side of the sherd per scan, the inner
side and the outer side have to be scanned separately to
reconstruct a complete 3D-model of the sherd.
Figure 3: Eyetronic’s ShapeCam consisting of a CCD-
camera (left) and a flashlight mounted on the top-right part
of the handling frame.
2.1 3D Data - VRML
The 3D-data acquired by the 3D-scanner is stored as 3D-
surface, which consists of 3D-points (vertices) that are con
nected in form of triangles (called faces). The 3D-model
contains the color information (called texture) for each face
for visualization. These vertices and faces are stored in an
indexed list (E).
There are different types of file formats for storing 3D-data
(e.g. AutoCAD, Wavefront, Openlnventor). We have cho
sen VRML (Nadeau, 1999), which is software independent
and can be viewed with a web-browser with a VRML-plugin
that is free of charge.
The pottery data set we use for experiments consists out of
the recorded objects described in table 1. The number of
pieces per view is 2, because for every piece an inner view
and an outer view has been acquired.
For storage in a database we use polygonal geometrical
Box 1
Box 2
Box 3
Number of pieces
21
12
26
Number of views/piece
2
2
2
Vertices/View
4.000-9.000
3.000-8.000
Table 1 : MURALE Pottery dataset
objects (Nadeau, 1999). These geometrical objects are de
scribed by an indexed list C v of n vertices p, = (Xi,yi,Zi) T
Xi ,yi and Zi are the coordinates of the vertices of the sur
face of the object in centimeter.
f-'v Pn} (1)
The vertices of C v are connected by faces /, which are
stored as list Ef.
£/ = {fi, ■ ■ ■, /m}, fk — {ki,k 2 , k 3 ) (2)
Optionally the normal vectors C n — {ni,..., n n } and the
color information C c = {ci,..., c n }, c* = (red,green, blue)
is stored, which is not necessary for further calculation, but
it is used for visualization purposes. The normal vectors
E n are used for the estimation of the rotational axis. If
they are not provided by the scanning software, they are
estimated by using the triangulated data.
So the 3D-model of the view is described by an indexed
list of vertices, faces, normal vectors and color informa
tion. sfieTcCyieuj {Ey, E f, Eji, Efi) .
To double the performance of processing the data, we use
two different types of coordinate systems. For translation
and rotation we use the cartesian coordinate system, be
cause the translation is done by an addition. To estimate
the two angles (azimuth and elevation), which is required
for rotation we use a spherical coordinate system. Points
p and vectors v in R3 using the spheric representation are
described by the azimuth 9, elevation 0 and the distance r
to the origin point in the cartesian coordinate system. This
representation has been chosen for the rotation, because ro
tation can be done by an addition. Cartesian p-(ai, y, z) T
and spheric: p=($, 0, r) T notation.
2.2 Profile line
The rotational axis rot leads to the exact position of a frag
ment on the original vessel. Therefore the rotational axis is
required for the estimation of the profile line and for regis
tration (Sablatnig and Kampel, 2002) of the inner and outer
view.
The profile line, which is used by archaeologists for clas
sification and reconstruction, is defined below:
• A profile line (profile) is the cross-section of the 3D-
model of the sherd {sherd) and an intersecting plane
ej. This intersecting plane e l , is defined by rot and the
direction i, so that e* intersects the sherd.
• The intersection at an index i maa: , where the sherd has
the maximum height h ma x = max{hi) is the profile
line with the longest arc length and is called longest
profile line (profile ). The index specifies the direc
tion.
• The height hi is defined as distance between two points
of the surface of the sherd parallel to the rotational
axis rot. The index i, where the height has its maxi
mum is called h rnax .
A sherd, its rotational axis and the estimated longest profile
line is shown in Figure 4.
