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Visual Perception

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Visual Perception Overview Vision & Visuals Human Visual Systems Visual Perceptions 3D Depth Cues 3D Stereographics Terminology Stereo Approximation 3D Displays & Auto-Stereoscopic Displays 029515 Desiggpyn Issues for VR Visual Displays 2010년 봄학기 3/29/2010 박경신 Human Visual System Human Eyes Light is focused by the cornea and the lens onto the retina Light passing through the center of the cornea and thlhe lens hi hihfts the fovea (or Macu l)la) Iris permits the eye to adapt to varying light levels, contro lling the amoun t o f lig ht en ter ing the eye. Retina is optically receptive layer like a film in a camera Retina translate light into nerve signals. Retina has photoreceptors (rods & cones) and inter- neurons. Photoreceptors (Rods & Cones) Rods & Cones Distribution Rods Rods & Cones Distribution Operate at lower illumination levels Fovea Luminance-only only cone receptors with The most sensitive to light very hihigh h dens itity At night where the cones cannot detect the light, the rods provide us with a black and white view of the world no rods The rods are also more sensitive to the blue end of the no S-cones (blue cones) spectrum responsible for high visual Cones acuity operate at higher illumination levels Blind Spot provide better spatial resolution and contrast sensitivity no receptors provide color vision (currently believed there are 3 types of Axons o f a ll gangli lion ce lls cones in human eye, one attuned to red, one to green and pass through the blind spot one to blue) [Young-Helmholtz Theory] on the way to the brain provide visual acuity Visual Spectrum Color Perception We perceive electromagnetic There are three types of energy having wavelengths cones, referred to as S, M, in the range 400 nm ~ 700 and L. They are roughly nm as visible light. equivalent to blue, green, ~450nm : violet and red sensors. Their peak 458nm ~ 480nm : blue sensitivities are located at 420 nm, 534 nm, 564 nm. 510nm ~ 550nm : green Color ppperception results 570nm ~ 590nm : ye llow Spectral Sensitivity of Cones from the simultaneous 590nm ~ 630nm : orange stimulation of the 3 cone 630nm ~ : re d types. Colorblindness results from a deficiency of one cone type. Chromatic Color Visual Perception Hue Visual Acuity distinguishes between colors Visual acuity is measured as the angle subtended by the human eye. Saturation Snellen fraction 20/X where the viewer sees at 20 feet detail how far is the color from a that the average person can see at X feet gray of equal intensity The standard definition of normal visual acuity (20/20 vision) vivid colors (red, blue) are is the ability to resolve a spatial pattern separated by an angle highly saturated, further from of 1 arc min gray A resolution of about 1 arc min is necessary to get to the pastltel co lors (ik(pink, s kky bl)blue) region of peak sensitivity are lightly saturated, closer to gray Brightness CIE (Commission Internationale perceived the intensity of a lum inous o bjec t d’Eclairage) Chromaticity Diagram 20/20 CRT 20/40 HMD 20/425 BOOM 20/85 CAVE 20/110 Visual Perception Visual Perception Temporal Resolution Color The real world doesn’t flicker. CRTs do flicker because the Most visual perceptual processes are driven by intensity not image is constantly being refreshed. color. We perceive flickering if the image on CRT isn’t refreshed fast Motion system is color blind, depth perception is color blind, enough. object recognition is color blind Most people stop perceiving the flicker between 15 Hz (for But uniquely colored objects are easy to find dark image) and 50 Hz (for bright images). Field of view (FOV) Some people can perceive flicking even at 60 Hz for a bright Each eye has a horizontal fov of 150° (60° towards the nose display with a large field of view. and90d 90° to the s ide ) an d a ver tica l fov o f 120° (50° up and70d 70° Very large display may require up to 85 Hz. down). Luminance Most HMDs have a FOV of about 40 to 60. The eye has a dynamic range of 7 orders of magnitude. Motion The eye is sensitive to ratios of intensities rather than to Motion of the visual field causes a sense of motion even abltdiffbsolute differences. BihtBrightness = Lum inance ^033^0.33. without physical motion. Studies have shown that a low To make something appear n times brighter the luminance resolution peripheral image contributes strongly to the effect must be increased by n^3. Visual Depth Cues Visual 3D Depth Cues - Pii“dh”ihPerceiving “depth” with one eye c ldlosed Linear perspective Objec ts ge t sma ller the fur ther away they are an d para lle l line converge in distance. Size of known objects We expect certain object to be smaller than others. Detail (texture gradient) Close objects appear in more detail, distant objects less. Occlusion (hidden surfaces) An object that blocks another is assumed to be in the foreground. Lighting and Shadows Closer objects are brighter, distant ones dimmer. Shadow is a ffliform of occlusion. Relative motion (motion parallax due to head motion) Objjyects further away seem to move more slowl yjy than objects in the foreground. 