International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
particular item and to formulate specific queries in order to
retrieve artefacts and sites of interest. In addition, the retrieval
system should be designed in such a way that it facilitates
comparative studies of sites and artefacts.
Once the content of interest has been identified, it must be
visualised. The visualisation process should be designed in
such a way that it provides a realistic view of the artefact under
consideration. In order to achieve such a high degree of
realism, the visualisation system should provide depth
perception, high spatial resolution, high refresh rate, immersion,
navigation, manipulation and, faithful reproduction of the
colours and geometry of the original [5].
This paper is organised as follow. Firstly, a cost effective stereo
visualisation system is described. Then, it is shows how the
composition of an image and the shape of an artefact can be
indexed. A retrieval system is presented. It is shown how to
combine this system with cluster analysis in order to perform
comparative studies. Finally, an integrated approach or
framework for virtual collections is presented. This framework
encompasses the acquisition, the creation of the models, virtual
model documentation, content = indexation, retrieval,
comparative studies and visualisation.
2. VISUALISATION OF VIRTUAL COLLECTIONS
2. Architecture of the system
In this section, we describe a cost-effective approach for the
stereo visualisation of virtual sites. When a site is virtualised, it
Is possible to make it available to a large variety of people, as
far as the intellectual property protection allows it.
While most users are content to visualise a site with a standard
computer, specialists and scholars are not. Most of the time,
scholars have access to high quality pictures of sites. Despite
the fact that the pictures provide a high-resolution
representation of the visual appearance of the site, they provide
limited and ambiguous information about the three-dimensional
shape. Furthermore, the pictures are taken from a very limited
subset of viewpoints, which are not necessarily the ones
required by the scholar. This is why, among many reasons,
archaeological sites need to be accessed. The situation is rather
different if the scholars can visualise the site in three
dimensions and navigate within the scene. This can be achieved
with dynamic stereo visualisation.
Most of the commercial stereo systems suffer either from high-
cost or from poor performances. In both cases, the access to the
data is limited. In the first case because a limited number of
users can afford the system, while in the second case, because
the system can only display a low-resolution version of the data.
For these reasons, we have created a cost-effective stereo
visualisation system suitable for the visualisation of large
amount of complex three-dimensional data. The system is made
of off-the-shelf components and is characterised by its
scalability, its distributed architecture and its portability. Let us
review its architecture.
The system is formed of two interconnected laptops and two
DLP video projectors. The first computer, the sender, receives
the events from the user and renders the right-eye view. The
second computer, the receiver is synchronised with the sender
and renders the left-eye view. Each computer is attached to an
ultra-compact DLP video projector. The left and right-eye
598
views are simultaneously projected though polarised filters on a
special screen that maintains polarisation. The polarisation is
circular right for the right-eye view and circular left for the lefi-
eye view. The circular polarisation insures that the orientation
of the user's head does not introduce cross talk in between the
right and the left view, which would deteriorate the stereo
effect. The scene can be visualised in three dimensions with the
corresponding passive stereo glasses.
Figure 1. Visualisation of the Crypt of Santa Cristina in
Capignano Salentino, Italy with the virtual theatre.
Details of a fresco on a column.
The software is written in Java/Java 3D and the same byte code
can run transparently on OpenGL and DirectX. The
synchronization is ensured as follows. Before it renders a
frame, the sender updates its navigation parameters from the
events generated by the user. It sends the navigation parameters
through a firewire to the receiver, which renders the left-eye
view of the scene. Meanwhile, the sender renders the right-eye
view of the scene. Then the sender sends a synchronisation
signal to the receiver and both computers swap their respective
buffer. In order to obtain a higher resolution, an arbitrary
number of receivers can be connected to the sender. Thus, it is
possible to render a scene at ultra-high resolution by simply
adding more computers and projectors: each computer-projector
pair being responsible for the rendering of a subset of the scene;
the so-called tiled wall display.
Currently, the system runs on Dell Precision M60 laptops with a
NVIDIA& Quadro FX Go700™ 4XAGP graphics card with
128 MB of texture memory. The selected DLP video projectors
are the NEC LT260K: their brightness is 2100 ANSI lumens
and their native resolution is 1024x768. These projectors have
a proprietary technology that corrects rapidly and efficiently the
horizontal and vertical keystones, which make the alignment
relatively straightforward even in the case of multiple displays.
The selected polarised filters are the 3M HNCP37. They have
been chosen because their light transmission curve is relativity
uniform over a wide range of visual wavelengths, which means
that the colour distortion is reduced to a minimum. This is an
important aspect to ensure the faithfulness toward the original.
2.2 Experimental results
The Crypt of Santa Cristina is a gin century Byzantine crypt
situated in Capignano Salentino in the South of Italy. It has an
irregular shape and contains one of the most ancient Byzantine
frescoes signed and dated. For instance, Theophylact painted
the Christ and the Annunciation in 959 A.D. The dimensions
are about 16.5 x 10.0 x 2.5 m.
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