DECEMBER 2001
24
In parallel with this trend, new auto-stereoscopic
techniques are now being developed
that hopefully will allow true 3D stereoscopic
views to be generated and viewed
without the need for external aids such as
spectacles and without undue strain on the
part of the user. In this particular context,
considerable funding for these new developments
in auto-stereoscopy has been
forthcoming from the entertainment industry,
since many content providers perceive
a large market developing for 3D stereoscopic
products in the cinema, TV and
video games sectors of their industry. There
is also a considerable interest from the
medical profession in the use of 3D stereoimages
in certain types of surgical procedures
and in the 3D stereo-visualization of
computer tomography and magnetic resonance
scans. To this can be added the further
strong interest in and the support for
the development of new 3D stereoscopic
technology from defence agencies. This is
related particularly to their training and
simulation requirements as well as those
for image interpretation for intelligence and
mapping purposes. If all of this current
activity and development of 3D stereoscopic
technology is successful, then of course
it will be very beneficial to the geoinformatics
industry as a whole as well as these
other fields of activity.
3D Stereo-Viewing for
Photogrammetry & Image
Interpretation
Within photogrammetry, the traditional
method of viewing hard-copy stereo-pairs
in analogue and analytical instruments
mostly involved the use of high precision
photo stages and elaborate and expensive
trains of high-quality optical components to
allow the binocular 3D stereo-viewing of
any part of the stereo-model. Often these
stereo-plotting instruments featured zoom
lenses or turreted optical systems with
alternative lenses to allow the magnification
of the stereo-model needed for high
precision measurements. Some less accurate
and less versatile stereo-plotting
instruments based on optical projection
(such as Multiplex, Balplex and Kelsh
Plotters) used simpler systems such as
anaglyphic (red/blue) filters or mechanically
operated alternating shutters to implement
the viewing of the resulting stereo-model.
On the image interpretation side, generations
of geologists and other field scientists
and military photo-interpreters utilized
simple lens or mirror stereoscopes in conjunction
with stereo-pairs of hard-copy aerial
photographs to carry out their interpretative
work. These traditional techniques are
still in widespread use.
3D Stereo-Viewing of Digital I
Is Auto-Stereoscopy the Future for 3D?
With the increasing availability of digital imagery for use in mapping and GIS operations,
the whole matter of the 3D stereoscopic viewing of aerial and space imagery
has been undergoing a radical change. The reasons are varied. On the one hand, we
have seen the widespread adoption of the digital photogrammetric workstation (DPW).
In association with this development, the computational power that now exists within
the DPW can handle comfortably the large amounts of digital image data that are
required to produce high-quality 3D stereoscopic images of the terrain interactively in
real time on its monitor screen. So the implementation of the new 3D display technology
is now much easier than before. Besides which, there is now an increasing realization
that the 2D visualizations of 3D terrain data that are currently being labeled as
"3D views" are insufficient and indeed deficient for many purposes - especially for
measurement and for image interpretation and image understanding. This has also
encouraged the use of truly 3D stereoscopic imagery.
By Prof. Gordon Petrie
Figure 1: Twin monitors viewed with a mirror stereoscope (Drawn by Mike Shand)
3D Stereo-Viewing of Digital I
DECEMBER 2001Latest News? Visit www.geoinformatics.com
25
3D Stereo-Viewing in
Remote Sensing & GIS
Turning next to remote sensing, although
digital image processing systems equipped
with display monitors have been used with
space images ever since the first Landsat
satellite came into operation nearly thirty
years ago, the data has almost always
been monoscopic. Indeed many users of
satellite remote sensing imagery have
never viewed a 3D stereoscopic image on
the display screens of their systems. Nor
have they used 3D stereoscopic data in
their analyses. As for the GIS community,
as noted above, what is often described by
them - even in GeoInformatics magazine! as
a 3D image or visualization is, in fact, a
2D perspective view or projection of the
terrain. This usually takes the form of a flat
2D image appearing either on a display
screen or in hard-copy form, even if 3D
data such as a DEM was used in its construction
or formation.
