focus cues kurt akeley cs248 lecture 20 6 december 2007
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Focus Cues
Kurt Akeley
CS248 Lecture 20
6 December 2007
http://graphics.stanford.edu/courses/cs248-07/
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Depth cues
Binocular geometric cues:
Stereopsis (retinal-image disparity)
Convergence of the lines of sight
Retinal-image disparity Vergence
Correct retinal-image disparity allows the viewer to “fuse” the scene
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Depth cues
Binocular geometric cues:
Stereopsis (retinal-image disparity)
Convergence of the lines of sight
Monocular geometric cues:
Motion parallax
Object size (relative and changing)
Perspective convergence
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Depth cues
Binocular geometric cues:
Stereopsis (retinal-image disparity)
Convergence of the lines of sight
Monocular geometric cues:
Motion parallax
Object size (relative and changing)
Perspective convergence
Color cues:
Occlusion
Lighting and shading
Atmospheric effects (attenuation, blue shift)
Texture gradient
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Focus matters too!
This photo is of a real scene, not of a model
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Outline
Focus
Focus cues
Fixed-viewpoint volumetric display
Experimental results
Practical implications
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Focus
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Focus
Focus defines a 1-to-1 correspondence between
Object points, which (may) radiate light, and
Image points, where the radiated light converges
In practice image points capture only the object-point radiation that passes through an aperture.
ApertureObject points
Image points
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CS248 Lecture 20 Kurt Akeley, Fall 2007
A lens provides the magic
Index of refraction is
greater than one
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CS248 Lecture 20 Kurt Akeley, Fall 2007
The correspondence
so si
f
dStraight line through the center of the lens
Object point
Image point
1 1 1
focal length of the lens (m)
object distance (m)
image distance (m)
o i
o
i
f s s
f
s
s
= +
=
=
=
Thin lens equation
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Diopters (D)
1Diopters
meters=
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Diopter distances
0.67 m
1.0 m
Diopter number-line assignments are
relative to a reference point
2 D
Diopter distances are differences between
number-line assignments:
2 D = 3 D – 1 D
Not computed as the reciprocal of the 0.67 m
“distance” !
1Diopters
meters=
0.5 m
0.33 m
2 D
1 D
3 D
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Thin-lens equation (using Diopters)
1 1 1
object distance (m)
image distance (m)
focal length of the lens (m)
o i
o
i
f s s
s
s
f
= +
=
=
=
so si
fObject point
Image point
1 object distance (D)
1 image distance (D)
1 power of the lens (D)
o i
oo
ii
P S S
Ss
Ss
Pf
= +
= =
= =
= =
Reference point is the center of the
lens
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Out-of-focus blur
so si
f
to ti
Image plane
1 1 (D)
o o
Et s
= - b ar r Eµ
Object point
ra
rb
E
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Depth of field (DOF)
(sometimes also called depth of focus)
DOF is the amount of focus error that is inconsequential
Recall that
Therefore:
DOF measured in Diopters is (almost) invariant with respect to focus distance (So)
For a given aperture radius (ra) and an acceptable blur radius (rb)
“Almost” because there is a slight dependence we are ignoring
DOF is inversely proportional to aperture radius Once the acceptable blur radius is determined
b ar r Eµ
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Focus Cues
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Focus cues
There are two focus cues:
Accommodation (the focus response of the eye)
Retinal-image blur
Neither is a quality of the light field
Instead they are conditions in the human body that are stimulated by the light field
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Accommodation
Accommodation is the focus response of the eye:
Resting focus is at infinity (0 D), or is
corrected with fixed lenses to infinity.
The ciliary muscles contract, allowing the lens to become
more spherical. This increases the power of the
lens, reducing the focal distance.
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Human accommodation range
18
0
∞
1
1
2Diopters
meters
3 4
12
13
8
14
All but one Diopter of focal range is within
arm’s reach!
12
112
ChildrenYoung adults
Me
…
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Human depth of field
Human depth of field is approximately +/- 0.3 D
The optics of the eye are not perfect
This corresponds to
A DOF from 2 m to infinity, or
A DOF from 10” to 12”
So near-field scenes (with differing depths) are blurry, while far-field scenes are not
18
0
∞
1
1
2Diopters
meters
3 4
12
13
8
14
12
112
…
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Tilt-shift miniaturization
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Why “tilt-shift”?
