practical purposes it is quite adequate. Experiments by
Mikhail (1992), Madani (1993) and Walker (1995) indicate,
however, that with image resolutions of perhaps 20 um or
better DPWSs lose little in measurement accuracy compared to
analytical plotters. Two modes of movement are possible:
fixed image moving cursor is much easier to implement than
fixed cursor moving image owing to the hardware limitations
noted above. Sub-pixel movement has been achieved by
custom built hardware in military systems but has been little
offered by commercial vendors. It is possible only by using
zoomed images, though Zeiss has apparently used aliasing, i.e.
small changes in the shape of the cursor, to achieve an optical
effect similar to sub-pixel movement.
2.3.3 Stereoscopic viewing. A useful account of all the
possibilities for stereoscopic viewing was given by Petrie
(1983) 13 years ago! A strong argument can be made that the
best way to achieve stereoscopic viewing is by displaying the
left and right images on separate monitors. This was achieved
in the expensive DSCC, for example, by means of optical
trains, but it has not been popular in commercial systems
partly because of cost and partly owing to technical
difficulties. Nevertheless, Matra used an arrangement of two
monitors at right angles, with horizontal and vertical
polarisations sheets in front of them and a semi-reflector
between them to give the required superimposition. The low
cost DVP system from Leica has split screen stereo, i.e. the
left and right images are displayed alongside one another and
viewed through a simple stereoscope. This works well enough
and is inexpensive, though the number of pixels in each
window may be quite small, but the use of the same
stereoscope has not proved popular with the Leica-Helava
systems, which also have split screen stereo as a standard
function on the console monitor. The split screen approach, of
course, permits only one operator to view the stereoscopic
image, whereas the methods described below enable several
people to see the imagery in stereo at the same time.
The most popular solution for stereoscopic viewing has proved
to be the display of left and right images, in quick succession
(60 Hz per image, giving the 120 Hz figure noted above), on
the same screen, which is viewed through a system developed
for just this purpose. One or two systems, such as the R-WEL
DMS and the early Terragon system, use an inexpensive
anaglyph approach, by displaying the two images in red and
green; as in its use in Multiplex and similar plotters, this
approach is limited to black and white imagery. Two
technologies are in more common use. The more popular, and
less expensive, is CrystalEyes® from the StereoGraphics
Corporation, where the screen is viewed through eyewear
containing PZLT or LCD alternating shutters which are
synchronised with the image display by direct wiring to the
controller or by an infrared emitter usually mounted on top of
the screen. The information to achieve this synchronisation is
obtained via a port on the graphics sub-system located beside
the connector for the RGB cable. Although the only
requirement of the monitor is the high refresh rate, rather few
monitors are available in sizes greater than 21 inches. In
addition to Intergraph’s excellent 27 inch units, there are only
one or two monitors in each of the 29, 33 and 37 inch sizes.
The alternative to CrystalEyes® is polarised viewing, where a
bezel containing an electronic prism is mounted on the front of
the monitor and is synchronised using the same data from the
graphics sub-system as was mentioned above; this polarises
the left and right hand images in clockwise and counter-
clockwise directions. The screen is viewed through polarised
spectacles. The addition of the bezel makes the monitor rather
expensive and the range of sizes available is small. The main
388
manufacturer of these systems is NuVision, a spinoff company
from Tektronix in Beaverton, Oregon. The 19 inch size has
been the most popular, though a number of 17 inch Tektronix
systems are in use and were offered by several vendors
including Matra. The spectacles are lighter weight than for
CrystalEyes®, do not need batteries and are much less
expensive. NuVision systems seem to be preferred at present
by vendors offering dual screen systems, whereas they are not
used at all in single screen ones.
Stereoscopic DPWs may be further classified according to the
number of monitors supplied. Some vendors offer single screen
systems, which obviously cost less than dual screen, at the
danger of having a rather cluttered screen. The CrystalEyes®
system is preferred for these systems, because it works with
virtually any fast refresh monitor and its basic cost is lower
than NuVision. Other vendors prefer dual screen approaches,
arguing that the second screen is used only for the stereoscopic
image and so is less congested and so more relaxing for the
user. Leica-Helava use the term “extraction monitor” for this
“second head”. Though the vendors of two screen systems
offer both viewing systems, there is a slight preference for
NuVision, probably owing the lighter, less “scifi” appearance
of the spectacles and the lower cost of replacement or
furnishing multiple sets for “plotting by committee”. Also, it is
common to suffer severe flickering when viewing the console
monitor through CrystalEyes® eyewear, because it is out of
synchronisation with the stereo monitor; thus the two screens
should be set with a significant angular distance between them,
so that the eyewear switches off when the operator turns
his/her head sufficiently away from the emitter. There is no
problem viewing the console monitor through the NuVision
spectacles. The comparative quality of the stereoscopic image
seen with the two systems seems to be a matter of taste,
depending as much on the individual operator and ambient
light conditions as on the viewing technology itself.
2.3.4 Superimposition. Since its first appearance early in the
1980s, superimposition of vector data over the stereoscopic
image has been an important topic for APs and, to a much
lesser extent, for analogue plotters. It has been acknowledged
that the role of superimposition for quality control through on-
line checks on both accuracy and completeness is a useful one
and that it makes photogrammetry a much more productive
tool for map revision, but it is an expensive option. On DPWs,
however, it is possible to have colour, stereo superimposition
as a standard feature. It is purely a matter of software to
achieve this and many of the graphics boards include hardware
or firmware to assist the process.
2.4 Control devices
There is a feeling in the industry that DPWs are computer
based and movement in the imagery should be performed by
means of computer devices, i.e. mouse and keyboard. This is
not necessarily ideal for photogrammetrists, who are used to
precise positioning and contouring using devices refined over
decades for maximum accuracy and comfort, such as free
moving hand controllers, hand wheels and foot disks. The use
of foot switches for data collection is also common as an
alternative to mouse buttons.
Not surprisingly, therefore, DPW vendors have been offering a
multiplicity of devices to meet this requirement. Again,
Intergraph have played a pioneering role by offering a variant
of their distinctive hand controller seen first on the IMA AP.
In digital photogrammetry, of course, the hand controller
should incorporate buttons to perform a wider range of tasks
than on APs, for example changing level in the image pyramid,
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996
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