Display Systems Anthony Steed Overview Critiques of Display

Display Systems Anthony Steed

Overview • Critiques of Display Technologies – Colour Gamut – Brightness & Contrast – Frame Rate • Depth Cues – Virtualisation – Cameras

CIE Chromaticity Diagram, source wikipedia

CRT Phosphor Spectrogram, source wikipedia

Brightness & Contrast • Brightness – Projectors 2000 lumens – Screen 500 candelas/m 2 – 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 24 Hz/96 Hz – PAL TV at 50 Hz – NTSC TV at 59. 97 Hz • Screens – CRTS have typically matched TV – Standard LCD panels up to 75 Hz – Newer panels up to 120 Hz (double NTSC/HDTV) • Projectors – CRT 120 Hz – DLP only recently matching this

Depth cues • Physiological – body has to do something in response to “reality” of the 3 D world: Doing that thing is a depth cue itself • Psychological – brain perceives some characteristics of a visual scene and infers its 3 D nature via empirical experience…baysian inference…

Levels of Virtualisation • Virtual Space – see a 3 D 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

Virtual Space Half-Life 2, Valve Software

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 headrelated and egocentric display • At this point, 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? • Binocular disparity (av. 65 mm) • 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 on the same cortical neurons in V 1 • Calculation of how different the 2 views are leads to model of depth 30 cm 6. 5 cm

Left eye Right eye Binocular percept Adapted from [Purves & Lotto]

Physiological Depth Cues • Accommodation – Focal length of eyes adjust to focus at diff. points in scene. – Change thickness of lens: relax or tense ciliary muscles. • Convergence – Rotation of eyes inwards: view near objects – 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

Level of Defn virtualization Cues ALL PSYCHOLOGICAL CUES: Virtual Space Linear persp. , 3 D obj. inscribed on a Shading, Shadows, flat sheet Pen & Paper + Perspective… ? ? Aerial persp. , Occlusion, Texture cues PHYSIOLOGICAL CUES Virtual Image Technology Stereoscopic Percepn. Of obj with depth disparity, Virtual Obs. slaved Environment motion parallax… “Stereoscope” 1830 s Charles Wheatstone Accomodation* Convergence* Stereo screens… All cues consistent with observer motion Cave, HMDs… ~1980 s - ‘ 90 s

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.

Setting up a stereo view Image plane Left Eye • Project diff. views of the same scene to each eye at the ~same time R 1 IPD P 1 Right Eye R 2 P 2 L 1 Stereo window Near side of image plane L 2 Adapted from [Slater, Steed, Chrysanthou] Far side of image plane • If want to present point as being in front of screen: (virtual point P 1) use L 1, R 1 • If want to present point as being behind screen: (P 2) use L 2, R 2 • Points L 1 & R 1 are homologous: same point in image space

Setting up a stereo view: Parallax 0. 0 Left Eye • IF: R-L > 0 : (P 2) R 1 IPD P 1 Right Eye L 2 R 2 P 2 • IF: R-L < 0 : (P 1) L 1 In front of stereo window Adapted from [Slater, Steed, Chrysanthou] Positive horizontal parallax: Points will be virtual points behind the stereo window. +ve Behind stereo window Negative horizontal parallax: Points will be virtual points in front of the stereo window.

Viewing Stereo pairs – what does it mean for your eyes? • Parallel setup: Left Eye R 1 IPD P 1 Right Eye L 2 R 2 Crosse d set up Parallel set up • Crossed setup: L 1 In front of stereo window Adapted from [Slater, Steed, Chrysanthou] – Right eye sees Right image, – Left eye sees left image – Requires focus beyond the images Behind stereo window – Right eye sees left image – Left eye sees right image – Requires crossing eyes • Viewing the opposite way around will reverse the sense of depth.

Presenting 3 D images: Ideals • Congruence L &R images should be same (except as caused by the horizontal parallax) Esp. 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 & accomodation To provide max. depth but lowest parallax: Place principal objects so that ~½ parallax values are +ve, ½ -ve

CAVE-style 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 X Y (out of screen) Z 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 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 (>100 Hz) – – 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 3 D 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