Three Dimensional Visual Display Systems for Virtual Environments

Three Dimensional Visual Display Systems for Virtual

Environments

Presented by Evan Suma

Part 3

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

• 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

where λ is the wavelength of light passing through the slit

θ = 2 asin ( λ / α )

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

where B0 is brightness of unblocked screen

Brightness = B0 * ( α / pitchslit )

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 variable-focus mirror can be used

Slice Stacking

• Common method uses acoustics• Vibrates a reflective membrane• Causes focal length to change• Uses reflection from monitor• Over time forms a truncated-

pyramid 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 low-power 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?