Eurographics Italian Chapter Conference (2006)
G. Gallo and S. Battiato and F. Stanco (Editors)
Easy Access to Huge 3D Models of Works of Art
M. Callieri, F. Ponchio, P. Cignoni, R. Scopigno
Istituto di Scienza e Tecnologie dell’Informazione, Consiglio Nazionale delle Ricerche†
Abstract
Automatic shape acquisition technologies evolved rapidly in recent years, and huge mass of 3D data can be easily
produced. The high accuracy of range scanning technology makes the Cultural Heritage domain one of the ideal
fields of use of these devices. Given this particular application domain, two issues arise: how to visualize at
interactive rates these complex data on commodity computers (both locally and on web), and how to improve the
ease of use of the visualization tools (as potential users are often not expert with interactive graphics).
We present a new visualization system designed to support easy implementation of multimedia kiosk for museums
or expositions, which has also been extended to web-based usage. The system allows naive users to inspect a large
complex 3D model at interactive frame rates on off-the-shelf PC’s, presenting the 3D model and all the multimedia
data that has been linked to selected points of its surface. A main goal in the design of the system was to provide the
user with a very easy and natural interaction approach, based on a straightforward “point and click” metaphor.
Visualization efficiency is obtained by adopting a continuous level-of-detail (LOD) representation, where on-line
automatic selection of the best-fit level of detail (according to the current view frustum) is coupled with visibility
culling and ready-to-render representation of the geometry.
Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Picture/Image Generation]: Digitizing and
scanning.
1. Introduction
Interactive rendering of complex 3D models is crucial for
many applications, such as architectural design, graphics
simulation and scientific visualization. These systems need
to provide the user with a realistic/accurate visual representation of the scene and real-time navigation/interaction. Cultural Heritage (CH) is a specific field of application of 3D
graphics, where a basic issue in the design of visualization
tools is ease of use: forecasted users usually possess limited
skills in managing 3D graphics technology (museum curators, art historians, restorers, ordinary visitors of a museum,
etc.). The design of the GUI and its overall usability play
a critical role in deciding if the tool will be just a nice toy
or a really useful technical instrument. Another problem is
† Area della Ricerca CNR, Via Moruzzi 1, 56126
Pisa,
ITALY.
WWW:
http://vcg.isti.cnr.it/
Email:
{m.callieri | f.ponchio | p.cignoni |
r.scopigno@isti.cnr.it}
c The Eurographics Association 2006.
that the accurate, realistic-looking models that we are dealing with usually contain much more graphics primitives than
the interactive capabilities of most graphics workstations,
since our focus is how to process and interactively visualize the huge meshes which are produced with 3D scanning
technologies [BR02].
The work described in this paper is two-fold: first, we
present the interface of a new visualization tool, specifically
oriented to the inspection of complex 3D representations of
works of art, enriched by interactive links to standard descriptive multimedia information. Second, we have designed
its internal architecture by taking into account two contrasting constrains: the system should support huge meshes inspection, and real-time behaviour should be ensured for both
local or remote access to the 3D data. Therefore, our goals
were:
• Graphics workstation tailoring and efficient rendering: gain maximal performances from inexpensive platforms (PCs with state of the art graphics boards), to ensure universal user community; support interactive frame
M. Callieri & F. Ponchio & P. Cignoni & R. Scopigno / Easy Access to Huge 3D Models
(let us cite just the seminal work [LPC∗ 00]). Most of them
considered data management issues, e.g. how to render at
interactive frame rates the high resolution meshes produced
with 3D scanning. Efficient data management and rendering is a central issue in processing huge dataset. Several
techniques have been developed to cope with this problem: some of them keep a triangle-based representation and
adopt geometry simplification and multiresolution representation [GH97, CMRS03] to reduce data complexity and rendering times; others adopt a point-based rendering approach
coupled with keen heuristic for dynamic data sub-sampling
[RL00, PZvBG00, BWK02, GM04].
Figure 1: User interface of the I NSPECTOR tool, showing
the Arrigo VII mesh.
•
•
•
•
rates even on huge models, by the adoption of efficient
multiresolution solutions;
User tailoring: easy of use and usability should be given
priority over flexibility and completeness;
Integration of multimedia data: it should support the addition of links to other MM information (hot spots over
the 3D model);
Local and Remote Access: the system should be able to
run both locally (e.g. acting as the 3D graphics component of a multimedia kiosk installed in a museum) and
remotely (providing access to the data from the web);
Data Protection over the WEB: high-quality 3D model
cost some effort to be scanned and could be considered
sensible data; therefore, 3D data should often be protected
and not distributed freely.
