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Head-Tracked Displays (HTDs)

This article provides an in-depth analysis of head-tracked displays (HTDs), including their characteristics, advantages, and disadvantages. It also explores the technical aspects of stereoscopic displays and issues related to retinal disparity and convergence angles. Additionally, it discusses different types of stereoscopic display technologies and their potential problems.

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Head-Tracked Displays (HTDs)

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  1. Head-Tracked Displays (HTDs) Sherman and Craig, pp. 140-151 Bowman, et al., pp. 34-49 Babu 2011

  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 Babu 2011

  3. Stationary Display 3D Glasses Head Tracker CRT HTD (Fishtank VR) Babu 2011

  4. Standard Display Stereoscopic Display Screen User C C A A B B Stereoscopic Display Babu 2011

  5. 3D Glasses 3D Display 3D Object Virtual Workbench Babu 2011

  6. CAVE Babu 2011

  7. Any Projected Large Stereoscopic Screen Babu 2011

  8. Characteristics • Large • Projection-Based (except for Fishtank VR) • Stereoscopic • Head Tracked • Stationary Display Screen(s) Babu 2011

  9. 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 Babu 2011

  10. 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 Babu 2011

  11. Stereoscopic Display Issues • Stereopsis • Stereoscopic Display Technology • Computing Stereoscopic Images • Stereoscopic Display and HTDs. Babu 2011

  12. Stereopsis The result of the two slightly different views of the external world that our laterally-displaced eyes receive. Babu 2011

  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. Babu 2011

  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. Babu 2011

  15. 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 Babu 2011

  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. Babu 2011

  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 Babu 2011

  18. Stereoscopic Display • Stereoscopic images are easy to do badly, hard to do well, and impossible to do correctly. Babu 2011

  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 Babu 2011

  20. 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 Babu 2011

  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 Babu 2011

  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. Babu 2011

  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. Babu 2011

  24. Stereographics Shutter Glasses Babu 2011

  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). Babu 2011

  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 Babu 2011

  27. Screen Parallax Zero parallax at screen, max positive parallax is i, max negative parallax is equal to -i halfway between eye and screen Babu 2011

  28. Stereoscopic Voxels Babu 2011

  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. Babu 2011

  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 Babu 2011

  31. 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. Babu 2011

  32. What doesn’t work • Each view has a different projection plane • Each view will be presented (usually) on the same plane Babu 2011

  33. i i What Does Work Babu 2011

  34. Setting Up Projection Geometry No Look at point Eye Locations Yes Eye Locations Look at points Babu 2011

  35. Screen Size Once computed, the screen parallax is affected by the size of the display screen Babu 2011

  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. Babu 2011

  37. Accommodation/ Convergence Display Screen Babu 2011

  38. Position Dependence (without head-tracking) Babu 2011

  39. Interocular Dependance True Eyes Modeled Eyes Projection Plane Perceived Point F Modeled Point Babu 2011

  40. Obvious Things to Do • Head tracking • Measure User’s Interocular Distance Babu 2011

  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?) Babu 2011

  42. Two View Points with Head-Tracking True Eyes Modeled Eyes Projection Plane Perceived Points Modeled Point Babu 2011

  43. Projection Plane Perceived Point True Eyes Modeled Point E F Modeled Eyes Maximum Depth Plane Maximum Depth Plane Babu 2011

  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. 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 ). Babu 2011

  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. Babu 2011

  46. 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 Babu 2011

  47. Ghosting (cont.) • Factors affecting perception of ghosting • Image brightness • Contrast • Horizontal parallax • Textural complexity Babu 2011

  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 Babu 2011

  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. Babu 2011

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