Introduction of Digital Imagery
However, with the introduction of digital
photogrammetric workstations (DPWs) in
the early 1990s and their increasing acceptance
and use in the late 1990s, the whole
situation began to change quite rapidly. In
particular, the development of the DPW
eliminated the need for the expensive highprecision
mechanical and optical components
of the previous analogue and analytical
instruments. Stereo-pairs in digital form
generated from the scanning of hard-copy
aerial photographs soon became commonly
available and into widespread use. So the
3D stereo-viewing technology had to be
changed in accordance with this development.
With the advent of the new airborne
digital imagers (e.g. HRSC-A, ADS40) and
spaceborne digital imagers (e.g. those
mounted on the IKONOS, EROS-A1,
OrbView-4, QuickBird-2 and SPOT-5 satellites)
that can generate overlapping pushbroom
scanner images in the along-track
direction, the situation can be expected to
change still further. Certainly more 3D
stereo-image data will become available in
the future. Thus it does seem appropriate
to review the current situation regarding
the 3D stereo-viewing of digital imagery
acquired from both airborne and spaceborne
platforms and to speculate on its
future development.
Why 3D Stereo-Viewing?
Based on the extensive experience of the
mapping community over many years, there
is no doubt that the use of 3D stereo-viewing
improves the interpretation and measurement
of imagery of almost all types of
terrain. Thus the provision of a 3D stereoviewing
facility is regarded by most photogrammetrists
as being a complete necessity
for their operations. In the first place,
this facility is required for the accurate
identification and measurement of the
ground control points needed for the absolute
orientation or geo-referencing of the
stereo-model. After which, it is then needed
for the subsequent measurement of the terrain
detail (nowadays called feature extraction!)
that is required for topographic map
compilation and the acquisition of the 3D
digital data needed for use in a GIS/LIS
environment. A 3D stereo-viewing facility is
also a vital element in carrying out map
revision and the editing of the digital elevation
data (DEMs) produced by automatic
image matching techniques. Last, but not
least, the availability of such a facility also
permits the stereo-superimposition of 3D
vector data over the stereo-model for quality
checks of the accuracy and completeness
of the 3D data. Thus virtually all
DPWs feature a stereo-viewing and measuring
capability - except for one or two systems
produced by companies that specialize
primarily in the remote sensing field
and appear to be less aware of the
attributes of 3D stereo-viewing.
Current Methods
Six methods of 3D viewing are currently in
use within the general area of photogrammetry
and geoinformatics. However many
others are possible - the number of
patents for 3D stereo-viewing systems run
into the many thousands. Whatever the
method actually used, the basic requirement
for 3D stereo-viewing is that the
observer's left eye must see the left image
only of the two overlapping images and
the right eye, the corresponding part of the
right image only.
• Binocular Viewing
(1) The first possibility is to utilize two flatscreen
display monitors displaying the left
and right images of the stereo-pair respectively.
These can be viewed using a mirror
stereoscope or a more complex optical
train (Fig. 1). This arrangement has been
magery
Figure 2: Single monitor display with split screen
viewing (Mike Shand)
Figure 3: Twin monitors viewed with polarizing spectacles (Mike Shand)
magery
DECEMBER 2001
26
used in the early I2S DPW, which has now
disappeared from the market. The matter of
ensuring the correct positioning and alignment
of the twin monitors and the exact
correspondence of the left and right images
displayed separately on the two monitors
appears to have discouraged the use of
this approach.