2. Previous Work
Many previous works concern the use of 3D technology either to reconstruct digital 3D models of Cultural Heritage
masterpieces or to present those models through digital media. An exhaustive description of those works goes well beyond the brief overview that we can draw in this section. We
prefer to cite here only some seminal papers on the technologies proposed for 3D scanning and interactive visualization.
Automatic 3D reconstruction technologies have evolved
significantly in the last few years [CS00]. Unfortunately,
most 3D scanning systems do not produce a final, complete 3D model but a large collection of raw data (range
maps) which have to be post-processed. The post-processing
pipeline is presented in the excellent overview paper by
Bernardini and Rushmeier [BR02] and some algorithmic
sub-tasks have been improved since this review paper (e.g.
surface reconstruction from samples).
Many significant projects concerning 3D scanning and
cultural heritage have been presented in the last few years
3. Virtual Inspector
The V IRTUAL I NSPECTOR browser has been designed to
give a solution to the issues introduced in Section 1. V IR TUAL I NSPECTOR evolved considerably from the preliminary version presented in [BCS01]. This section presents
briefly its architecture and main features (described more in
detail in the following sections).
The system architecture has been designed by choosing a triangle-based approach to 3D data management. To
support interactive presentation of massive models, V IR TUAL I NSPECTOR adopts a multiresolution approach where
view-dependent variable resolution representations are extracted on the fly using a new and highly efficient approach [CGG∗ 05]. For each frame, the best-fit variable resolution LOD is selected according to the current view frustum and the requested visualization accuracy. LOD selection and rendering are very efficient since we adopt a patchbased representation, where a coarse-grain multiresolution
hierarchy is visited on the fly and ready-to-render geometry
patches are associated to each logical node of the variable
LOD produced. 3D data are therefore not reconstructed on
the fly at the single triangle grain, but triangle chunks are efficiently fetched from disk on demand and copied on GPU
memory for maximal rendering efficiency.
V IRTUAL I NSPECTOR is mainly oriented to the visualization of single works of art (sculptures, pottery, architectures,
etc.), and adopts a very intuitive approach to guide the virtual manipulation and inspection of the digital replica, based
on a straightforward metaphor (see Figure 2).
Other important characteristics of V IRTUAL I NSPECTOR
are its flexibility and configurability. All main parameters of
the system can be easily specified via XML tags contained in
a initialization file, such as: which are the 3D models to be
rendered (a single mesh or multiple ones), the system layout characteristics (i.e. how the different models and GUI
components will be presented on the screen), the rendering modes (e.g. standard Phong-shaded per-vertex colors or
BRDF rendering) and the interaction mode (e.g. model manipulation via a standard virtual trackball, via the dummybased “point and click" interaction, or both).
c The Eurographics Association 2006.
M. Callieri & F. Ponchio & P. Cignoni & R. Scopigno / Easy Access to Huge 3D Models
Figure 2: The new gaze point, selected by the user by a simple mouse click on the corresponding location on the dummy, is
marked for illustration purposes by a red circle (see the two zoomed image fragments).
Finally, V IRTUAL I NSPECTOR supports the specification
of hot-spots. Hot spots are a very handy resource to associate
multimedia data (e.g. html pages) to any point or region of
a 3D model. This allows to design interactive presentations
where the 3D model is also a natural visual index to historical/artistic information, presented using standard HTML
format and browsers.
4. Geometric Data Management
A lot of different solutions have been proposed in literature
for the efficient visualization of large, complex digital 3D
models. In the design of such a rendering system we must
face, among the others, the following issues: choosing the
right resolution with a dynamic data extraction criterion;
culling unnecessary geometry, since only the visible geometry should be rendered; keeping in memory only the currently
viewed portion of the model, possibly in a format which allows maximal rendering performances on modern GPU’s.
Until recently, the vast majority of view-dependent LOD
methods were based on multiresolution structures taking decisions at the triangle/vertex primitive level. The cons is a
constant CPU workload for each triangle that with current
GPU evolution makes the CPU the bottleneck of the whole
rendering process. To overcome this bottleneck and to fully
exploit the capabilities of current graphics hardware it is
therefore necessary to select and send batches of geometric
primitives to be rendered with just a few CPU instructions.
