Human Visual SystemUnderstanding HVS, Why?Illustration of Human EyeHuman Eye StructureImage formationVisual PsychophysicsSlide 7Brightness AdaptationWeber RatioSimultaneous ContrastMach Band EffectVisual MaskingTemporal VisionFrequency Threshold VisionOptical Illusions© 2002-2003 by Yu Hen Hu1Human Visual System© 2002-2003 by Yu Hen Hu2Understanding HVS, Why?Image is to be SEEN!Perceptual Based Image Processing»Focus on perceptually significant information»Discard perceptually insignificant informationIssues:»Biological»Psychophysical© 2002-2003 by Yu Hen Hu3Illustration of Human EyeThe amount of light entering the eye is controlled by the pupil, which dilates and contracts accordingly. The cornea and lens, whose shape is adjusted by the ciliary body, focus the light on the retina, where receptors convert it into nerve signals that pass to the brain. A mesh of blood vessels, the choroid, supplies the retina with oxygen and sugar. http://psych.athabascau.ca/html/Psych402/Biotutorials/22/intro.shtml© 2002-2003 by Yu Hen Hu4Human Eye StructureLight passes through cornea, iris, lens and form image on retina.Two types of photoreceptors on retina:»Cones cluster at Fovea, detect color at bright light - photopic vision»Rods spread at back of eye, general vision - scotopic vision© 2002-2003 by Yu Hen Hu5Image formationDistance between center of lens and retina (focal length) vary between 14-17 mm. When object is 3 m or more away, f = 17mm with lowest refractive power. Image length h = 17(mm) x (15/100)© 2002-2003 by Yu Hen Hu6Visual PsychophysicsModel vision "system" as an input-output system»visual stimuli: input »prescribed sensations: output.Visual psychophysics: »Characterize the response of HVS to different stimuli© 2002-2003 by Yu Hen Hu7Visual PsychophysicsBrightness AdaptationSpatial Threshold Vision»Weber ratio»Visual Masking»Mach EffectTemporal visionFrequency Threshold Vision© 2002-2003 by Yu Hen Hu8Brightness AdaptationHVS can view large intensity range (1010)But simultaneous perceived intensity range is much smaller. If one is at Ba intensity (outside) and walk into a dark theater, he can only distinguish up to Bb. It will take much longer for eye to adapt for the scotopic vision to pick up. http://www.cs.berkeley.edu/~efros/java/vision/vision.html© 2002-2003 by Yu Hen Hu9Weber RatioHVS’s sensitivity to intensity difference differ at different background intensities.Weber ratio: I/I: Just noticeable intensity difference versus background intensity. It is a function of log I.II+ I© 2002-2003 by Yu Hen Hu10Simultaneous ContrastThe perceived brightness of inner circle are different due to different background intensity levels even they are identical.© 2002-2003 by Yu Hen Hu11Mach Band EffectPerceived Brightness changes around strong edges.© 2002-2003 by Yu Hen Hu12Visual MaskingThreshold intensity increases at background with large non-uniform spatial, temporal changes.© 2002-2003 by Yu Hen Hu13Temporal VisionPerceived spatial resolution reduced sharply at scene changeFlicker fusion: the basis of movie and TVEye is more sensitive to flicker at high luminance than low luminance.© 2002-2003 by Yu Hen Hu14Frequency Threshold VisionUsing spatial grating, it is found that contrast sensitivity is a function of spatial and temporal frequencies. In general, the contrast sensitivity decreases as spatial and temporal frequencies increases.© 2002-2003 by Yu Hen Hu15Optical
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