(2) An alternative and more popular
approach is to display the left and right
images side-by-side on a single display
monitor and to view these through a simple
lens or mirror stereoscope (Fig. 2) - the
so-called split-screen stereo method. This
was the method used in the pioneering
Kern DSP1 DPW from the late 1980s. It is
currently in quite extensive use on many of
the DPWs produced by the Canadian company,
DVP Geomatic Systems. Other DPWs
employing this arrangement include LH
Systems' 600 series DPWs running the
SOCET SET software suite, GeoSystems'
Delta Workstation and some examples of
the KLT Associates Atlas DPW. The increasing
availability of the wide-screen television
sets that have been developed recently for
use with HDTV would appear to be very
suitable for the display of side-by-side
images for 3D stereo-viewing.
• Use of Complementary Filters
(3) Another possibility is to use two flatscreen
display monitors set at right angles
to each other, the one with its axis pointing
horizontally, the other with its axis
pointing vertically upwards. One has a horizontal
polarization sheet placed in front of
its screen; the other has a vertical polarization
sheet placed over it. A large semireflecting
mirror set at a 45° angle between
the two monitors acts as a simple beam
splitter. This allows the two component
images of the stereo-pair to be superimposed
on each other, while the observer
wears appropriate spectacles fitted with the
corresponding horizontally and vertically
polarizing filters to allow stereo-viewing
(Fig. 3). This arrangement was used originally
by Matra in its Traster analytical plotters
employing hard-copy photographs. The
same basic arrangement has been used in
the Topcon PI-1000 DPW and in
Galileo/Siscam's Stereodigit and Microdigit
DPWs. In each case, quite small monitors
have been used for the image display which
rather limits their resolution; on the
other hand, the method does not involve
dynamic alternating imaging which carries
the possibility of flickering. The Microdigit
DPW utilises twin liquid crystal (LC) displays;
however the other two DPWs in this
group both utilize conventional CRT-based
monitors. But it must be said that these
DPWs have only had a very limited penetration
of the market.
(4) A low-cost solution that provides a 3D
stereo-viewing capability is simply to
superimpose the two component images of
the stereo-pair on the screen of a single
colour monitor with the one image displayed
in red and the other in blue or
green using the well-known anaglyphic
technique. As noted above, this method is
familiar to most photogrammetrists from its
use on analogue stereo-plotting instruments
such as the Multiplex, Balplex, Kelsh
Plotter, Zeiss DP-1, etc. based on optical
projection. With DPWs, users view the
resulting 3D stereo-model appearing on the
screen of the display monitor wearing spectacles
equipped with the corresponding
red/blue or red/green filters (Fig. 4). This
method has been used quite a lot with the
R-WEL DMS system. Previously the method
has been limited to the use of
monochrome (black and white) stereoimagery.
However recent developments
have shown that some limited colour rendition
of the stereo-model from colour aerial
photographs is possible using the
anaglyphic technique.
• Alternating Images
(5) Probably the most commonly used 3D
stereo-viewing system on DPWs is to display
the left and right component images
alternately at high speed, e.g. 50 to 60
Hz per image, on a single monitor screen.
This makes use of the capabilities of the
human visual system to store and merge
the two component images of a stereopair
across a time gap of 50ms. The actual
3D stereo-viewing is then carried out
using so-called "active" spectacles
equipped with alternating shutters. These
are synchronised with the alternate left
and right images being displayed on the
monitor screen so that the left eye sees
the left image only and the right eye, the
right image only (Fig. 5). Again, as noted
above, the basic idea is familiar to those
photogrammetrists who have used systems
such as the Stereo-Image Alternator
(SIA) on older optical projection instruments
such as the Balplex and Kelsh
Plotter. However these systems used
mechanically operated shutters. The modern
versions that are in use with DPWs
use pairs of liquid crystal (LC) alternating
shutters mounted in spectacle frames.
The open/shut operations of these shutters
are synchronized electronically with
the display image. Thus the spectacles
need power and some control circuitry,
together with a timing signal from the
display monitor that triggers the operation
of the appropriate shutter. This signal
is provided either by direct wiring to
the display controller or using a wireless
technique employing an infra-red (IR)
emitter mounted on top of the display
monitor.
