10/21/2008 Display Systems Anthony Steed Anthony Steed Overview - - PDF document

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10 21 2008
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10/21/2008 Display Systems Anthony Steed Anthony Steed Overview - - PDF document

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10/21/2008 Display Systems Anthony Steed Anthony Steed Overview Critiques of Display Technologies Colour Gamut Brightness & Contrast Frame Rate Frame Rate Depth Cues Virtualisation Cameras CIE

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10/21/2008 1

Display Systems

Anthony Steed Anthony Steed

Overview

  • Critiques of Display Technologies

– Colour Gamut – Brightness & Contrast Frame Rate – Frame Rate

  • Depth Cues

– Virtualisation – Cameras

CIE Chromaticity Diagram, source wikipedia

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CRT Phosphor Spectrogram, source wikipedia

Brightness & Contrast

  • Brightness

– Projectors 2000 lumens – Screen 500 candelas/m2 Luminance may be different for different colours – Luminance may be different for different colours

  • Contrast Ratio

– Ratio between black level and white – 1000:1 is good – Very difficult to measure accurately – Also depends on response time (time to change between any two levels)

Frame Rate

  • Image sources are various rates

– Film at 24Hz/96Hz – PAL TV at 50Hz – NTSC TV at 59.97Hz

  • Screens

– CRTS have typically matched TV – Standard LCD panels up to 75Hz – Newer panels up to 120Hz (double NTSC/HDTV)

  • Projectors

– CRT 120Hz – DLP only recently matching this

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10/21/2008 3

Depth cues

  • Physiological – body has to do something in

response to “reality” of the 3D world: Doing that thing is a depth cue itself P h l i l b i i

  • Psychological – brain perceives some

characteristics of a visual scene and infers its 3D nature via empirical experience…baysian inference…

Levels of Virtualisation

  • Virtual Space – see a 3D object when looking at

something inscribed on a flat sheet

– Perspective cues, lighting, shading

  • Virtual Image – perception of an object with depth

Virtual Image perception of an object with depth

– Stereo disparity

  • Virtual Environment – perception of a surrounding

environment

– head-slaved parallax, accommodation and vergence, surrounding display

Virtual Space

Half-Life 2, Valve Software

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10/21/2008 4

Stereo Pairs

Keystone View Company - Ruins of the Granite Temple, the Sphinx and the Great Pyramid (Oliver Wendell Holmes Stereoscopic Research Library)

Virtual Image Virtual Environment

  • Requires (almost) full

immersion because of the requirements for head- related and egocentric display

  • At this point, user can

actively explore the (visual) environment by moving naturally

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Psychological Depth Cues

  • Linear perspective
  • Shading
  • Shadows
  • Aerial Perspective
  • Occlusion
  • Texture gradient
  • Fogging

How do we see in stereo?

  • Binocular disparity (av. 65mm)
  • Each eye has different view of

same object

  • Perceptual fusion of 2 views

(cyclopean union) of 2 viewpoints: perpn. of depth

  • Inputs from 2 eyes converge
  • n the same cortical neurons

in V1

  • Calculation of how different

the 2 views are leads to model

  • f depth

30cm 6.5cm Left eye Right eye Binocular percept

Adapted from [Purves & Lotto]

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Physiological Depth Cues

  • Accommodation

– Focal length of eyes adjust to focus at

  • diff. points in scene.

Change thickness of lens: relax or tense – Change thickness of lens: relax or tense ciliary muscles.

  • Convergence

– Rotation of eyes inwards: view near

  • bjects

– Rotation of eyes outwards (~parallel): view far objects – Powerful cue

Adapted from [Slater, Steed, Chrysanthou]

Accommodation and Convergence

  • Usually work in conjunction with each other.
  • This correspondence is not physiologically

determined.

