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50 4. Building a Practical VR System
makes most LCDs unsuitable for stereopsis. The best technology cur-
rently available for manufacture of desktop and laptop LCD panels is
the back-lit, thin film transistor, active matrix, twisted nematic liquid
crystal described in Figure 4.2. The main problem with LCD film is
that the color can only be accurately represented when viewed straight
on. The further away you are from a perpendicular viewing angle, the
more the color will appear washed out. However, now that they can
be made in quite large panel sizes and their cost is dropping rapidly,
several panels can be brought into very close pr oximity to give a good
approximation of a large wide-screen desktop display.
As illustrated in Figure 4.2, the LCD screen is based on a matrix (i, j)
of small cells containing a liquid crystal film. When a voltage is applied
across the cell, a commensurate change in transparency lets light shine
through. Color is achieved by using groups of three cells and a filter
for either red, green or blue light. Each cell corresponds to image pixel
(i, j) and is driven by a transistor switch to address the cells as shown
in Figure 4.2(a). A voltage to give the required transparency in cell
(i, j) is set on the column data line i. By applying another voltage to
row address line j, the transistor gate opens and the column signal is
applied to the LCD cell (i, j). The structure of the LCD cell is depicted
in Figure 4.2(b). The back and front surfaces are coated with a 90
◦
phased polarizing layers. This would normally block out the light.
However, when no voltage is applied across the cell, the liquid crystals
tend to line up with grooves in the glass that are cut parallel to the
column data lines (i)
transistor
LCD cell
(a)
v=0
(b)
transparent
opaque
v>0
row address lines (i)
glass
polarizer
backlit LCD cell
Figure 4.2. The operation of an LCD screen: (a) electrical layout, (b) physical layout.
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4.1. Technology of Visualization 51
direction of the polarizing filters, and so a twisted orientation in the
liquid crystals develops across the cell. This gradual twist in crystal
orientation affects the polarization of the light to such an extent that
in passing from back to front, its polarization has rotated by 90
◦
and it
matches the polarizing filters, and so the light shines through. When a
voltage is applied to the cell, the crystals align with the electric field, the
twist effect on the polarizing light is lost, and the cell becomes opaque.
Touch screens offer another alternative for pointer-type in-
teraction. Their cost is not prohibitively expensive and their
accuracy, whilst not down to the pixel level, can certainly
locate a finger pointing to an onscreen button. The LCD
or CRT surface of a touch screen is coated with either a
resistive or capacitive transparent layer. In a capacitive sen-
sor, when a finger touches the screen, the capacitance of the
area around it changes, causing a small change in charge
stored at that point. The (AC) electrical current flowing
through the corners to accommodate this change in charge
is proportional to their distance from the point of contact.
Touch screens have wide use in machines that need only
a small amount of user interaction, or in systems where
their greater robustness compared to a keyboard or mouse
is helpful. From a practical point of view, the touch screen
connects to the computer via the serial or USB port, and its
software driver emulates a mouse. Some models will even
fit over an existing LCD panel.
• Plasma displays. Fluor escent display panels based on gas plasma tech-
nology have proved popular in the TV market. They have the advan-
tage that they can deliver very large screen sizes, have a high uniform
brightness from all viewing angles and allow for a higher refresh rate
than most LCDs. They do not have resolution as good as a desk-
top LCD, they generate a lot more heat and may suffer from burn-in,
so they are not normally used for desktop display. They are almost
big enough to constitute a video wall with only a couple of panels.
The technology underlying the plasma display panel, illustrated in Fig-
ure 4.3, is like the LCD system in that it has a matrix of cells which
are addressed by applying a voltage to a row address i and column ad-
dress j. Each cell contains a gas (neon or xenon), and when the voltage
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52 4. Building a Practical VR System
Figure 4.3. The structure of a plasma display panel and cell cross-section.
across cell (i, j) is high enough, the gas will ionize and create a plasma.
Plasma is a highly energetic state of matter that gives off ultraviolet
(UV) photons of light as it returns to its base state. The UV photons
excite a phosphor coating on the cell walls to emit visible light. Some
cells are coated with a phosphor that emits either red, green or blue
light. In this way, a color image can be generated. The plasma generat-
ing reaction is initiated many times per second in each cell. Different
visible light intensity in a cell is generated by changing the number of
times per second that the plasma is excited.