Most of the suppliers of DPWs that utilize
this type of alternating shutter technology
have adopted the CrystalEyes system from
the StereoGraphics Corporation based on
the use of LC shutters and an IR emitter.
However the rival MacNaughton company which
is best known for its NuVision system
(see below) - also offers this type of
system via its 60GX wireless spectacles,
again used in conjunction with an IR emitter.
Many much cheaper alternatives can
now be sourced from various suppliers in
the Far East and the U.S.A. that are serving
the huge 3D video games market. Formerly
the method was mainly in use on those
DPWs based on up-market SGI graphics
work stations which were "stereo-ready",
i.e. they did not need additional hardware
to implement the method. However this no
longer applies since PCs are readily adapted
for the purpose using "stereo-ready"
cards that are produced in large numbers
for use with 3D video games. Thus the
technology is now in widespread use by
almost all DPW suppliers.
Figure 4: 3D
stereo-viewing
with
anaglyphic
spectacles
(Mike Shand)
DECEMBER 2001Latest News? Visit www.geoinformatics.com
27
(6) The final method that is in current use
on DPWs also uses alternating (left/right)
overlapping images displayed on a single
monitor screen. However, in this case, each
successive image displayed on the screen
has a different (clockwise/anti-clockwise)
polarisation pattern induced by a LC-based
modulator - sometimes described as an
electronic "prism" or "panel" - that is
bezel-mounted on the front of the display
monitor (Fig. 6). Users viewing the 3D
stereo-model wear spectacles equipped
with the corresponding polarising filters.
Again these ensure that the left eye only
sees the left image and the right eye, the
right image only. Moreover these simple
and inexpensive spectacles do not need
power to activate them. Thus they are usually
described as being "passive" spectacles
to distinguish them from the so-called
"active" spectacles equipped with alternating
shutters described in the previous paragraphs
in (5) above. The principal supplier
of this type of viewing system has been
the MacNaughton company with its
NuVision system that was developed originally
by the Tektronix company. This is
available both in the form of an integrated
monitor or as kits that can be fitted over
the screen of an existing display monitor.
Recently the rival StereoGraphics company
has also adopted this technology with the
introduction of its so-called ZScreen 2000
monitor series.
Multiple Viewing
It will be noted that those 3D stereo-viewing
systems that use spectacles equipped
with filters or alternating shutters - as
described in (3) to (6) above - all allow a
number of users to view the stereo-images
simultaneously. Whereas those systems
employing binocular mirror or lens stereoscopes
- types (1) and (2) above - restrict
the viewing to a single observer. However
doubtless, if required, this restriction could
be overcome to some extent through the
use of dual binocular systems, as was
done in the past with a number of analogue
or analytical stereo-plotting instru-
ments.
Hard Copy Prints & Transparencies
For many years, the principal method of
producing 3D hard-copy prints and transparencies
for publication or widespread use
has been to use the anaglyphic method.
The two component images of the stereopair
are printed in red and blue respectively,
superimposed on a common base sheet.
Usually this printing is done using a photographic
or lithographic process. 3D viewing
of the printed stereo-pair is then achieved
using spectacles equipped with complementary
red/blue filters. Quite recently, the
StereoJet process has been introduced
which allows 3D stereo-viewing of fullcolour
hard-copy images produced from
overlapping digital imagery. With this process,
the left and right images of the
stereo-pair are registered and printed on
the opposite sides of a transparent base.
This is done using an inkjet printer applying
proprietary polarizing inks on to a proprietary
substrate. The two printed images
have polarizing patterns that are oriented
at right angles to one another. As with the
corresponding screen displays described
above, the observer obtains the 3D stereoscopic
image by viewing the prints or
transparencies with spectacles equipped
with the appropriate polarizing filters.
