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Head-Tracked Displays (HTDs). Sherman and Craig, pp. 140-151 Bowman, et al., pp. 34-49. Head-tracked Displays. Displays in which the user’s head is tracked and the image display screen is located at a fixed location in physical space. CRT Virtual Workbench or ImmersaDesk CAVE

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head tracked displays htds

Head-Tracked Displays (HTDs)

Sherman and Craig, pp. 140-151

Bowman, et al., pp. 34-49

C Babu 2008

head tracked displays
Head-tracked Displays
  • Displays in which the user’s head is tracked and the image display screen is located at a fixed location in physical space.
    • CRT
    • Virtual Workbench or ImmersaDesk
    • CAVE
    • Many large screen displays

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crt htd fishtank vr

Stationary Display

3D Glasses

Head

Tracker

CRT HTD (Fishtank VR)

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stereoscopic display

Standard Display

Stereoscopic Display

Screen

User

C

C

A

A

B

B

Stereoscopic Display

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virtual workbench

3D Glasses

3D Display

3D Object

Virtual Workbench
slide6
CAVE

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characteristics
Characteristics
  • Large
  • Projection-Based (except for Fishtank VR)
  • Stereoscopic
  • Head Tracked
  • Stationary Display Screen(s)

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large screen projection
Larger field-of-view than HDM

Field of regard is smaller than HMD but larger than a typical CRT Display

Projectors must be aligned properly.

Architectural Statement!

Front Projection (user may be in the way).

Back Projection (takes up even more space)

When multiple screens are arranged at or near 90 degree angles, reflection between screens may be a problem

Large Screen Projection

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large screen projection1
Advantages

Viewer not isolated from real objects or other people in the virtual world space.

Less physical gear to wear than HMD

Potentially better resolution than HMD

Large field of view compared to HMD or CRT.

Disadvantages

Usually one one person is head-tracked.

Real objects may occlude virtual objects in inappropriate ways

Multiple screens require more computation

At least one direction is not part of the virtual world.

Large Screen Projection

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stereoscopic display issues
Stereoscopic Display Issues
  • Stereopsis
  • Stereoscopic Display Technology
  • Computing Stereoscopic Images
  • Stereoscopic Display and HTDs.

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stereopsis
Stereopsis

The result of the two slightly different views of the external world that our laterally-displaced eyes receive.

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retinal disparity
Retinal Disparity

If both eyes are fixated on a point, f1, in space, then an image of f1 if focused at corresponding points in the center of the fovea of each eye. Another point, f2, at a different spatial location would be imaged at points in each eye that may not be the same distance from the fovea. This difference in distance is the retinal disparity.

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disparity
Disparity
  • If an object is closer than the fixation point, the retinal disparity will be a negative value. This is known as crossed disparity because the two eyes must cross to fixate the closer object.
  • If an object is farther than the fixation point, the retinal disparity will be a positive value. This is known as uncrossed disparity because the two eyes must uncross to fixate the farther object.
  • An object located at the fixation point or whose image falls on corresponding points in the two retinae has a zero disparity.

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convergence angles
Convergence Angles

+a+c+b+d = 180

+c+d = 180

- = a+b = 1+2 = Retinal Disparity

f1

a

D1

f2

b

a

D2

b

c

d

2

1

i

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miscellaneous eye facts
Miscellaneous Eye Facts
  • Stereoacuity - the smallest depth that can be detected based on retinal disparity.
  • Visual Direction - Perceived spatial location of an object relative to an observer.

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horopters
Horopters
  • Corresponding points on the two retinae are defined as being the same vertical and horizontal distance from the center of the fovea in each eye.
  • Horopter - the locus of points in space that fall on corresponding points in the two retinae when the two eyes binocularly fixate on a given point in space (zero disparity).

Vieth-Mueller Circle

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stereoscopic display1
Stereoscopic Display
  • Stereoscopic images are easy to do badly, hard to do well, and impossible to do correctly.

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stereoscopic displays
Stereoscopic Displays
  • Stereoscopic display systems create a three-dimensional image (versus a perspective image) by presenting each eye with a slightly different view of a scene.
    • Time-parallel
    • Time-multiplexed

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time parallel stereoscopic display
Two Screens

Each eye sees a different screen

Optical system directs each eye to the correct view.

HMD stereo is done this way.

Single Screen

Two different images projected on the same screen

Images are polarized at right angles to each other.

User wears polarized glasses (passive glasses).

Time Parallel Stereoscopic Display

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passive polarized projection issues
Passive Polarized Projection Issues
  • Linear Polarization
    • Ghosting increases when you tilt head
    • Reduces brightness of image by about ½
    • Potential Problems with Multiple Screens (next slide)
  • Circular Polarization
    • Reduces ghosting but also reduces brightness and crispness of image even more

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problem with linear polarization
Problem with Linear Polarization
  • With linear polarization, the separation of the left and right eye images is dependent on the orientation of the glasses with respect to the projected image.
  • The floor image cannot be aligned with both the side screens and the front screens at the same time.

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time multiplexed display
Time Multiplexed Display
  • Left and right-eye views of an image are computed and alternately displayed on the screen.
  • A shuttering system occludes the right eye when the left-eye image is being displayed and occludes the left-eye when the right-eye image is being displayed.

