Visual Display for VR

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Visual Display for VR
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Stereoscopy and Display Systems

• Human Eye
• Stereoscopy
• Depth Cues
• Optics / Display Systems
• Error Sources

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Human Eye
Cornea
Retina
Fovea
Light
Image Formed Here
Optic Nerve (To the Brain)
Iris
Lens
Muscles
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Human Eye

• Retina in the back of the eye, where there are
  photoreceptors, cells that react to light
  intensity
• Rods and Cones
• More Rods in the Peripheral Region (react to low
  intensity light)
• More Cones in the Central Region (Fovea, where
  focused images exists, react to high intensity
  light)
• Two Light Refractions
• At the Cornea (75 )
• At the Lens (23 )
• Within the Eyeball (liquid kind of thing inside
  the eye)
• Iris adjusts amount of incoming light
• Pupil Opening of the Iris
• Lens adjusts its size to change shape lens using
  the occulomotor muscles

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Eye Parameters

• FOV (Field of View) How wide the human Eyes can
  see (measured in angles)
• Foveal Acuity Minimum Lateral Length or Depth
  Human Eyes can Perceive (Acuity drops at the
  Periphery)
• Known to distinguish elements subtended by 15
  arc min from 20 ft.

1 5 arc min
20 ft
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Display Parameters want to match eye and display
parameters as close as possible

• FOV (Field of View) Angle subtended by the
  viewing surface from a given observer location
• Humans 120H x 180V
• Related to Spatial Resolution
• Related to Angular Resolution
• Spatial Resolution No. of pixels that can be
  displayed in unit area
• Pitch How wide and Tall the Pixel is
• Angular Resolution Visual angle the pixel
  subtends from a particular viewing distance
• Critical Flicker Frequency ( 60 Hz)
• Refresh Rate (15 20 Hz)
• Brightness / Contrast
• Color

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Stereoscopy and Depth Perception

• Occulo-motor Cues
• Accommodation changing shape of eye lens to
  focus
• Convergence rotate eyes to fixate on object
• Accommodation and Convergence work together (when
  eyes converge to a certain distance,
  automatically accommodates and vice versa)
• This is physiological cue, not visual cue
  (sense of tension on muscles that control eye
  movement and lens focus)
• Effective cue when object within 1020m

Viewing Axes
Convergence Angle
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Stereoscopy and Depth Perception
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Stereoscopy and Depth Perception

• Binocular Disparity Difference in retinal images
  due to projection of objects at different depth
• Depth is felt with respect to an object in focus
• Screen Parallax must be (re) computed based
  current fixation point to get right disparity
  feeling
• Stereopsis Depth Perception due to Binocular
  Disparity

-
Disparity in the Left Eye

Focused point
Other point

Total Disparity

Disparity in the Right Eye
-
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Stereoscopy and Depth Perception

• Retinal Disparity and Convergence Angle Difference

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Horopter

• In this picture, the lifeguard is fixating on
  Ralph. The curved, dashed line represents the
  horopter all points on this line are the same
  distance from the lifeguard as the fixation
  point. So, Susan and Harry (and Ralph) fall on
  the horopter Carol and Charlie do not.

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Horopter

• The red, green, and blue lines show where the
  images of Susan, Ralph, and Harry, respectively,
  are cast on the retinae of the two eyes. Using
  point F/F' as a reference (the foveae for the
  left and right eyes, respectively), you can see
  that the image of Susan falls on the same point
  on both eyes. In other words, A is the same
  distance to the left of F (left eye) as A' is
  from F' (right eye). Similarly, the image of
  Harry falls on the same point in both eyes (G is
  the same distance to the right of F, as G' is
  from F'). Because the images of these objects
  (Susan, Ralph, and Harry) fall on the same
  positions in both eyes, there is ZERO disparity.
  All objects will be perceived as being the same
  distance from the observer.
• Potential Problem In graphics with flat image
  plane, Ralph and Susan will have different depth
  values ..!?..
• (http//www.cquest.utoronto.ca/psych/psy280f/ch7/h
  oropter.html)

The red, green, and blue lines show where the
images of Susan, Ralph, and Harry, respectively,
are cast on the retinae of the two eyes. Using
point F/F' as a reference (the foveae for the
left and right eyes, respectively), you can see
that the image of Susan falls on the same point
on both eyes. In other words, A is the same
distance to the left of F (left eye) as A' is
from F' (right eye). Similarly, the image of
Harry falls on the same point in both eyes (G is
the same distance to the right of F, as G' is
from F'). Because the images of these objects
(Susan, Ralph, and Harry) fall on the same
positions in both eyes, there is ZERO disparity.
All objects will be perceived as being the same
distance from the observer.
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Psychological Depth Cues