Examples of this new process were shown
at the recent ASPRS 2001 Annual
Conference held in St. Louis. Several service
bureaux have also been set up in the
U.S.A. (e.g. San Francisco Imaging Services)
and Canada (e.g. Aero Geometrics) to supply
this product to the mapping community
in North America.
Auto-Stereoscopic Displays
At the present time, an enormous research
effort is being undertaken and huge sums
of money are being spent on the development
of auto-stereoscopic displays using
digital image data. These devices display
(or attempt to display) the 3D spatial
image of an object to an observer without
the use of spectacles, filters or other external
viewing aids. Thus commentators now
talk and write about having the possibility
of having "free viewing" systems in the
future instead of the "aided viewing" systems
employing spectacles and filters that
are currently being used on DPWs and in
other applications utilizing 3D stereoscopic
viewing. Much of this research effort is
directed specifically towards medical, CAD,
industrial inspection, flight simulator and
multi-media applications. However the
results will be of considerable importance
for the geoinformatics industry. At the present
time, there are a wide variety of alternative
techniques being investigated or
developed to form the basis of auto-stereoscopic
displays. These are being carried out
both by a number of small enterprising
start-up companies and by some large and
well-established companies such as Philips,
Zeiss, Sharp, Sony, Sanyo, etc., that are
well known in the fields of optics and consumer
electronics. Besides which, a number
of research groups in various universities in
the U.S.A., Japan and Europe (especially in
Germany and the U.K.) are also involved in
this development. Even attempting to sumFigure
5: Alternating images on the monitor screen with alternating shutters for 3D stereo-viewing (Mike Shand)
Figure 6: Alternating
images on the monitor
screen with an electronic
prism set in front of the
screen: 3D viewing using
polarizing spectacles
(Mike Shand)
DECEMBER 2001
28
marize briefly all the various different
approaches would fill a whole issue of
GeoInformatics magazine: it really is a wild
and somewhat confusing scene! Besides
which, many of the concepts are quite
complex and far from easy to understand.
Thus only a couple of simple examples
based on the use of gratings and/or lenticular
sheets will be outlined here to illustrate
the basic idea of auto-stereoscopic
displays. Many more complex approaches
are being worked on at the moment.
• Parallax Stereograms
The starting point may be taken as the socalled
parallax-barrier method, well known
for nearly a century and forming the basis
of the rather crude 3D viewing cards that
come free in packets of breakfast cereals. A
much more refined version of this basic
idea is being used with digital image data.
This utilizes a very finely ruled grating that
is placed in front of a specially produced
image. The spacings in the grating form a
series of parallel transparent slits or gaps.
Each slit in the grating acts as a window to
a specific vertical column or strip of the
image placed behind it. This image comprises
two sets of interlaced and alternating
vertical columns derived from the two
images of the stereo-pair. This striped
interlaced image and the vertical grating
with its slits are arranged so that the
observer's left eye can only see the oddnumbered
columns or strips derived from
the left image; while the right eye can only
see the even-numbered columns or strips
from the right image. The opaque lines of
the grating block the unwanted images
from being viewed. Thus both images of
the stereo-pair are displayed simultaneously
and reach the appropriate eyes of the
observer. The result is that the observer
sees a 3D stereoscopic image.
• Lenticular Sheets
An extension to this basic concept involves
replacing the grating with its set of parallel
slits by a lenticular sheet. This comprises
an array of long parallel cylindrical lenses
or lenslets that are formed from a transparent
plastic material using a special mould.
Each cylindrical lens in the array takes the
place of an individual slit. The result is a
much improved set of images with a considerable
reduction in the loss of light
caused by the opaque lines of the grating
used in the previous method.
