Display Systems Will Steptoe (Based on slides by Anthony Steed) - - PowerPoint PPT Presentation

Display Systems

Will Steptoe

(Based on slides by Anthony Steed)

Overview

• Critiques of Display Technologies

– Colour Gamut – Brightness & Contrast – Frame Rate

• Depth Cues

– Virtualisation – Cameras

International Commission on Illumination (CIE) Chromaticity Diagram (1931)

Human-Perceptible Colour Gamut

• Colour is the perceptible

proportion of the electromagnetic spectrum

• International Commission on

Illumination (CIE) Chromaticity Diagram (1931). Describes ‘average’ properties of human eye.

• Curve is monochromatic

(single-wavelength, nm) light

Standard Computer Display Gamut

• Only a portion of the human

colour gamut is reproduced by typical computer displays.

• Image shows typical CRT/LCD
• display. Different devices have

different gamuts: type (printer, projector, HDR display...), manufacturer, model.

• Image on previous slide

describes colours outside the sRGB gamut, so depending on the display calibration, they may not be displayed properly!

CRT Phosphor Spectrogram

Primary Colours for Computer Displays

• Emission spectra of CRT

displays

• Displays mix light instead of

colour: red, blue, green (RGB)

• Light is additive rather than

subtractive as used by artists

• Additive colour primaries are

secondary colours of subtractive and vice versa.

Additive (displays) Subtractive (paint)

Brightness & Contrast

• Brightness

– Projectors 2000 lumens (power of light + human eye) – Screen 500 candelas/m2 (intensity + direction + eye) – 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). Static vs Dynamic ratio.

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 – LCD now up to120Hz (double NTSC/HDTV for stereo)

• Projectors

– CRT 120Hz – DLP only recently matching this (digital cinema rear projection)

Depth Cues

• Physiological

– Body has to do something in response to “reality” of the 3D world: doing that thing is a depth cue itself

• Psychological

– Brain perceives some characteristics of a visual scene and infers its 3D nature via empirical experience (Baysian inference)

Three 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

– Stereo disparity

• Virtual Environment – perception of a surrounding

environment

– head-slaved parallax, accommodation and vergence, surrounding display (describes CAVE)

Level 1: Virtual Space

Half-Life 2, Valve

Level 1: Virtual Space

Shadow of the Colossus, Sony

First implementation of stereoscopy in 1840 by presenting two slightly different images to each eye Keystone View Company - Ruins of the Granite Temple, the Sphinx and the Great Pyramid (Oliver Wendell Holmes Stereoscopic Research Library)

Level 2: Stereo Pairs

Level 2: Virtual Image

• Combines multiple cues:
• stereopsis, accomodation, and vergence
• Actual perceptual scale of space is not arbitrary

Level 3: Virtual Environment

• Co-ordinated multisensory display with observer-slaved motion

parallax (head tracking)

• Depth-of-focus variation and wide field of view without visible

restriction in field-of-view, postural reflex, consistent near-reflex, vestibular ocular-reflex

• Requires (almost) full immersion because of the requirements for

head-related and egocentric display

• User can actively explore the (visual) environment by moving

naturally

Psychological Depth Cues

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

How do we see in stereo?

• Inter-Pupilary Distance (IPD) or

binocular disparity ~ 65mm

• Each eye has different view of

same object

• Perceptual fusion of two views

(cyclopean union), subsequent perception of depth

• Inputs from 2 eyes converge on

the same cortical neurons in visual cortex (V1)

• Calculation of how different the 2

views are leads to model of depth

30cm 6.5cm

Left eye Right eye Binocular percept

Adapted from [Purves & Lotto]

Physiological Depth Cues

• Accommodation

– Focal length of eyes adjust to focus at different points in scene. – 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

Autostereograms

Override vergence by forcing your eyes parallel

Virtual Space Virtual Image Virtual Environment Level of virtualization Definition Cues Pen & Paper + Perspective… ?? Technology 3D objects inscribed on a flat sheet Perception of

• bjects with

depth Objects slaved motion parallax

ALL PSYCHOLOGICAL CUES:

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

PHYSIOLOGICAL CUES

Stereoscopic disparity, Accomodation* Convergence* All cues consistent with

• bserver motion

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

Cameras: 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 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.

Stereo window Left Eye Right Eye

L2 R2

Setting up a stereo view

• Project different views of the

same scene to each eye at the ~same time

• 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)

Image plane

R1 L1 Near side of image plane Far side of image plane P2 P1 Adapted from [Slater, Steed, Chrysanthou]

Setting up a stereo view: Parallax

• IF: R-L > 0 : (P2)

Positive horizontal parallax: Points will be virtual points behind the stereo window.

• IF: R-L < 0 : (P1)

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

Left Eye Right Eye

L2 R2 L1 R1 In front of stereo window Behind stereo window P2 P1 1

• 1

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

• 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.

Left Eye Right Eye

L2 R2 L1 R1 In front of stereo window Behind stereo window P1

Parallel set up Crossed 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) Especially colour & brightness same for homologous points

• 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 & accommodation To provide maximum depth but lowest parallax, place principal objects so that ~½ parallax values are +ve, ½ -ve

CAVE-like Displays

• Screens surround the user
• Modelled as a series of

cameras, two per wall

• Each camera defined by

corners of 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 over HMD or semi-

immersive VR systems

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

Cruz-Neira et al.’s Discussion

• Disadvantages

– Expensive and complex to configure

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

– User occludes the screen with their own body – Other users can occlude screen – Floor shadows – Typically 4 walls – Only one perspective-correct user at once

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 portray

robust illusions of 3D objects

VR Systems UK

Next Week!

• Lab session on Tuesday from 2-4pm
• Lecture (given by Mel Slater) on Friday,

time to be confirmed.