Transcript 1992 (Part III) - Personal Web Pages

Three Dimensional Visual
Display Systems for Virtual
Environments
Part 3
Presented by Evan Suma
Parallax Barrier
• Vertical slit plate
• Blocks part of the screen from each eye
• Screen displays images in vertical strips
Parallax Barrier
• More than two images can be displayed
• Creates multiple views from side to side
Parallax Barrier
Horizontal res = display res / # of 2D views
Multiple projecting
monitors can be used
to maintain higher
horizontal resolution.
Parallax Barrier: Drawbacks
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Not commonly used
Barrier blocks most of light to eye
Causes dim image
Small slit widths can result in
diffraction of spreading light rays
Parallax Barrier: Diffraction
• Angular spread of light through slit
of width α is approximately
θ = 2 asin ( λ / α )
where λ is the wavelength of light
passing through the slit
α = pitchslit / N
Parallax Barrier: Diffraction
• More diffraction than lenticular display
• Caused by loss of directivity of barrier
• Parallax barrier is only a fraction of
lenticular pitch
Parallax Barrier: Brightness
• Reduce light which reaches eye
Brightness = B0 * ( α / pitchslit )
where B0 is brightness of unblocked screen
Parallax Barrier: Rate and Bandwidth
• Horizontal resolution is reduced
(same as lenticular displays)
• Bandwidth must be increased to maintain
high visible resolution
• Or sacrifice other parameter
– Vertical resolution
– Refresh rate
Parallax Barrier: Rate and Bandwidth
• Horizontal resolution is reduced
(same as lenticular displays)
• Bandwidth must be increased to maintain
high visible resolution
• Or sacrifice other parameter
– Vertical resolution
– Refresh rate
Slice Stacking
• Building a 3D volume by layer 2D images
• Also called multiplanar displays
• Rather than use a planar mirror, a variablefocus mirror can be used
Slice Stacking
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Common method uses acoustics
Vibrates a reflective membrane
Causes focal length to change
Uses reflection from monitor
Over time forms a truncatedpyramid viewing volume
Slice Stacking
• Traces out a luminous volume
• Objects are transparent
• Objects further in depth cannot be
obscured
Slice Stacking
• Ideal for volumetric data sets and modeling
problems
• Poorly suited to “photographic” or realistic
images with hidden surfaces
Slice Stacking: Resolution and FOV
• Spatial resolution and FOV the same as
underlying 2D display
• Varifocal mirrors limited to approximately
20 inches due to acoustic and mirror
characteristics
Slice Stacking: Depth Resolution
• Depth of reflected CRT is constantly
changing
• Very fine resolution of depth spots can
potentially be imaged
• Limited by bandwidth of CRT and
persistence of phosphors
Slice Stacking: Accommodation
• One of the few displays that support
ocular accommodation
• Actually displays points in 3D space
either directly or optically
Slice Stacking: Refresh Rate
• Refresh rate is twice frequency of
vibration
• Typically 30 Hz signal drives mirror
• Results in 60 Hz refresh rate
Slice Stacking: Brightness
• Short persistence phosphors must be used
• Prevents smearing of image in depth
• Brightness somewhat reduced from “typical”
2D display
• Phosphors of short enough duration only
available in green (circa 1986)
Slice Stacking: Viewing Zone
• Viewing zone limited by
position of display CRT
• Obstructs viewing zone
• Can use beam splitter to
move CRT below, but
lowers brightness by at
least 75%
Slice Stacking: Viewing Volume
• Magnification of
mirror changes size of
reflected CRT
• Results in truncated
pyramid volume
instead of rectangle
Slice Stacking: Volume Extent
• Mirrors have leverage
of approximately 85
• Distance h in mirror
• Movement 85h in
reflected image
Slice Stacking: Number of Views
• Number of views essentially unlimited
• Horizontal and vertical parallax are
both supported
Holography: CG Stereograms
• Recorded optically from a set of 2D views
of a 3D scene
• Projects each 2D image into a viewing zone
• Stereo views with horizontal parallax
• Full-color, high resolution images
• Non-real time
• Requires a huge amount of information
(100-300 views)
Holography: CG diffraction patterns
• Generates a diffraction pattern
• Hologram creates a 3D wavefront when
illuminated
• Images 3D objects and light sources in space
• Traditional methods were complex and
computationally expensive
• New method (circa 1992) allows generation
to be displayed in real-time
Holography: Spatial Resolution
• Very high horizontal resolution is needed
• Vertical resolution can be lower
• High horizontal resolution is not resolution of
displayed holographic image
– Horizontal resolution of image points is
diffraction limited
– Beyond human perceptual limits
Holography: Miscellaneous
• Like slice-stacking displays, holograms
support ocular accommodation
• Good brightness and contrast using lowpower laser (a few milliwatts)
• Both monochromatic and color have been
demonstrated
• Very high bandwidth compared to other
systems
Holography: Miscellaneous
• Depth resolution is beyond human perceptual
capabilities
• Provides many views from side-to-side
• No vertical parallax
• Viewing zone angle is determined by the frequency of
diffraction pattern
• MIT system’s depth range limited to approximately
100mm
• MIT system’s refresh rate is 36 Hz with a little flicker
Any Questions?