• Perspective
• Motion Parallax Differential angular velocity of
  objects at different depth from observer (close
  objects move more rapidly than far objects)
• Height in Field of View and Relative Size
• Occlusion and Texture Gradient
• Color and Haziness
• Shadows
• Cues are usually additive

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Display Systems

• We want Autostereoscopy (display stereo without
  any extra aids)
• Anaglyphs / Polarized Glasses
• HMD
• Time Multiplexed (Crystal Eye)
• Boom Hand(s) Occupied
• Retinal Display (Laser Scanning)
• Fully / Semi / Non Immersive

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Screen Parallax Using One Image Plane
Display Screen
Positive Parallax (uncrossed)
Eye
Zero Parallax(where eyes actually focus)
Negative Parallax (crossed)
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Screen Parallax Using One Image Plane
Display Screen
Eye
IOD6.5cm
D
Parallax IPD (D d) / D
d Viewing Distance
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Generating Right and Left Images Assumption
Fixated on Infinitely Far Objects
Display Screen
Eye
IOD6.5cm
Parallax between Right and Left Image
View Vectors and View Frustum of Each Eye offset
by IOD/2 from the Usual Center of Projection
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Autostereoscopy

• Parallax Barrier
• Lenticular Sheet

Slits
L
R
Image
lens
L
R
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Time Multiplexed

• Switch between right and left images very fast
  (60 Hz, Critical Flicker Frequency)
• Field Sequential (Use odd line scanning for right
  image and vice versa)
• When right image shown, block left eye and when
  left image shown block right eye using a LCD
  Shutter Glass (need to synchronize LCD glass and
  monitor)

Left image shown
Right eye blocked
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Anaglyphs and Polarized Glass

• Chromatic Anaglyphs Display right image in red
  and left image in blue, and use color filtering
  glass (right eye sees only blue and vice versa
• Light Polarization Display right and left image
  using two orthogonal light waves, and use
  filtering glass that passes one or the other.
  Because each image is generated without the
  complementary wave component, the brightness is
  reduced.

Right/Left image shown
Right passes red Left passes blue only Or Right
passes one wave and Left passes the orthogonal
wave
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Head Mounted Display (Two Image Planes)

• Putting LCD / CRT Projection in front of the eye
  causes strain (excess accommodation)
• Use of magnifiers to place virtual image at
  comfortable range

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Optics

• Law of Reflection and Refraction

Reflection angle is same (th1 th2) Refraction
angle (angle coming in and coming out is related
by sine function, sin (th1) sin (th3)
th2
th1
th3
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Real Image vs. Virtual Image
My eye thinks the object is underneath the mirror
(because of the light direction), but there is no
light underneath the mirror, therefore the image
is virtual On the other hand, real image is
formed by light coming to focus
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Optical System

• Includes any number of reflecting and/or
  refracting surfaces of any curvature that alter
  the directino of rays leaving the object O
• Rays spread out radially in all directions from O
  (object space), and enters image space where
  rays converge to a common point I
• Every ray that start at O must arrive at I with
  same travel time (some rays that do not reach I
  at the same time, then loss in brightness)
• Aberration Optical system can not produce one to
  one mapping between O and I
• Diffraction Lights scatter and light intensity
  at I decreases

Optical System
O
I
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Optical Elements

• Apertures, Stops, Pupil Limits amount of passing
  light
• Entrance Pupil The limiting aperture (could be a
  physical stop or even the size of lens) looking
  into the optical system from object space
• Exit Pupil The limiting aperture looking into
  the optical system from the image space, need to
  match the exit pupil to where the eye is so that
  maximum light enters the eye (no clipped vision)
• Eye Relief Distance from eye to the optical
  system (closest optical element from eye)

Aperture / Stop / Pupil
Lens
Eye
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Simple Magnifier
Display
Cannot see
50 Light Received only
Lights refracted almost parallel as to make
virtual image formed at infinity (collimated
image) Flight simulator application
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Display Considerations

• Different tasks might require different display
  characteristics
• Flight simulator with mostly far objects
  (accommodation and convergence cues will not be
  so effective), use motion parallax
• Manipulation task is the other way around, Use
  binocular disparity
• If possible, put objects near zero parallax
  screen so that there is less conflict between
  accommodation and convergence

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Display Considerations

• If screen is relatively small, objects that must
  have negative parallax can be clipped and feel it
  has positive depth (because of occlusion). Avoid
  this kind of situation if possible.
• There are inherent errors from optical
  distortion, IPD difference, failure to use off
  center projection when needed (as user is
  tracked), and resolution
• When user is tracked and free to move, we must
  rescale the image because image perception has
  inverse squared relationship to viewing distance.

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CAVE
Error among users
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ImmersaDesk / Workbench
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