Saturday, July 14, 2012
New 3-D Imaging Technology Could Prove More Practical Than Holography
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In a new paper featured as a research highlight at this summer's
Siggraph computer-graphics conference, the MIT Media Lab's Camera
Culture group offers a new approach to multiple-perspective, glasses-
free 3-D that could prove much more practical in the short term.
As striking as it is, the illusion of depth now routinely offered by
3-D movies is a paltry facsimile of a true three-dimensional visual
experience. In the real world, as you move around an object, your
perspective on it changes. But in a movie theater showing a 3-D movie,
everyone in the audience has the same, fixed perspective - and has to
wear cumbersome glasses, to boot.
Despite impressive recent advances, holographic television, which
would present images that vary with varying perspectives, probably
remains some distance in the future.
Instead of the complex hardware required to produce holograms, the
Media Lab system, dubbed a Tensor Display, uses several layers of
liquid-crystal displays (LCDs), the technology currently found in most
flat-panel TVs. To produce a convincing 3-D illusion, the LCDs would
need to refresh at a rate of about 360 times a second, or 360 hertz.
Such displays may not be far off: LCD TVs that boast 240-hertz refresh
rates have already appeared on the market, just a few years after 120
-hertz TVs made their debut.
"Holography works, it's beautiful, nothing can touch its quality,"
says Douglas Lanman, a postdoc at the Media Lab and one of the new
paper's co-authors. "The problem, of course, is that holograms don't
move. To make them move, you need to create a hologram in real time,
and to do that, you need - little tiny pixels, smaller than anything
we can build at large volume at low cost. So the question is, what do
we have now? We have LCDs. They're incredibly mature, and the're
The Nintendo 3DS uses two layered LCD screens to produce the illusion
of depth, with the bottom screen simply displaying alternating dark
and light bands. Two slightly offset images, which represent the
different perspectives of the viewer's two eyes, are sliced up and
interleaved on the top screen. The dark bands on the bottom screen
block the light coming from the display's backlight in such a way that
each eye sees only the image intended for it.
This technique is in fact more than a century old and produces a
stereoscopic image, the type of single-perspective illusion familiar
from 3-D movies. The bottom screen displays the same pattern of light
and dark bands no matter the image on the top screen. But Lanman,
graduate student Matthew Hirsch and professor Ramesh Raskar, who leads
the Camera Culture group, reasoned that by tailoring the patterns
displayed on the top and bottom screens to each other, they could
filter the light emitted by the display in more sophisticated ways,
creating an image that would change with varying perspectives. In a
project they dubbed HR3D, they developed algorithms for generating the
top and bottom patterns as well as a prototype display, which they
presented at Siggraph Asia in 2010.
The problem is that, whereas a stereoscopic system such as a 3-D movie
projector or the 3DS needs to display only two perspectives on a
visual scene - one for each eye - the system the Media Lab researchers
envisioned had to display hundreds of perspectives in order to
accommodate a moving viewer. That was too much information to display
at once, so for every frame of 3-D video, the HR3D screen in fact
flickered 10 times, displaying slightly different patterns each time.
With this approach, however, producing a convincing 3-D illusion would
require displays with a 1,000-hertz refresh rate.
To get the refresh rate down to 360 hertz, the Tensor Display adds
another LCD screen, which displays yet another pattern. That makes the
problem of calculating the patterns exponentially more complex,
however. In solving that problem, Raskar, Lanman and Hirsch were
joined by Gordon Wetzstein, a new postdoc in the Camera Culture group.
The researchers? key insight was that, while some aspects of a scene
change with the viewing angle, some do not. The pattern-calculating
algorithms exploit this natural redundancy, reducing the amount of
information that needs to be sent to the LCD screens and thus
improving the resolution of the final image.
As it turns out, the math behind the Tensor Display is similar to that
behind computed tomography, or CT, an X-ray technique used to produce
three-dimensional images of internal organs. In a CT scan, a sensor
makes a slow circle around the subject, making a series of
measurements of X-rays passing through the subject's body. Each
measurement captures information about the composition of tissues at
different distances from the sensor; finally, all the information is
stitched together into a composite 3-D image.
"The way I like to think about it is, we're building a patient whose
CT scan is the view," Lanman says.
At Siggraph, the Media Lab researchers will demonstrate a prototype
Tensor Display that uses three LCD panels. They?ve also developed
another prototype that uses only two panels, but between the panels
they introduce a sheet of lenses that refract light left and right.
The lenses were actually developed for stereoscopic display systems;
an LCD panel beneath the lenses alternately displays one image
intended for the left eye, which is diffracted to the left, and
another for the right eye, which is diffracted to the right. The MIT
display also takes advantage of the ability to project different
patterns in different directions, but the chief purpose of the lenses
is to widen the viewing angle of the display. With the three-panel
version, the 3-D illusion is consistent within a viewing angle of 20
degrees, but with the refractive-lens version, the viewing angle
expands to 50 degrees.
"The paper reveals how you would greatly improve the realism, and
image depth, and physical simplicity of 3-D display systems,
particularly those that don't require you to wear glasses," says Gregg
Favalora, a principal at the engineering consultancy Optics for Hire
and co-chair of the SPIE Stereoscopic Displays and Applications
Conference. "It's only possible when you have these really good
mathematicians and signal-processing people and optics experts all
sitting in the same room."
"It's definitely suitable for commercial applications, because each
component is commonplace, and it sounds easy to manufacture, so this
ought to be something that a consumer-electronics company would
license," Favalora adds. "Honestly, this is a really big deal."