Focus Cues Kurt Akeley CS248 Lecture 20 6 December 2007

Focus Cues

Kurt Akeley

CS248 Lecture 20

6 December 2007

http://graphics.stanford.edu/courses/cs248-07/

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

Depth cues

Monocular geometric cues:

Motion parallax

Object size (relative and changing)

Perspective convergence

Depth cues

Motion parallax

Color cues:

Occlusion

Lighting and shading

Atmospheric effects (attenuation, blue shift)

Texture gradient

Focus matters too!

This photo is of a real scene, not of a model

Outline

Focus cues

Fixed-viewpoint volumetric display

Experimental results

Practical implications

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

A lens provides the magic

Index of refraction is

greater than one

The correspondence

dStraight line through the center of the lens

Object point

Image point

focal length of the lens (m)

object distance (m)

image distance (m)

Thin lens equation

Diopters (D)

1Diopters

meters=

Diopter distances

0.67 m

Diopter number-line assignments are

relative to a reference point

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=

0.33 m

Thin-lens equation (using Diopters)

object distance (m)

image distance (m)

focal length of the lens (m)

fObject point

Image point

1 object distance (D)

1 image distance (D)

1 power of the lens (D)

Reference point is the center of the

Out-of-focus blur

Image plane

1 1 (D)

= - b ar r Eµ

Object point

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

Focus Cues

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

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.

Human accommodation range

2Diopters

meters

All but one Diopter of focal range is within

arm’s reach!

ChildrenYoung adults

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

2Diopters

meters

Tilt-shift miniaturization

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:

Make a model appear real

Make a real scene appear miniaturized

A stereo display gets all these right …

Motion parallax

Color cues:

Occlusion

Texture gradient

But the focus cues are all wrong

No retinal-image blur

Incorrect accommodation

Vergence and accommodation are decoupled

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

But they fail in other critical ways

Motion parallax

Color cues:

Occlusion

Texture gradient

No view-dependent shading is possible,

because viewer position is not known

Fixed-viewpoint Volumetric Display

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

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 !

Prototype display design

Prototype display

Bite bar

Depth blending

Retinal image of a sine wave grating

Eye image from www.wikipedia.com

Lower contrast

Modulation transfer function

Retinal-image contrast with summed images

Experimental Results

Work done at UC Berkeley

Marty Banks

Simon Watt

Ahna R. Girshick

David M. Hoffman

http://bankslab.berkeley.edu/

Stimuli

Forced-choice

This ?

Or this ?

Experimental design

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

Practical implications

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?

Fixed optics

Adaptive optics

Images from www.wikipedia.com

Summary

Focus cues are

Accommodation (focus response)

Retinal blur

Correct focus cues matter

Tilt-shift miniaturization

Display typeCorrect

geometric depth cues

Correct shading depth

Correct focus depth cues

Stereo Autostereoscopic volumetric

Fixed-viewpoint volumetric

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

Final exam

Location:

Gates B01

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

Office hours

Not available today after class

Next Tuesday 1:30 pm to 3 pm

Not available next Thursday (the day of the final exam)

Check the web site

Will monitor and respond to e-mail questions

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

Focus Cues Kurt Akeley CS248 Lecture 20 6 December 2007

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