Touching Data: Creating
an Interactive Narrative

To bring volume rendering into the galleries of public venues we used several building blocks to design and implement a prototype of a visualization table for volumetric data termed “Inside Explorer” (see the sidebar “Historical Notes on Virtual Autopsy”). As starting points we used: state-of-the-art DVR software based on our research and development for high-quality visualization of large-scale volume data; large display screens with multi-touch interfaces; interaction design targeted at minimizing visual clutter and hiding of advanced concepts (such as the TF); and challenging large-scale volumetric data obtained from the scanning of humans and animals, as well as various artifacts.

It should be noted that interactive DVR has rarely been deployed at public venues to enable a general audience to explore scientific content. It is primarily a tool for research and limited professional use, requiring complex settings and domain-specific knowledge. The freedom and flexibility provided in professional medical workstations and experimental research software pose interesting challenges. For example, see Yu et al. 15 and Jian et al. 8 who both use multi-touch tables but primarily target expert users. These tools allow free exploration desired by expert domain users. However, for the general public of novice users, built-in guidance, simplified visual representations, and interaction are all necessary for efficiently supporting interactive narrative exploration. Here, we provide several insights gained with respect to the constraints and demands imposed on visualization at public venues. Our discussion focuses on four critical areas.

Performance. In a public installa-
tion, all aspects of performance must
be considered in order to optimize the
user experience. Such installations
require fast loading times, as case-
specific data is being loaded at runtime,
and fast rendering times, with at least
30 frames per second at consistent high
image quality. There are several rea-
sons behind this requirement. In con-
trast with the medical- and scientific-
visualization domains, where accuracy
is more important than fast rendering
and responsiveness, users at public
venues are eager to get started, not
necessarily technically knowledgeable,
and can potentially lose patience and
interest. They are also not aware of the
demanding nature of the datasets that
are being displayed and often compare
performance with the latest computer
games. However, the key realization
for software developers with respect to
performance is the extreme sensitiv-
ity of human perception to large-scale
touch interfaces and lagging visual up-
dates. While the system is capable of
creating the notion of touching a real
artifact, that experience is lost as soon
as users perceive lag in the interaction
or rendering is perceived.
The Gebelein Man dataset has a size
of 512 × 512 × 1,834 voxels, 16-bit inte-
gers, and rendering image size of 1,920
× 1,080 (full HD). To ensure high frame
rates with sufficient image quality, the
renderer employs a set of optimizations:
generation of a tight bounding surface
for ray entry points and skipping empty
blocks of data; termination of ray inte-
gration when opacity is saturated along
the ray; and use of fast simplified shad-
owing using a “planesweep” technique
that diffuses shadows from the light
source and creates a shadow volume. 14
The frame rates achieved range from
32Hz to 60Hz for the four different TFs
used, as in Figure 4.
Robustness. From both a hardware
and a software perspective a public
environment poses particular robust-
ness challenges in terms of number
and type of user, hours of operation,
and physical handling. The software
must be almost as robust as high-avail-
ability systems and not provide users
any opportunity to bypass the user
interface and access lower layers of
data or software. During hours of op-
eration, the equipment must be able
to endure handling by a wide range of
visitors and sustain substantial wear
and tear, as well as intentional damage
caused by some of the visitors. Our ex-
perience is that multi-touch interfaces
are indeed very robust, with all inte-
grated technology hidden behind tem-
pered glass and difficult to damage,
even in environments like the British
Museum, the leading visitor attraction
in the U.K., with more than six million
visitors per year.
Interaction. At public venues, the
threshold time for “walk up and use”
A design constraint
is the limitation
of interactions
that cause rapid
context switches
and dramatic
scene changes.