V IRTUAL I NSPECTOR is based on a new solution
[CGG∗ 05]: a batched multiresolution framework based on
the Multi-Triangulation (MT) [Pup96]. The MT is a very
general framework that encompasses a wide class of mulc The Eurographics Association 2006.
tiresolution algorithms, but, like the techniques proposed in
the 90’s, it was originally designed to minimize the number of triangles to be rendered, at the expense of CPU time.
Therefore, we have redesigned in a GPU-friendly fashion
the MT scheme, by moving the granularity from triangles to
optimized triangle patches, and by redefining the construction and rendering algorithm to work on external memory (a
mandatory approach to manage huge scanned meshes).
Our Batched Multi-Triangulation (BMT) represents the
3D model by building a DAG, where each single nodes represent a batch of triangles. The per-frame workload is reduced by reconstructing at run-time an adequate puzzle of
pre-assembled optimized surface patches, making it possible to employ the retained-mode rendering model instead of
the less efficient direct rendering approach. The basic idea
is grouping together sets of triangles (and representing them
in the more GPU-efficient manner) in order to alleviate the
CPU/GPU bottleneck. Since the granularity is much smaller
than the one of a standard MT representation, the processor
workload for multiresolution data structure management is
very small and we can keep the data in a form that has a
small memory footprint.
Extracting a variable resolution model means extracting a
cut over the DAG, action which can be performed efficiently
when the DAG size is small. For the sake of interactivity the
multiresolution extraction process should be able to support
a constant frame rate, given the available time and memory
resources. Our reconstruction algorithm selects a new cut
over the DAG within a predetermined budget of time and
memory resources, always ending with a consistent result,
or in other words, it is interruptible.
M. Callieri & F. Ponchio & P. Cignoni & R. Scopigno / Easy Access to Huge 3D Models
The extraction and rendering of a dynamic continuous
LOD with the BMT scheme have been evaluated over several
inspections, rotating and abruptly zooming in and out the
model. All the tests were done with rendering window size
800x600 on a Windows machine equipped with an AMD
Athlon 64, 2 GHz, 512 MB Ram, SCSI hard disk, bus AGP
8x and graphics card GeForce 6800 GT [CGG∗ 05]. The sustained rendering rate is around 4M triangles per frame at 35
fps with less than one pixel error.
5. GUI Design
The layout of the V IRTUAL I NSPECTOR tool is shown in
Figure 1. The major choices of the GUI design remained
nearly unchanged from the original first version of the system [BCS01]. The output window of the tool is divided in
two main frames: the one on the right is dedicated to the interactive selection of the desired view and to the GUI; the
main visual output region is the one on the left. The object portion visible at run time in the leftmost frame usually
depends on the viewing parameters that the user selects by
means of the rightmost window region.
A complete model of the inspected object, called dummy,
is visualized in the rightmost frame. It is conceived as a sort
of interactive and intuitive 3D map whose role is to allow an
easy selection of the view specs. The user disposes of constrained rotation of the dummy (on its vertical axis), to see it
from any side view. The selection of the viewing parameters
is implemented according to a very simple direct manipulation approach. A mouse click on any point of the dummy
(see Figure 2) updates the current view as follows: the point
selected on the dummy surface becomes the gaze point, and
the view direction is set by default to be equal to the surface
normal in the gaze point. The field of view is set initially using a default value (an example is shown in Figure 2, where
approximately 20% of the object is in the current view volume), or takes the value used/set in the previous interactive
actions. It can be updated (zooming-in/zooming-out) clicking on the corresponding GUI buttons (see Figure 2).
6. Rendering modes
In terms of rendering mode, V IRTUAL I NSPECTOR supports:
enhanced Phong-based rendering mode, BRDF-based rendering and protected remote rendering over the Web.
Figure 3: Rendering cast shadows greatly affects the resulting expression of the David’s face. On the left a standard OpenGL rendering with Phong lighting, on the right a
OpenGL rendering with a pre-computed diffuse lighting.
Current graphics hardware render self-cast shadows with
multiple rendering passes, but with a sensible performance
penalty. Moreover even with those approaches it is hard to
render soft lighting environments where lights are not pointsized and shadows are not very sharp. For the above reasons
and for the sake of efficiency we have chosen to partially fix
the lighting environment and to compute off-line cast shadows and diffuse shading over the surface; the resulting data
is a vector field evaluated on the mesh and stored as a pervertex color. The fine tesselation of the mesh guarantees a
sufficiently good sampling and rendering of the shadow map
onto the surface. Even if we use statically cast shadows, the
visual results can be made less static by adding one headlight to the lighting environment (a common metaphor for a
dynamic light source: move always the light together with
the observer during the dynamic object inspection). Therefore, we have chosen an hybrid approach: we use a headlight,
but we substitute the standard constant ambient lighting term
with the cast shadows field. The latter approximates the exact lighting that each face of the mesh would receive from an
anisotropic diffuse lighting environment. This diffuse lighting can be evaluated in an approximate manner by computing the solid angle of the sky that can be seen from each face.
