Transcript Seeing 3D from 2D Images

Seeing 3D from 2D
Images
William and Craig 115 - 164
How to make a 2D image appear as
3D!
► Output
is typically 2D Images
► Yet we want to show a 3D world!
► How can we do this?
 We can include ‘cues’ in the image that give our
brain 3D information about the scene
 These cues are visual depth cues
Visual Depth Cues
► Monoscopic
Depth Cues (single 2D image)
► Stereoscopic Depth Cues (two 2D images)
► Motion Depth Cues (series of 2D images)
► Physiological Depth Cues (body cues)
Monoscopic Depth Cues
►
Interposition
 An object that occludes another is closer
►
Shading
 Shape info. Shadows are included here
►
Size
 Usually, the larger object is closer
►
Linear Perspective
 parallel lines converge at a single point
►
Surface Texture Gradient
 more detail for closer objects
►
Height in the visual field
 Higher the object is (vertically), the further
it is
►
Atmospheric effects
 further away objects are blurrier
►
Brightness
 further away objects are dimmer
Stereoscopic Display Issues
► Stereopsis
► Stereoscopic
Display Technology
► Computing Stereoscopic Images
► Stereoscopic Display and HTDs.
► Works for objects < 5m. Why?
Stereopsis
The result of the two slightly different views of the
external world that our laterally-displaced eyes
receive.
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.
f2
f1

 



Lef t Eye
Right Eye
Ret inal disparit y=
  
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.
Convergence Angles
a+a+c+b+d = 180
b+c+d = 180
a-b = a+(-b) = 1+2
= Retinal Disparity
f1
a
D1
f2
a
b
c
1
D2
b
d
i
2
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.
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).
► Points on the horopter
appear at the same depth
as the fixation point.
►
f1
f2
Vieth-Mueller
Circle

Stereoscopic Display
► Stereoscopic
images are easy to do badly,
hard to do well, and impossible to do
correctly.
Stereoscopic Displays
► Stereoscopic
display systems create a threedimensional image (versus a perspective
image) by presenting each eye with a
slightly different view of a scene.
 Time-parallel
 Time-multiplexed
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).
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
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.
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 righteye image is being displayed.
Stereographics Shutter Glasses
Screen Parallax
Display
Screen
P
Pleft
Left eye
position
Right eye
position
Pright
Object w ith
positiv e
parallax
P
Pright
Pleft
Object w ith
negativ e 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).
Screen Parallax
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
(cont.)
f1
D
Proje cti on
Plan e
p
i
Le ft
e yepoi n t
Ri gh t
e yepoi n t
d
Screen Parallax
5.00
Screen Parallax
-5.00 0
50
100
150
200
250
300
350
-15.00
-25.00
-35.00
-45.00
-55.00
-65.00
Distance from Eye
Zero parallax at screen, max positive parallax
is i, negative parallax is equal to I halfway
between eye and screen
Stereoscopic Voxels
Left Eye
Point
1
2
3
1
2
3
1
1
2
1
1
Right Eye
Point
4
2
1
2
1
2
5
5
4
5
3
2
B
A
3
2
1
4
4
3
3
3
4
4
5
5
Screen Parallax and Convergence
Angles
► Screen
parallax depends
on closest distance to
f2
screen.
► Different convergence
angles can all have the
b
same screen parallax.
► Also depends onProje ction
assumed eye
Plan e
separation.
f1
a
f3

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
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.
What doesn’t work
•Each view has a different
projection plane
•Each view will be presented
(usually) on the same plane
What Does Work
i
i
Setting Up Projection Geometry
No
Look at point
Eye
Locations
Yes
Eye
Locations
Look at points
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
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.
Accommodation/ Convergence
Display Screen
Position Dependence
(without head-tracking)
Interocular Dependance
Projection Plane
Perceived
Point
Modeled Point
F
Obvious Things to Do
► Head
tracking
► Measure User’s Interocular Distance
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?)
Two View Points with Head-Tracking
True Eyes
Modeled Eyes
Projection Plane
Perceived Points
Modeled Point
Maximum Depth Plane
True Eyes
Projection
Plane
Perceived
Point
F
Modeled Eyes
Modeled
Point
E
Maximum Depth Plane
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 ).
Point of fixation
0.40
0.30
Symmetric convergence
0.25
0.20
Convergence 20 centimeters to the left of the left eye
0.15
0.10
0.05
150
140
130
120
110
100
90
80
70
60
50
40
30
20
0.00
10
separat ion in centimeters
Change in eyepoint
0.35
Distance in centimeters from eye plane
Change in eyepoint separation with change in point of fixation.
Centers of rotation of the eyes are assumed to be 6.4 centimeters apart.
Ghosting
► Affected
by the amount of light transmitted
by the LC shutter in its off state.
► Phosphor persistence
► Vertical screen position of the image.
Ghosting
Extinction Ratio =
Image Position
Top
Middle
Bottom
(cont.)
Luminance of the correct eye image
-----------------------------------------------------------Luminance of the opposite eye ghost image
Red
61.3/1
50.8/1
41.1/1
White
17/1
14.4/1
11/1
Ghosting
► Factors




(cont.)
affecting perception of ghosting
Image brightness
Contrast
Horizontal parallax
Textural complexity
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.
Motion Depth Cues
► Parallax
created
by relative head
position and
object being
viewed.
► Objects nearer to
the eye move a
greater distance
Pulfrich Effect
► Neat
trick
► Different levels of illumination require
additional time (your frame rates differ base
of amount of light)
► What if we darken one image, and brighten
another?
► http://dogfeathers.com/java/pulfrich.html
► www.cise.ufl.edu/~lok/multimedia/videos/p
ulfrich.avi
Physiological Depth Cues
► Accommodation
– focusing adjustment
made by the eye to change the shape of
the lens. (up to 3 m)
► Convergence – movement of the eyes to
bring in the an object into the same location
on the retina of each eye.
Summary
► Monoscopic
– Interposition is strongest.
► Stereopsis is very strong.
► Relative Motion is also very strong (or
stronger).
► Physiological is weakest (we don’t even use
them in VR!)
► Add as needed
 ex. shadows and cartoons