13 Perspective Size The observance that parallel lines converge at a single Compare the size of objects with our memory of similar vanihiishing po int. objec ts to approx ima te how far away the o bjec t is from Relies on the assumption that the object being viewed is us. construc te d o f para lle l lines, suc h as mos t bu ildings. Comparing the size of objects with respect to other objects of the same type to determine the relative distance between objects. Detail Occlusion Our eyes cannot discern as much detail of a texture at a An object occludes our view of another distance as compare d w it h up c lose. The strongest depth cue Atmospheric effects, such as haze and fog, cause more dis tan t o bjec ts to be v isua lly less dis tinc t. Liggghting and Shadow Motion Parallax Brighter objects are perceived as being closer. As an observer moves, nearby objects appear to move rapidly while far objects appear to move slowly. With one source of light, all shadows lie in same Come from the parallax created by the changing relative direction. ppjgosition between the head and the object being The object covered by the shadow is perceived to be observed. further away than the object in the light. Generally more important than stereoscopy for VR. A form o f s ha ding tha t in dica te the pos itiona l relationship between two objects. Visual Depth Cues Stereoscopy - UiUsing bo th eyes Binocular vision occurs when Binocular cues: binocular disparity (stereopsis) two eyyges look at the same thing This is the difference in the images projected onto the back of at a slightly different angle, the eye (and then onto the visual cortex) because the eyes are resulting in two slightly different seppyyarated horizontally by the interocular distance. images. Oculomotor cues: accommodation & convergence The brain must match points Based on information from muscles in the eye between the two separate Accommodation (focus) images seen by the two eyes. left right This is the muscle tension needed to change the focal length of the The slight difference between eye lens in order to focus at a particular depth. the v iewpo in ts o f your two eyes Convergence is called binocular disparity This is the muscle tension required to rotate each eye so that it is fifacing t hfhe foca l po int. Stereopsis is depth perception Accommodation and Convergence work together (when eyes due to binocular disparity converggye to a certain distance, automatically accommodates and Possiblyypp 12% of people have no vice versa) stereo vision or some problem with stereo vision. Accommodation (()focus) Convergence Some Terminology Homologous Points Horizontal Parallax (Binocular Disparity) When the re tina l images o f an o bjec t fa ll on dispara te po in ts on the two retinas, these points differ only in their horizontal position. Thlhe value gi ibven by R – L Stereo Window (Stereo Plane) The point at which there is no difference in parallax between the two eye views Usually at the same depth as the monitor screen or the projection surface. Scan line Homologous Points Points which correspppyond to each other in the separate eye views. Vertical Displacement Vertical parallax between homologous points relative to the line Stereo Plane that the two eyes form. Some Terminology Some Terminology Interocular Distance Positive Parallax The distance between the left and right eyes, usually about The point lies behind the stereo window (On the opposite side 2.5 inches (i.e., 6.5 cm) from the observer) Hypostereo/Giantism Negative Parallax Decreasing the distance between the left and right eyes to The point lies in front of the stereo window (On the same side as show stereoscopic detail on small items the observer) Hyperstereo/Lilliputism Zero Parallax Increasing the distance between the left and right eyes to The point is at the same depth as the stereo window (Both eyes shidililhow stereoscopic detail in large scenes see thi)he same image) Interocular Crosstalk (Ghosting) Each eye should only see it’ s view but sometimes it can see part of the other eye view as well. This is distracting and causes eye fatigue. Positive Parallax Negative Parallax AdtiAccommodation-convergence mithismatch AdtiAccommodation-convergence mithismatch R L R L Iffff Objects are too close in front of the projection plane, negative parallax will increase.
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