Larger the solid angle more light will flow onto the face.
6.2. BRDF-based rendering
6.1. Enhanced local rendering
A common problem in computer graphics is the evaluation
of the trade-off between quality and efficiency in the rendering process. In the context of the visualization of models of
real statues the standard lighting model used in interactive
graphics is not satisfactory, since the effects of cast shadows
are not taken into account. Figure 3 show the major change
when considering cast shadows or not.
The bi-directional reflectance distribution function (BRDF)
describes how light is reflected off the surface of an object.
V IRTUAL I NSPECTOR has been extended to be able to render 3D models which comes with the specification of their
BRDF. More in detail, standard BRDF are used to describe
the behaviour of ideal objects whose surface is made of a
single homogeneous material. Most objects however consists of several different materials. This is especially true
c The Eurographics Association 2006.
M. Callieri & F. Ponchio & P. Cignoni & R. Scopigno / Easy Access to Huge 3D Models
ing more sophisticated rendering algorithms) without having to change the client. A single server can satisfy multiple clients and deal with different high resolution models;
or the remote rendering server can be implemented with a
rendering farm (multiple servers managed by a renderer dispatcher). The user interaction with the 3D model and the the
application behavior, beside the latency introduced by the
network interaction, remains identical to the one of the local
rendering mode.
Figure 4: V IRTUAL I NSPECTOR showing the Minerva head,
rendered on the basis of the BRDF model sampled from the
real statue.
for works of art or archeological objects (an example of the
Minerva head rendered using a sampled BRDF is shown in
Figure 4). A very precise way to represent these details is to
assign a different BRDF to each surface point which leads
to a spatially varying BRDF. Without these details, objects
tend to look artificial and unrealistic. The spatially varying
BRDFs approach included in V IRTUAL I NSPECTOR follows
the sampling and rendering approach proposed by Lensch
et al. [LKG∗ 03]. It has been implemented, in collaboration
with MPI colleagues, in an efficient manner by exploiting
the programmable features of modern programmable GPU.
6.3. Protected remote rendering
Since managing big and accurate data is a problem on the
web, we designed V IRTUAL I NSPECTOR to be used on the
web as well. Moreover, a protected rendering approach is
also useful if the 3D mesh is valuable and has not to be transmitted to the user. To support web-based visualization, Virtual Inspector has been extended by adding a remote rendering mode and a corresponding dedicated rendering service
module. The local machine does not receive the full resolution model but only a reduced resolution model for user
interaction; when a viewpoint is selected by the user, the local rendering client sends a request over the net to a rendering server (that has copy of the full resolution model), the
server renders the model according to the view parameters
and sends back the resulting image to the client. Therefore,
we follow the client-server approach presented in [KTL∗ 04],
which presents several benefits: low ram footprint on the remote client; no limitation on 3D model resolution; data protection. The only drawback of this approach is the cost of
the network communication, in terms of latency and network
load.
The remote server has been implemented using the same
multiresolution technology outlined in Sect. 4 and therefore can manage models of arbitrary complexity. Moreover,
it is possible to apply server-side changes in order to enhance rendering (by augmenting the model resolution or usc The Eurographics Association 2006.
7. XML-based specification of input, GUI layout and
rendering modes
A basic innovation with respect to the first version of V IR TUAL I NSPECTOR is the improved configurability of the system. This has been obtained by designing a simple interpreted language, called NSP, that is used to build up the interface with its behavior. The language exploits XML for all
the well known advantages of this technology (availability of
syntax aware parsers, human readability, extendibility etc.)
At startup, V IRTUAL I NSPECTOR reads the .nsp file and
configure itself opportunely accordingly to the instructions
there specified. With this approach, the designer of the multimedia application does not have to compile a new version
of the system to obtain a new type of layout, but he can
simply design a new GUI layout by simply typing a new
.nsp file. The elements of the .nsp file are divided into two
classes, declarations and framework elements. Declarations
are a sequence of XML elements that declares all the 3D entities (models, viewers, lights) that will be displayed, while
the framework is the tag which declares the current screen
configuration (e.g. resolution used or other parameters).