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screen parallax
Screen Parallax

The screen parallax is the distance between the projected location

of P on the screen, Pleft, seen by the left eye and the projected

location, Pright, seen by the right eye (different from retinal disparity).

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screen parallax cont
Screen Parallax (cont.)

p = i(D-d)/D

where p is the amount of screen parallax for a point, f1, when projected onto a plane a distance d from the plane containing two eyepoints.

i is the interocular distance between eyepoints and

D is the distance from f1 to the nearest point on the plane containing the two eyepoints

d is the distance from the eyepoint to the nearest point on the screen

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screen parallax1
Screen Parallax

Zero parallax at screen, max positive parallax is i, max negative parallax is equal to -i halfway between eye and screen

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screen parallax and convergence angles
Screen Parallax and Convergence Angles
  • Screen parallax depends on closest distance to screen.
  • Different convergence angles can all have the same screen parallax.
  • Also depends on assumed eye separation.

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how to create correct left and right eye views
How to create correct left- and right-eye views
  • To specific a single view in almost all graphics software or hardware you must specify:
    • Eyepoint
    • Look-at Point
    • Field-of-View or location of Projection Plane
    • View Up Direction

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basic perspective projection set up from viewing paramenters
Basic Perspective Projection Set Up from Viewing Paramenters

Y

Z

X

Projection Plane is orthogonal to one of the major axes (usually Z). That axis is along the vector defined by the eyepoint and the look-at point.

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what doesn t work
What doesn’t work
  • Each view has a different projection plane
  • Each view will be presented (usually) on the same plane

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what does work

i

i

What Does Work

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setting up projection geometry
Setting Up Projection Geometry

No

Look at point

Eye

Locations

Yes

Eye

Locations

Look at points

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screen size
Screen Size

The size of the window does

not affect the retinal disparity

for a real window.

Once computed, the screen parallax

is affected by the size of the display

screen

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visual angle subtended
Visual Angle Subtended

Screen parallax is measured in terms of visual angle. This is a screen

independent measure. Studies have shown that the maximum angle

that a non-trained person can usually fuse into a 3D image is about

1.6 degrees. This is about 1/2 the maximum amount of retinal disparity

you would get for a real scene.

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accommodation convergence
Accommodation/ Convergence

Display Screen

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slide39

Interocular Dependance

True Eyes

Modeled Eyes

Projection Plane

Perceived

Point

F

Modeled Point

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obvious things to do
Obvious Things to Do
  • Head tracking
  • Measure User’s Interocular Distance

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another problem
Another Problem
  • Many people can not fuse stereoscopic images if you compute the images with proper eye separation!
  • Rule of Thumb: Compute with about ½ the real eye separation.
  • Works fine with HMDs but causes image stability problems with HTDs (why?)

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slide42

Two View Points with Head-Tracking

True Eyes

Modeled Eyes

Projection Plane

Perceived Points

Modeled Point

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slide43

Projection

Plane

Perceived

Point

True Eyes

Modeled

Point

E

F

Modeled Eyes

Maximum Depth Plane

Maximum Depth Plane

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can we fix this
Can we fix this?
  • Zachary Wartell, "Stereoscopic Head-Tracked Displays: Analysis and Development of Display Algorithms,"  Ph.D. Dissertation, Georgia Institute of Technology, August 2001.
  • Zachary Wartell, Larry F. Hodges, William Ribarsky.   "An Analytic Comparison of Alpha-False Eye Separation, Image Scaling and Image Shifting in Stereoscopic Displays,"   IEEE Transactions on Visualization and Computer Graphics, April-June 2002, Volume 8, Number 2, pp. 129-143. (related tech report is GVU Tech Report 00-09 ( Abstract , PDF , Postscript .)
  • Zachary Wartell, Larry F. Hodges, William Ribarsky.   "Balancing Fusion, Image Depth, and Distortion in Stereoscopic Head-Tracked Displays." SIGGRAPH 99 Conference Proceedings, Annual Conference Series. ACM SIGGRAPH, Addison Wesley, August 1999, p351-357. (Paper: Abstract ,  PDF ,  Postscript ; SIGGRAPH CD-ROM Supplement, supplement.zip,supplement.tar.Z ).

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ghosting
Ghosting
  • Affected by the amount of light transmitted by the LC shutter in its off state.
  • Phosphor persistence
  • Vertical screen position of the image.

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ghosting cont
Ghosting (cont.)

Luminance of the correct eye image

------------------------------------------------------------

Luminance of the opposite eye ghost image

Extinction Ratio =

Image Position RedWhite

Top 61.3/1 17/1

Middle 50.8/1 14.4/1

Bottom 41.1/1 11/1

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ghosting cont1
Ghosting (cont.)
  • Factors affecting perception of ghosting
    • Image brightness
    • Contrast
    • Horizontal parallax
    • Textural complexity

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refresh rate versus resolution
Refresh Rate versus Resolution
  • Current raster graphics workstation stereoscopic display techniques half the screen resolution in order to quadruple the image update rate.
    • Gives the equivalent of four frame buffers
    • Complex images are not as detailed

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time parallel stereoscopic images
Time-parallel stereoscopic images
  • Image quality may also be affected by
    • Right and left-eye images do not match in color, size, vertical alignment.
    • Distortion caused by the optical system
    • Resolution
    • HMDs interocular settings
    • Computational model does not match viewing geometry.

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