• DTI 3D Auto-Stereoscopic Displays
These concepts and technologies have
been used as the basis for a series of 15
and 18 inch flat-panel colour LCD displays
produced by a small American company,
Dimension Technologies Inc. (DTI) based in
Rochester, New York. Instead of the grating
being placed in front of the image, it takes
the form of an array or lattice of very thin
but very bright vertical lines of light placed
behind the transparent liquid crystal display
(LCD) panel on which the strip images
are being displayed (Fig. 8). This illumination
lattice is generated by a lenticular
sheet which focuses the light from fluorescent
lamps and forms the parallel series of
thin light lines on a translucent diffuser.
These illuminate the alternating image
columns so that each eye receives only the
appropriate strip images. In this way, a 3D
stereoscopic impression is formed - though
it can only be viewed by a single person
from a fixed position in front of the display
panel. Thus they are single user displays.
These DTI 3D auto-stereoscopic displays
have been supplied both to NASA and to
various U.S. defence agencies concerned
with intelligence gathering and mapping.
The displays are in use with Recon Optical,
the American manufacturer of airborne digital
reconnaissance cameras, and have also
been evaluated recently by Z/I Imaging.
• Philips 3D Auto-Stereoscopic Displays
In the case of the Philips 3D-LCD autostereoscopic
displays, the sheet of finepitched
cylindrical lenticular lenses is
placed in front of a high-resolution LC
colour display panel in such a position that
the LCD image plane lies in the focal plane
of the lenses (Fig. 8). If the stereo-pair of
images of the terrain are divided up into
alternating vertical strips or columns from
the left and right images respectively, then,
once again, all the columns of the left
image are seen by the left eye only, while
the columns of the right image are only
seen by the right eye. Combining the two
sets of image data, the observer obtains a
full 3D stereoscopic impression of the terrain.
Recent further developments have
seen the technology enhanced to allow
several persons to view the 3D stereoscopic
image at the same time. Philips appear
to be concentrating their efforts on selling
this technology and its accompanying software
on an OEM basis to companies that
are already established in specific application
fields. This could well suit the system
suppliers to the geoinformatics industry.
DEMs & Orthoimages
With the advent of overlapping airborne and
spaceborne digital imagery has come the
highly automated production of DEMs and
orthoimages. The DEMs are produced using
automatic image matching (correlation) techniques.
This operation is then followed by
the ortho-rectification of one of the overlapping
images using the 3D coordinate data
produced by the DEM to generate the orthoimage.
This capability is provided on almost
all modern DPWs. For some time now, this
procedure has been implemented both for
stereo-pairs of aerial photographs and with
the overlapping cross-track scanner imagery
acquired by the SPOT and IRS-1C/D satellites.
With the current introduction of overlapping
imagery acquired along-track by
pushbroom scanners operating both from
airborne platforms (e.g. the HRSC-A and
ADS40 imagers) and spaceborne platforms
(e.g. the IKONOS and EROS-A1 imagers), this
type of automated operation can be expected
to become still more widespread. Indeed
already both Space Imaging with its IKONOS
space image data and ISTAR with its HRSC-A
airborne image data offer DEMs and
orthoimages as standard products from their
overlapping pushbroom scanner imagery.
Stereomates
Of course, the generation and supply of
fully rectified individual orthoimages or
orthoimage mosaics means that users canFigure
7: (a) The basis of the DTI 3D auto-stereoscopic display. (b) DTI's 15 inch and (c) 18 inch display monitors
(Source: Dimension Technologies Inc.)
DECEMBER 2001Latest News? Visit www.geoinformatics.com
29
not then enjoy the benefits of 3D stereoviewing.
In the past, this led to the development
in Canada, France and Austria of
stereomates. These used the DEM elevation
data to re-introduce parallaxes in a
controlled manner into the rectified
imagery, so creating a second image (the
stereomate) that could be utilised together
with the orthoimage to create and view a
3D stereo-image of the terrain. However
only one or two DPWs offer this possibili-
ty.
