AVOCADO - The Virtual Environment Framework
by Henrik Tramberend, Frank Hasenbrink, Gerhard Eckel
and Uli Lechner
AVOCADO is a software framework designed to allow the rapid development
of virtual environment applications for immersive and non-immersive
display setups like the CAVE (CAVE Automatic Virtual Environment),
CyberStage, Responsive Workbench and Teleport. It supports the
programmer in all tasks involved with these types of applications.
AVOCADO integrates a variety of different interface devices, is
easily extensible and adaptable for new devices to be invented.
It is highly interactive and responsive, supports a rapid prototyping
style of application development, will enable the development
of truly distributed applications, and is targeted at high-end
Silicon Graphics workstations and aims to deliver the best performance
these machines are capable of.
AVOCADO includes the following concepts:
- Viewer: All kinds of configurations of input and output devices
can be assembled to viewers. A viewer builds up the interface
between the user and the virtual world. Typical elements of a
viewer are the visual, auditory and tactile displays as output
devices and spatial trackers, audio or video sources as input
devices. In a multi-user environment every user configures his
own viewer.
- Scripting: All relevant parts of the system’s Application Programming
Interface are mapped to Scheme, an interpreted scripting language.
This enables the user to specify and change scene content, viewer
features and object behavior in a running system.
- Streaming: All objects know how to read and write their state
to and from a stream. This is the basic facility needed to implement
object persistence and network distribution. Persistence together
with streaming support for objects enable the user to write the
complete state of the system to a disk file at any time. An initial
system state can be read from a disk file as well.
- Distribution: All AVOCADO objects are distributable, and their
state is shared by any number of participating viewers. Object
creation, deletion and all changes at one site are immediately
and transparently distributed to every participating viewer.
- Extensions: The System is extendible by subclassing existing C++
system classes. This concerns object classes as well as classes
which encapsulate viewer features. Compiled extensions can be
loaded into the system at runtime via Dynamically Shared Objects.
- Interaction: Viewers provide input/output services which can be
mapped to objects in the scene. Objects can respond to events
generated from input devices or other objects and can deliver
events to output devices.
- Visual Rendering: The different displays all have their own rendering
mechanism applied to the modeling hierarchy. Only the visual rendering
has a direct access through the Performer pipeline. The auditory
and the tactile rendering can be calculated on a second computer,
connected to the ‘master’ by a fast network.
Visual Data Processing
The visual data processing is organized in a pipeline and computed
in parallel by Performer. This rendering pipeline consists of
a set of optional units, for:
- a database connection
- a user application
- the visual culling of the scene
- the intersection of objects,
- the drawing of the scene.
After the modeling hierarchy is updated to its actual state in
the application process it is passed on to the culling process
which strips all invisible objects. It is important to support
this technique by dividing large geometry into smaller, cullable
objects. The part of the scene left over after the culling is
passed on to the drawing process where it is rendered to the screen
with OpenGL. For configurations with more than one visual display
system, the appropriate number of pipelines is used.
Auditory Rendering
Rendering the auditory scene has to take into account the position
of the observer´s head in the virtual world and in the auditory
display as well as the characteristics of the auditory display
(ie the loudspeaker configuration). The auditory rendering process
is a two stage process. In the first stage a source signal is
synthesized and in the second stage it is spatialized. In the
first stage only the sound model parameters are needed by the
rendering process. In the second stage the signals driving the
auditory display are computed as a function of the distance between
observer and sound source, the radiation characteristics of the
source and the signature of the acoustic environment.
With these signals the auditory display produces the illusion
of a sound source emitted from a certain position in a certain
acoustic environment shared by the observer and the source. The
sound rendering is a dynamic process that takes into account movements
of the observer in the display, movements in the virtual world,
and movements of the sound source. If these movements are faster
than about 30 km/h, the pitch changes due to Doppler shift are
simulated as well.
Tactile Rendering
The CyberStage display has a set of low-frequency emitters built
into its floor. This allows vibrations to be generated, which
can be felt through the feet and legs. There are two main areas
of application of this display component. First, low frequency
sound (which cannot be localized) can be emitted to complement
the loudspeaker projection. Second, specially synthesized low
frequency signals can be used to convey attributes such as roughness
or surface texture.
The vibration display is handled like sound in the rendering process.
Sound models are used to generate the low-frequency signals. Sound
synthesis techniques, generally referred to as granular synthesis,
are very well suited to produce band-limited impulses that may
represent surface features. Such features can be displayed through
user interaction. For instance, a virtual pointing device can
be used to slide or glide over a surface and produce vibrations.
Additionally, higher-frequency sound can also be produced if necessary.
Some of what can be felt usually through the skin of our fingers
when sliding over an object is presented to our feet. This sensation
can complement sound and vision dramatically.
Please contact:
Henrik Tramberend - GMD
Tel: +49 2241 14 2364
E-mail: henrik.tramberend@gmd.de
Frank Hasenbrink - GMD
Tel: +49 2241 14 2051
E-mail: frank.hasenbrink@gmd.de
Gerhard Eckel - GMD
Tel: +49 2241 14 2968
E-mail: gerhard.eckel@gmd.de
Uli Lechner - GMD
Tel: +49 2241 14 2984
E-mail: ulrich.lechner@gmd.de
