1. Head-Tracked Displays (HTDs)
Sherman and Craig, pp
Bowman, et al., pp
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2. 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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3. CRT HTD (Fishtank VR) Stationary Display 3D Glasses Head Tracker
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4. Stereoscopic Display User Screen Standard Display Stereoscopic DisplayB
Screen
Standard Display
Stereoscopic Display
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5. Virtual Workbench
3D Glasses
3D Display
3D Object
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6. CAVE
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7. Any Projected Large Stereoscopic Screen
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8. Characteristics Large Projection-Based (except for Fishtank VR)
Stereoscopic
Head Tracked
Stationary Display Screen(s)
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9. 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
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10. Large Screen Projection
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.
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11. Stereoscopic Display Issues
Stereopsis
Stereoscopic Display Technology
Computing Stereoscopic Images
Stereoscopic Display and HTDs.
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12. Stereopsis
The result of the two slightly different views of the external world that our laterally-displaced eyes receive.
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13. 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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14. 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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15. Convergence Angles +a+c+b+d = 180 +c+d = 180
- = a+b = 1+2 = Retinal Disparity f1 a D1 f2 a b D2 b c d 1 i 2
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16. 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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17. 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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18. Stereoscopic Display
Stereoscopic images are easy to do badly, hard to do well, and impossible to do correctly.
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19. 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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20. 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).
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21. 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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22. 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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23. 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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24. Stereographics Shutter Glasses
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25. 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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26. 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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27. 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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28. Stereoscopic Voxels
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29. 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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30. 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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31. Basic Perspective Projection Set Up from Viewing ParamentersY 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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32. 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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33. What Does Work i i
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34. Setting Up Projection GeometryNo
Look at point
Eye
Locations
Yes
Eye
Locations
Look at points
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35. Screen Size Once computed, the screen parallax
is affected by the size of the display
screen
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36. 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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37. Accommodation/ Convergence
Display Screen
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38. Position Dependence (without head-tracking)
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39. Interocular Dependance
True Eyes
Modeled Eyes
Projection Plane
Perceived
Point F
Modeled Point
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40. Obvious Things to Do Head tracking Measure User’s Interocular Distance
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41. 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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42. Two View Points with Head-Tracking
True Eyes
Modeled Eyes
Projection Plane
Perceived Points
Modeled Point
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43. Maximum Depth Plane Projection Perceived Plane True Eyes Modeled Point
Modeled Eyes
True Eyes E F
Modeled
Point
Perceived
Projection
Plane
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44. 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 (related tech report is GVU Tech Report ( 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, p (Paper: Abstract ,  PDF ,  Postscript ; SIGGRAPH CD-ROM Supplement, supplement.zip, supplement.tar.Z ).
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45. 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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46. Ghosting (cont.) Extinction Ratio = Image Position Red White
Luminance of the correct eye image
Luminance of the opposite eye ghost image
Extinction Ratio =
Image Position Red White
Top /1 17/1
Middle / /1
Bottom /1 11/1
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47. Ghosting (cont.) Factors affecting perception of ghosting
Image brightness
Contrast
Horizontal parallax
Textural complexity
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48. 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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49. 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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