  • Learned by experience
  • Is broken when looking at eg screen based stereo

views.

http://www.eleves.ens.fr:8080/home/massimin/Images/teapot.gif

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10/21/2008 7

Virtual Space Level of virtualization Defn Cues Pen & Paper + Perspective… ?? Technology 3D obj. inscribed on a flat sheet

ALL PSYCHOLOGICAL CUES:

Linear persp., Shading, Shadows, Aerial persp., Occlusion,

Virtual Image Virtual Environment

  • Percepn. Of
  • bj with depth
  • Obs. slaved

motion parallax…

, Texture cues

PHYSIOLOGICAL CUES

Stereoscopic disparity, Accomodation* Convergence* All cues consistent with

  • bserver motion

“Stereoscope” 1830s Charles Wheatstone Stereo screens… Cave, HMDs… ~1980s - ‘90s

Stereo Graphics Creation

  • Measure positions of the left and right eye
  • For each display surface render the image, with

the field of view that matches the eye’s view of th t f that surface

  • Easy in a CAVE

– Image remains flat

  • More difficult in head-mounted displays

– Image often distorted by the optics, no longer square when seen.

Left Eye

Setting up a stereo view

  • Project diff. views of the same

scene to each eye at the ~same time If t t t i t b i

Image plane

R1

Stereo window Right Eye

L2 R2

  • If want to present point as being

in front of screen: (virtual point P1) use L1, R1

  • If want to present point as being

behind screen: (P2) use L2, R2

  • Points L1 & R1 are homologous:

same point in image space

L1 Near side of image plane Far side of image plane P2 P1 Adapted from [Slater, Steed, Chrysanthou]

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SLIDE 8

10/21/2008 8 Setting up a stereo view: Parallax

  • IF: R-L > 0 : (P2)

Positive horizontal parallax: Points will be virtual points

Left Eye

L2 2 R1 P2 P1 0.0

behind the stereo window.

  • IF: R-L < 0 : (P1)

Negative horizontal parallax: Points will be virtual points in front of the stereo window.

Right Eye

R2 L1 In front of stereo window Behind stereo window +ve Adapted from [Slater, Steed, Chrysanthou]

Viewing Stereo pairs – what does it mean for your eyes?

  • Parallel setup:

– Right eye sees Right image, – Left eye sees left image – Requires focus beyond the images

Left Eye

L2 2 R1 P1

Parallel Crosse d set up images

  • Crossed setup:

– Right eye sees left image – Left eye sees right image – Requires crossing eyes

  • Viewing the opposite way around

will reverse the sense of depth.

Right Eye

R2 L1 In front of stereo window Behind stereo window

set up

Adapted from [Slater, Steed, Chrysanthou]

Presenting 3D images: Ideals

  • Congruence

L &R images should be same (except as caused by the horizontal parallax) Esp.colour & brightness same for homologous points

V ti l ll Z

  • Vertical parallax -> Zero

(If>0, uncomfortable to fuse images)

  • Parallax (view separation) trade off…

Wide parallax: good depth, but too wide leads to discomfort. Parallax should be less <= IPD Closer the homologous points…less disparity between convergence & accomodation To provide max. depth but lowest parallax: Place principal objects so that ~½ parallax values are +ve, ½ -ve

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10/21/2008 9

CAVE-style Displays

  • Screens surround

the user

  • Modelled as a

series of cameras, two per wall

  • Each camera

defined by corners

  • f the wall and

centre of eye

CAVE Projection

Z Y (out of screen) X Straight forward to show that:

Adapted from [Cruz-Neira et al]

Cruz-Neira et al.’s Discussion

  • Advantages of a CAVE

– Wide field of view – Less rotational instability See yourself – See yourself – Higher quality images – Less optical distortion

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10/21/2008 10

Cruz-Neira et al.’s Discussion

  • Disadvantages

– Expensive and complex to configure

  • Need to align several projectors
  • Very high refresh rate needed (>100Hz)

y g ( )

– User occludes the screen with their own body – Other users can occlude screen – Floor shadows – Typically 4 walls

Summary

  • Current display technology is limited

– Colour, brightness, frame-rate, contrast

  • However 3D computer graphics is very successful
  • Brain uses a variety of cues

– Physiological – Psychological

  • Properly configured immersive systems portary

robust illusions of objects

VR Systems UK VR Systems UK