Recall that focus is a correspondence
Before Photoshop the effect was created by tilting the image plane of the camera off the main axis:
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Make a model appear real
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Make a real scene appear miniaturized
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CS248 Lecture 20 Kurt Akeley, Fall 2007
A stereo display gets all these right …
Binocular geometric cues:
Stereopsis (retinal-image disparity)
Convergence of the lines of sight
Monocular geometric cues:
Motion parallax
Object size (relative and changing)
Perspective convergence
Color cues:
Occlusion
Lighting and shading
Atmospheric effects (attenuation, blue shift)
Texture gradient
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CS248 Lecture 20 Kurt Akeley, Fall 2007
But the focus cues are all wrong
No retinal-image blur
cues
Incorrect accommodation
cue
Vergence and accommodation are decoupled
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Volumetric displays fix the focus cues …
And they are autostereoscopic:
Require no tracking of the viewer’s position or orientation
Support multiple simultaneous viewers
Stereopsis is “free”
References:• Downing et al. 1996• Favalora et al. 2002• Lightspace Tech. 2003
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CS248 Lecture 20 Kurt Akeley, Fall 2007
But they fail in other critical ways
Binocular geometric cues:
Stereopsis (retinal-image disparity)
Convergence of the lines of sight
Monocular geometric cues:
Motion parallax
Object size (relative and changing)
Perspective convergence
Color cues:
Occlusion
Lighting and shading
Atmospheric effects (attenuation, blue shift)
Texture gradient
No view-dependent shading is possible,
because viewer position is not known
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Fixed-viewpoint Volumetric Display
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Fixed-viewpoint volumetric display
Fixed-viewpoint:
All geometric and color depth cues are correct
Volumetric:
All focus cues are near-correct
No need for gaze tracking
What’s the catch?
Display is head-mounted
Must track viewer position and orientation Latency is a challenge
Must overcome ergonomic issues
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Required volumetric resolution
Autostereoscopic volumetic displays have huge pixel-count requirements in all three dimensions
Fixing the viewpoint allows spatial and depth resolutions to be optimized independently:
Spatial pixel density requirements are unchanged Foveal limit requires 2 pixels/arc min
But depth pixel density is determined by depth of field +/- 0.3 D is more than satisfied by two pixels per diopter
A display with 4 D range has a depth pixel-count of 7 !
1000s
1000s
7
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Prototype display design
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Prototype display
Bite bar
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Demo
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Depth blending
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Retinal image of a sine wave grating
Eye image from www.wikipedia.com
Lower contrast
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Modulation transfer function
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Retinal-image contrast with summed images
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Experimental Results
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Work done at UC Berkeley
Marty Banks
Simon Watt
Ahna R. Girshick
David M. Hoffman
…
http://bankslab.berkeley.edu/
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Stimuli
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Forced-choice
This ?
Or this ?
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Experimental design
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Experimental results
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Results summary
Correct focus distance results in
Shorter time to “fuse” a depth-corrugation stereogram
Ability to fuse finer depth corrugations
Better estimations of depth People consistently underestimate depth in VR
environments
Less fatigue No forced decoupling of vergence and accommodation
There is no performance penalty for depth blending
For details refer to the publications:
http://bankslab.berkeley.edu/publications.html
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Practical implications
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Without a fixed-viewpoint volumetric display
Use long viewing distances when possible
Flight simulators use either Large done display
Collimated (infinite focus distance) optics
Minimize accommodation/vergence conflict
High-quality stereo headsets have adjustable focal distance
Set it to the best average distance
Is there hope of a practical fixed-viewpoint volumetric display?
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Fixed optics
f
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Adaptive optics
Images from www.wikipedia.com
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Summary
Focus cues are
Accommodation (focus response)
Retinal blur
Correct focus cues matter
Tilt-shift miniaturization
Experimental results
Display typeCorrect
geometric depth cues
Correct shading depth
cues
Correct focus depth cues
Stereo Autostereoscopic volumetric
Fixed-viewpoint volumetric
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Project 3 and game competition
Source code and write-ups due tomorrow at 5 pm
Congratulations to the game competition winners:
1st Place
Balloo
Belinda Gu, Edward Luong, and Joel Galenson
2nd Place
Shootout
Vincent Gire and David Lissmyr
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Final exam
Location:
Gates B01
Time:
Thursday 13 December, 7 pm to 9 pm
Material:
We’ll test your mastery of the content of lectures 11-18
But we’ll assume you understand and can apply the material of lectures 1-10 too
Review session:
Gates B03
Friday 7 December (tomorrow), 4:15 pm to 5 pm
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Office hours
Kurt:
Not available today after class
Next Tuesday 1:30 pm to 3 pm
Not available next Thursday (the day of the final exam)
CAs:
Check the web site
All:
Will monitor and respond to e-mail questions
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CS248 Lecture 20 Kurt Akeley, Fall 2007
Good-bye
Thanks to the CAs !
Andrew, David, and Justin
My goals:
Learn a lot (accomplished)
Convey my understanding to you
Final requests:
Please share your thoughts with me about how the course or slides can be improved In person or e-mail
Please complete the on-line Axess course evaluation The system is open now
It closes at 11:59 pm on Sunday 16 December
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CS248 Lecture 20 Kurt Akeley, Fall 2007
End