The XML approach allows to change very easily the look
and feel of V IRTUAL I NSPECTOR. As an example, compare
the different layout and graphics of intermediate versions
(see Figures 4 and 5) with the one of the more professional
Arrigo VII’s installation (Figure 7), where a professional
graphic designer has redesigned the layout of the application, all icons and the background graphics elements. This
has been done by the easy specification of the new images
and location on the screen of all icons and elements of the
GUI in the XML initialization file and did not required neither programming nor recompilations. It is a task that can be
easily assigned to an operator with very limited IT competence.
8. Interoperation with a web browser and other
multimedia data
The specification of hot spots is extremely easy in V IRTUAL
I NSPECTOR, since modifications to the 3D models are not
required. We provide a simple 3D browser to the person in
charge of the implementation of the multimedia presentation, which allows to query the 3D coordinates of any point
on the surface of the artifact (by simply clicking with the
M. Callieri & F. Ponchio & P. Cignoni & R. Scopigno / Easy Access to Huge 3D Models
Figure 5: V IRTUAL I NSPECTOR showing two models of the
Minerva statue, scanned at different times during restoration.
mouse on the corresponding point). Then, a new hot spot is
specified by introducing a new XML tag in the V IRTUAL
I NSPECTOR specification file. The hot spot XML tag specifies basically the 3D location and the action that has to be
triggered when clicking on the hot spot (e.g. the name of the
html file, if we want to open a multimedia page). After activation, the control passes to the html browser, while V IR TUAL I NSPECTOR remains sleeping in the background and
regains automatically the control of the interaction whenever
the html page is closed.
A museum installation can be organized with introductory
HTML pages, which present some general artistic/historic
information on the work of art; some of these may provide
links to activate V IRTUAL I NSPECTOR on a single or multiple artifacts (see an example concerning the Arrigo VII installation [BBC∗ 04] in Figure 6).
9. Evaluation and use of the V IRTUAL I NSPECTOR tool
The V IRTUAL I NSPECTOR tool was originally conceived in
the framework of a cooperation with the Centro di Restauro
(Restoration Laboratory) of the Soprintendenza Archeologica Toscana in Florence, a cooperation aimed at the restoration of the Minerva statue and started in year 2000. This cooperation made clear the need of an easy-to-use tool which
could allow the restorers to access the accurate 3D model.
One basic requirement was obviously the capability to show
the 3D data selectively, giving to the user the capability to access even the high resolution model interactively and without loosing accuracy and detail. The first experimentation
of the V IRTUAL I NSPECTOR tool were very encouraging,
fulfilling the above requirements. A first release of the system was evaluated by the restorers and was considered very
handy. Since then, the various version of V IRTUAL I NSPEC TOR have been routinely used in the scanning projects of our
group
Figure 7: V IRTUAL I NSPECTOR: the “Arrigo VII enthroned” statue rendered with active hot spots (top); a short
popup panel with a short info, describing the missing hand,
appears when the mouse passes over the hotspot (middle).
10. Conclusions
We have presented the V IRTUAL I NSPECTOR tool, an interactive system for the visualization of complex and accurate digital 3D models. It has been designed according to
the specification and needs of both restorers and art curators,
and qualifies as a very handy tool for discovering the beauty
and complexity of works of art. From a computer graphics
point of view, the innovation degree of this proposal does not
rely in the single techniques used, but rather in the design
of a complex system based on state-of-the-art technologies.
To our knowledge, V IRTUAL I NSPECTOR is the only visualization tool specifically designed for CH applications which
fulfills the specification listed in the introduction section.
Acknowledgements We would like to thank: the Stanford Computer Graphics Group and Marc Levoy; the graphics group at MPI
Saarbrucken (H. Lensch, M. Goesele, H.P. Seidel); our partners
in the CH domain: Franca Falletti, the Director of the Galleria
dell’Accademia Museum, Clara Baracchini of the CH Superintendency in Pisa, and the archaeologists and restores of the Archeological Restoration Laboratory in Florence. We acknowledge the financial support of the EU IST-2001-32641 “ViHAP3D" project and the
EU NoE EPOCH.
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M. Callieri & F. Ponchio & P. Cignoni & R. Scopigno / Easy Access to Huge 3D Models
Figure 6: The initial screen of the Arrigos VII’s multimedia kiosk and one of the following sub-index pages are shown above;
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