Chromo-Stereoscopy
Over the last few years, an alternative
approach has been to use the chromostereoscopic
technique originally developed
by Chromatek Technologies in the
U.S.A. This method has been developed
still further by Dr. Toutin of the Canada
Centre for Remote Sensing (CCRS) for use
with remotely sensed image data. The
basis of the method is completely different
to all of those discussed above. The
different elevation values given by the
DEM are encoded into the orthoimage as
varying colour values using the wide
range of colours available across the
whole of the visible spectrum. Thus it
uses only a single image (the orthoimage)
instead of utilizing two images as
required by all the other stereo-viewing
methods discussed above. This colour
encoding of the orthoimage in accordance
with the corresponding elevation
value for each pixel position is carried
out in a precise and highly structured
manner through a preliminary radiometric
processing step prior to stereo-viewing.
Once this step has been completed, the
resulting colour-encoded image is viewed
by the observer when it is then decoded
using quite simple and inexpensive prismatic
spectacles to give a strong 3D
impression of the terrain covered by the
ortho-image. Thus the chromo-stereoscopic
approach is very different to that
of the conventional 3D stereoscopic techniques
- which are based on the use of
two images containing disparities or parallaxes
(resulting from relief displacements)
related to the terrain elevation to
produce the required depth. By contrast,
the chromo-stereoscopic orthoimage
retains its map geometry but still gives a
3D stereoscopic impression of the terrain
to an observer wearing the appropriate
ChromoDepth spectacles. The chromostereoscopic
images can either be printed
in hard-copy form or displayed on colour
monitor screens. Readers of
GeoInformatics have already had the
opportunity to view the various examples
of chromo-stereoscopic images that were
included in the September 2001 issue of
the magazine.
• ChromoDepth 3D Stereo-Viewing
The basis of the actual stereoscopic viewing
is to use the so-called ChromoDepth
spectacles. These are equipped with pairs
of very thin double prisms made from plastic
film. One of the two prisms in each
double prism is made of a highly dispersive
material with a refractive index that is
strongly wavelength (i.e. colour) dependent
(Fig. 9b). The second prism in the double
prism has a low refractive index. When
combined together, the resulting double
prism acts like a thick glass prism (Fig. 9a):
however they are actually produced in the
form of a thin plastic film. When the pair of
double prisms are mounted in the appropriate
orientation in the frame of the viewing
spectacles, the colour encoded image
will then appear to the observer in stereoscopic
3D giving depth to the scene (Fig.
9c). Thus the terrain objects with the highest
elevation (encoded in red) will appear
to be nearer (i.e. higher) to the observer
than those at the lowest elevations (encoded
in blue) that appear to lie further away
(i.e. at a lower elevation). Terrain objects at
intermediate elevation values (encoded in
orange, yellow, green, etc.) will appear to
lie at the appropriate depth (or elevation)
between the highest and lowest elevation
values.
Conclusion
With the ever-increasing availability of
overlapping stereo-imagery acquired in
digital form from airborne and spaceborne
platforms, the benefits of 3D stereo-viewing
can be experienced by ever more people
working in the geoinformatics field.
Thus the current developments in 3D
stereo-viewing technology - such as the
auto-stereoscopic, StereoJet and chromostereoscopic
techniques - are of considerable
interest. They promise to make it
easier for users to utilize 3D stereo-viewing
of the terrain with all the advantages
that go with it.
Professor G. Petrie (g.petrie@geog.gla.ac.uk),
Department of Geography & Topographic Science,
University of Glasgow, Glasgow, G12 8QQ,
Scotland, U.K.
Web Pages - http://www.geog.gla.ac.uk/~gpetrie
Figure 9:
The basis of chro-
mo-stereoscopic
viewing.
(a) The refraction
of white light
through a
glass prism.
(b) The superchromatic
double
prism.
(c) ChromaDepth
viewing spectacles
using a pair of
double prisms
(Drawn by Mike
Shand)
Figure 8: The
basis of the
Philips 3D
auto-stereoscopic
displays
using a lenticular
sheet
(Source:
Philips
Research)