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

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 Chromaticity Diagram, source wikipedia 1

10/21/2008 CRT Phosphor Spectrogram, source wikipedia Brightness & Contrast • Brightness – Projectors 2000 lumens – Screen 500 candelas/m 2 – 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 2

10/21/2008 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 P h l i l b i i 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 3

10/21/2008 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 4

10/21/2008 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 30cm • Perceptual fusion of 2 views (cyclopean union) of 2 viewpoints: perpn. of depth • Inputs from 2 eyes converge 6.5cm on the same cortical neurons in V1 • Calculation of how different the 2 views are leads to model of depth Left eye Right eye Binocular percept Adapted from [Purves & Lotto] 5

10/21/2008 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 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 6

10/21/2008 Level of Defn Cues Technology virtualization ALL PSYCHOLOGICAL CUES: Pen & Paper Linear persp., 3D obj. + Perspective… Shading, inscribed on a Virtual ?? Shadows, flat sheet Space Aerial persp., Occlusion, , Texture cues PHYSIOLOGICAL “Stereoscope” CUES 1830s Stereoscopic Percepn. Of Virtual Charles Wheatstone disparity, obj with depth Image Accomodation* Stereo screens… Convergence* Virtual Obs. slaved All cues Cave, HMDs… Environment motion consistent with ~1980s - ‘90s parallax… observer motion 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 that surface f • 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 • Project diff. views of the same scene to each eye at the ~same time Left Eye R1 • If If want to present point as being t t t i t b i L2 in front of screen: (virtual point P1 P2 R2 P1) use L1, R1 Right Eye L1 • If want to present point as being behind screen: (P2) use L2, R2 Near side of Stereo window Far side of image plane image plane • Points L1 & R1 are homologous: same point in image space Adapted from [Slater, Steed, Chrysanthou] 7

10/21/2008 Setting up a stereo view: Parallax 0.0 Left Eye • IF: R-L > 0 : (P2) R1 Positive horizontal parallax: L2 Points will be virtual points P1 P2 R2 2 behind the stereo window. • IF: R-L < 0 : (P1) Right Eye L1 Negative horizontal parallax: +ve Points will be virtual points in In front of Behind stereo stereo window window front of the stereo window. Adapted from [Slater, Steed, Chrysanthou] Viewing Stereo pairs – what does it mean for your eyes? • Parallel setup: Crosse – Right eye sees Right image, Left Eye d set R1 – Left eye sees left image up – Requires focus beyond the L2 Parallel P1 images images R2 2 set up Right Eye • Crossed setup: L1 – Right eye sees left image In front of Behind stereo – Left eye sees right image stereo window window – Requires crossing eyes • Viewing the opposite way around will reverse the sense of depth. 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 • Vertical parallax -> Zero V ti l ll Z (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 8

10/21/2008 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 See yourself – Higher quality images – Less optical distortion 9

10/21/2008 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 10