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60 4. Building a Practical VR System
DLP projectors have high refresh rates and deliver a high resolution. At
the core of DLP projectors is the Texas Instruments MEMS (microelectrome-
chanical system) chip and the digital micromirror device (DMD). Figures 4.8
and 4.9 illustrate the DMD and how DLP projectors work. The word mir-
ror in DMD gives a vital clue to the idea behind the DLP. The DMD chip
is an array of microscopic mirrors, one mirror for each pixel in the image.
The microscopic mirrors are attached to the chip’s substrate by a small hinge,
and so it can be twisted into two positions. In one of these positions, it is
angled in such a way that light from the projector’s lamp is reflected through
the projector’s lens and onto the screen. In the other position, it is reflected
away from the lens and no light then appears on the screen. Thus, we have
a binary state for the projection of each pixel. By flicking the micromirror
back and forth hundreds of times a second, intermediate shades of gray can
be simulated. Color is easily added by using filters and three DMDs for the
RGB primaries. F or a detailed description of the structure of the DMD and
a comparison of its performance ag ainst LCD and other display technology,
see Hornbeck [9].
Figure 4.8. (a) The Texas Instruments DMD chip. (b) Close up of the micromirrors.
(c) 3D impression of the micromirror structure. (d) Micrograph of the mechanism
without mirror. (e) Principle of operation. When a pixel is on, the mirror reflects
the light into the lens. When a pixel is off, the light is reflected onto a black body
absorber. (Images courtesy of Texas Instruments.)
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4.1. Technology of Visualization 61
Projection Registration and Blending
If you are using a projection-based cave, it is certain that one projector will be
insufficient. A cuboid environment will require at least four. It might seem
that one projector per wall will do the job, but unless the aspect ratio of the
wall matches that of the projector, it will be necessary to use two projectors
to span the wall. Later in the book (Chapter 18), we look at practical code
to allow you to pre-process images, movies and synthetic virtual 3D environ-
ments in real time using multiple projectors displaying on a half-cylindrical
wall. A cylindrical display surface, such as that depicted in Figure 4.10(a),
introduces an element of nonlinearity in the registration of the projected im-
age. However, we shall see shortly that such distortions can be corrected for.
This opens up the possibility of combining projected images to form a large
panoramic view across non-flat display surfaces.
This brings us to the important issues of how to break up the rendered
environment into pieces for projection, how to process the projection so that
the pieces line up correctly without distortion and how to blend the output
so that no join is visible. We will now address these one by one:
• Sl icing the display. The easiest way to divide the output for use with
multiple projectors is to use a computer video adapter with two or more
outputs. The outputs can usually be configured so that one shows the
Figure 4.9. DLP projectors are built with either one or three DMD chips. In the
single-chip projector, a color filter on a rotating wheel passes red, green and blue fil-
ters in the light path. In the three chip projector, a dichroic prism filters the light into
primary colors, which are processed separately and reflected into the l ens. (Images
courtesy of Texas Instruments.)
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62 4. Building a Practical VR System
Figure 4.10. (a) Projecting onto a curved surface distorts the displayed pictures. (b)
To produce a wide-screen projection, a single desktop image is divided in two by a
dual-output video card and presented by two projectors. (c) If the projectors are not
arranged in parallel, a significant distortion will result. The application software can
be changed [14] to compensate for the distortion.
left side of the display and the other the right side (Figure 4.10(b)). It
is also possible to configure networked computers so that each node on
the network renders one piece of projection. More programming effort
is required to make sure the renderer synchronizes the views it is gen-
erating. Synchronizing the video signals sent to the projectors is vital
for stereoscopic imaging, and quite a few of the stereo-ready graphics
adapters offer an interface to facilitate this. In fact, the primary use of
these adapters is in synchronizing rendering for ultra high resolution
displays.
• Distortion. Unless the direction of projection is perpendicular to the
screen, the display will be distorted. You are probably familiar with
the idea of correcting for keystone distortion. However, if we are using
two projectors as shown in Figure 4.10(c) then we will have to arrange
that our software corrects for the fact that the projection is not at right
angles to the screen.
2
2
Projecting two images in the way depicted in Figure 4.10(c) is often used to minimize the
shadow casting effect that arises in front projection. Because the beams are directed from the
side, a person standing in the middle will not cast a shadow on the screen. This is especially
true if we allow for some overlap.
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4.1. Technology of Visualization 63
The problem becomes even more complex if the screen is curved [21].
In a semi-cylindrical cave, a projection will exhibit distortion, as shown
in Figure 4.11(a). To overcome this, the rendering engine is pro-
grammed to negate the effect of the distortion (Figure 4.11(b)). We
show how to do this in practice in Section 18.5. We can use the same
idea to correct for all sorts of distortion. In a multi-projector cave,
aligning adjacent images is made much easier by interactively adjusting
the software anti-distortion mechanism rather than trying to mechan-
ically point projectors in exactly the right direction (it also helps when
building a portable cave).
• Blending. In theory, projecting an image as a set of small tiles (one tile
per projector) should appear seamless, provided the distortion has been
corrected. Unfor tunately, in practice, there is always this annoying lit-
tle gap. Often it arises because of minor imperfections in the display
screen; a cylindrical shape approximated by small planar sections, for
example. This annoying artefact can be removed by arranging that the
tiles have small overlapped borders. Inside the border area, brightness is
gradually reduced in one tile while it is increased in the other. Having
to build tiles with overlapping edges complicates the projection soft-
ware drivers, but as Figure 4.12 demonstrates, large panoramic images
Figure 4.11. (a) A panoramic projection from three sources gives rise to nonlinear
distortion on a curved screen. To correct for the distortion, the projected images
are pre-processed. The image in (b) shows the changes made to the projections
to correct for the distortion in (a). After pre-processing, the nonlinear distortion is
greatly reduced (c). (The rectangular grid in the images is used to manually configure
the pre-processor.)
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64 4. Building a Practical VR System
Figure 4.12. When projecting onto a non-flat surface, the edges rarely line up. Small
imperfections are very noticeable to the human eye, so a better approach is to overlap
the adjacent projections and gradually blend between them. In (a) we see the result
of including a small overlap. By applying a gradual blend (using a sigmoid function
(c)) from one projection to the other, the area of overlap becomes indistinguishable
from the rest of the projection (b).
can be obtained using tiled sections that ov erlap and blend to make the
join appear seamless.
Figure 4.13 illustrates a 180
◦
cylindrical stereoscopic panoramic display
built using low-cost components. It is driven by a pair of PCs, each sending
Figure 4.13. A semi-immersive wide-screen VR environment. Four low-cost DLP pro-
jectors are driven by two networked computers, each with dual video outputs. The
system uses custom display software and can be interactively configured to correct
for distortion.
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