Desktop Projection
Projection is not normally something one thinks about when considering
desktop VR. But if you want something to really stand out then it should
be. In situations where binocular stereopsis does not provide a sufficient sim-
ulation of depth perception, or you really do want to look around the back
of something, projecting an image of a virtual object onto an a pproximately
shaped surface or even onto panes of coated glass may provide an alternative.
Some work on this has been done under the topic of image-based illumina-
tion [15]. The basic idea is illustrated in Figure 4.4, where a model (possibly
clay/wood/plastic) of an approximate shape is placed on a tabletop and il-
luminated from three sides by projectors with appropriate images, possibly
animated sequences of images. This shows surface detail on the blank shape
and appears very realistic. Architectural models are a good candidate for this
type of display, as are fragile or valuable museum exhibits, which can be real-
istically displayed on open public view in this manner. Some distortion may
be introduced in the projection, but that can be corrected as illustrated in
Figure 4.10.
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4.1. Technology of Visualization 53
If you are projecting onto a surface, there are a couple of issues that need
to be addressed:
• The number of projectors it takes to provide the illumination.Whenpro-
jecting onto a surface that is primarily convex, good coverage will be
achieved with the use of three projectors.
• Images which are being projected may need to be distorted.Ifthesurface
of projection is not flat and perpendicular to the direction of projec-
tion then any projected image will be distorted. The distor tion may
be relatively simple, such as the keystone effect, or it may be very non-
linear. Whether to go to a lot of trouble to try to undistort the image
is a difficult question to answer. We will examine distortion in detail
shortly, but for cases where different shapes are being projected onto a
generic model (for example, different textures and colors of fabric pro-
jected onto a white cloth), imperfections in alignment may not be so
important.
4.1.2 Immersive VR
To get a sense of being immersed in a virtual environment, we have to leave
the desktop behind, get up out of the chair and gain the freedom to move
about. This poses a major set of new challenges for the designers of immersive
technology. To deliv er this sense of space, there are two current approaches,
Figure 4.4. Projection of a image of a wine bottle onto a white cylinder gives the
illusion of a real wine bottle being present in the room.
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54 4. Building a Practical VR System
the cave and the head-mounted display (HMD). However, neither are perfect
solutions.
Head-Mounted Displays
An HMD is essentially a device that a person can wear on her head in order
to have images or video information directly displayed in front of her eyes.
As such, HMDs are useful for simulating virtual environments. A typical
HMD consists of two miniature display screens (positioned in front of the
user’s eyes) and an optical system that transmits the images from the screens to
the eyes, thereby presenting a stereo view of a virtual world. A motion tracker
can also continuously measure the position and orientation of the user’s head
in order to allow the image-generating computer to adjust the scene represen-
tation to match the user’s direction of view. As a result, the viewer can look
around and walk through the surrounding virtual environment. To allow this
to happen, the frame refresh rate must be high enough to allow our eyes to
blend together the individual frames into the illusion of motion and limit the
sense of latency between movements of the head and body and regeneration
of the scene.
The basic parameters that are used to describe the performance of an
HMD are:
• Field of view . Fieldofviewisdefinedastheangularsizeoftheimageas
seen by the user. It is usually defined by the angular size of the diagonal
of the image.
• Resolution. The quality of the image is determined by the quality of the
optics and the optical design of the HMD, but also by the resolution of
the display. The relation between the number of pixels on the display
and the size of the FOV will determine how “grainy” the image appears.
People with 20/20 vision are able to resolve to 1 minute of an arc. The
average angle that a pixel subtends can be calculated by dividing the
number of pixels by the FOV. For example, for a display with 1280
pixels in the horizontal plane, a horizontal FOV of 21.3
◦
will give 1
pixel per minute of arc. If the FOV is increased beyond this, the image
will appear grainy.
• Luminance. Luminance, or how bright the image appears, is very im-
portant for semi-transparent HMD systems which are now being used
to overlay virtual data onto the user’s view of the outside world. In this
case, it is important that the data is bright enough to be seen over the
light from the ambient scene.
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