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New perspectives to save cultural heritage
Altan, M. Orhan

CI PA 2003 XIX 11 ' International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
Figure 7: Screenshot of WebCAME embedded into a web
page displayed in the Mozilla web browser showing an
arch of the Roman baths at Sagalassos.
the entire data is visible and close to the virtual camera:
objects which are close to the viewer have to be rendered
at high detail, while more distant objects, where the de
tail is no longer visible, should be simplified. Since the
viewpoint constantly changes in interactive applications,
the mesh has to be adapted in real-time. The progressive
meshes method (Hoppe, 1996) and some more involved
techniques, such as (El-Sana and Chiang, 2000), belong to
this class of algorithms.
Second, geometry information stored in a straightforward
way (i.e. coordinates for points and indices for connecting
lines and surfaces) contains a large amount of redundancy.
By exploiting coherence within the data set the storage size
can be reduced. This principle has first been described by
(Deering, 1995), more efficient techniques have been de
veloped later, e.g. (Täubin and Rossignac, 1998, Alliez and
Desbrun, 2001b).
Third, if the simplified and compressed model is still too
large to be displayed immediately after the user requested
it, progressive transmission can be used to give the user
a fast first impression and refine it progressively, as new
data arrives. The user can start to operate on the (possibly
low-quality) object with almost no delay. This feature is
often integrated into simplification and compression tech
niques, e.g. (Khodakovsky et al., 2000, Alliez and Des
brun, 2001a).
In our visualization system we combine all three concepts.
As stated above, we use triangle meshes as the represen
tation of 3D objects. However, even under this restric
tion there are some properties that affect the choice of a
particular method: generic meshes cannot be guaranteed
to be two-manifold, and even simplifying a two-manifold
mesh can create a non-manifold one. Therefore we will
not rely on the two-manifold property (which greatly sim
plifies mesh simplification), but rather use a technique that
treats non-manifold meshes and topology simplifications
in a natural way (see Section 4.2).
4.2 Compressed Adaptive Multiresolution Encoding
The CAME data structure (Grabner, 2002), which we have
developed, is an extension and enhancement of the meta
node approach (El-Sana and Chiang, 2000). It achieves a
compression ratio of approximately 1:25 compared to the
uncompressed meta-node data, while still providing view-
dependent access to multiresolution triangle meshes based
on an estimation of the screen-space error.
The key idea in CAME is to identify mesh vertices by bit
strings indicating the path to be taken in the simplification
hierarchy to reach the leaf node corresponding to the ver
tex. Topology compression is achieved by omitting redun
dant prefixes of bit strings. Mesh connectivity is implicitly
maintained by independently traversing the simplification
hierarchy according to the bit string stored with each trian
gle. For details we refer to the original publication.
After reconstruction the original meshes are converted to
the CAME format by a fully automatic encoder and stored
into a database. The rendering system used to view the 3D
models progressively reads the multiresolution mesh from
the database and provides a view-dependent rendering.
4.3 Internet-based Application
The CAME technology has been integrated into a light
weight web browser plugin called WebCAME (Grabner,
2003b). It allows to progressively transmit compressed
textured 3D models with view-dependent simplification over
the Internet. Although being a general-purpose tool, the
navigation facilities have been designed with virtual ar
chaeology applications in mind. The requirements of ar
chaeologists and of visitors an archaeological sites are re
flected in the navigation: both viewpoint-centered navi
gation to move around in the virtual world and object-
centered navigation for careful exploration of single ob
jects are provided. Figure 7 shows a screenshot of the
WebCAME plugin with an arch from the Roman baths of
4.4 Smart Web Objects
To increase the semantics of the models, the traditional
multiresolution concept, which only handles pure geom
etry, has been extended to preserve object identities and
enable access to attribute data associated with the objects.
The object identities are included in the CAME structure
to support their compressed adaptive transmission.
Application-specific properties of objects in the scene (meta- 1
data) are typically stored at a separate place, such as in our i
example the 3D MURALE multimedia database. In order <
to access this data, it must be possible to associate each i
piece of geometry in the scene with the appropriate meta
data record. A simple way to solve this problem is to add
a unique object identifier to every triangle in addition to
color and texture information. This allows to perform the
following important operations on the client side: *
• Querying additional data about any part of the mul- t
tiresolution mesh r
• Manipulating (e.g. translating or rotating) parts of the
mesh c