1. © 2002-2003 by Yu Hen Hu 1 Human Visual System

2. © 2002-2003 by Yu Hen Hu 2 Understanding HVS, Why? l Image is to be SEEN! l Perceptual Based Image Processing »Focus on perceptually significant information »Discard perceptually insignificant information l Issues: »Biological »Psychophysical

3. © 2002-2003 by Yu Hen Hu 3 Illustration of Human Eye The 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 http://psych.athabascau.ca/html/Psych402/Biotutorials/22/intro.shtml

4. © 2002-2003 by Yu Hen Hu 4 Human Eye Structure l Light passes through cornea, iris, lens and form image on retina. l 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

5. © 2002-2003 by Yu Hen Hu 5 Image formation l Distance between center of lens and retina (focal length) vary between 14-17 mm. l When object is 3 m or more away, f = 17mm with lowest refractive power. l Image length h = 17(mm) x (15/100)

6. © 2002-2003 by Yu Hen Hu 6 Visual Psychophysics l Model vision "system" as an input-output system »visual stimuli: input »prescribed sensations: output. l Visual psychophysics: »Characterize the response of HVS to different stimuli

7. © 2002-2003 by Yu Hen Hu 7 Visual Psychophysics l Brightness Adaptation l Spatial Threshold Vision »Weber ratio »Visual Masking »Mach Effect l Temporal vision l Frequency Threshold Vision

8. © 2002-2003 by Yu Hen Hu 8 Brightness Adaptation l HVS can view large intensity range (10 10 ) l But simultaneous perceived intensity range is much smaller. l If one is at B a intensity (outside) and walk into a dark theater, he can only distinguish up to B b. 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

9. © 2002-2003 by Yu Hen Hu 9 Weber Ratio l HVS’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. I I+  I

10. © 2002-2003 by Yu Hen Hu 10 Simultaneous Contrast l The perceived brightness of inner circle are different due to different background intensity levels even they are identical.

11. © 2002-2003 by Yu Hen Hu 11 Mach Band Effect Perceived Brightness changes around strong edges.

12. © 2002-2003 by Yu Hen Hu 12 Visual Masking Threshold intensity increases at background with large non- uniform spatial, temporal changes.

13. © 2002-2003 by Yu Hen Hu 13 Temporal Vision l Perceived spatial resolution reduced sharply at scene change l Flicker fusion: the basis of movie and TV l Eye is more sensitive to flicker at high luminance than low luminance.

14. © 2002-2003 by Yu Hen Hu 14 Frequency Threshold Vision l Using spatial grating, it is found that contrast sensitivity is a function of spatial and temporal frequencies. l In general, the contrast sensitivity decreases as spatial and temporal frequencies increases.

15. © 2002-2003 by Yu Hen Hu 15 Optical Illusions