lecture19_2

lecture19_2 - CS 519 Signal Image Processing Human Vision...

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1 CS 519 – Signal & Image Processing Human Vision Human Vision Some properties of human vision that affect image perception: Linear and non-linear parts Non-linear (approx. logarithmic) encoding of input Adaptation Relative-contrast encoding Varying sensitivity to spatial frequencies Generally treats brightness and color separately

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2 Discrimination Experiments Many vision experiments involve comparisons “Two alternative, forced choice” (2-AFC) experiments: Is there a difference? (yes/no) Which is brighter, farther apart, etc. (top/bottom, left/right, etc.) Random guessing without bias: 50% correct Pick some percentage above which the observer must get it right: often 75% (half the time they “see it”, half the time they guess) Vary experimental parameter to determine the threshold T above which the observer reaches this desired level of confidence This is called the just noticeable difference (JND) Sensitivity: 1/ T Weber’s Law Many visual properties obey Weber’s Law For intensity discrimination: for some constant c In other words, the JND I for intensity is proportional to the intensity itself Also applies to distance judgments, spatial frequency discrimination, and many others c I I =
3 Weber’s Law and Logarithmic Encoding Differences of logarithmic encoding produces Weber’s Law The human intensity sensitivity function isn’t exactly logarithmic, but it’s close enough to be a useful model ( ) ( ) constant 1 log log log log = + = ¸ ¹ · ¨ © § + = + c I I I I I I Adaptation Our eyes have an incredible ability to adapt to lighting conditions Total JND steps for the eye is about 1000 Total JND steps for fixed adaptation is about 200

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4 Contrast Encoding The response of the eye to light isn’t absolute – it’s relative to the surrounding intensities This causes the Mach effect at strong intensity transitions Even our color perception seems to be, to a degree, based on relative differences Intensity: measurable light Brightness: the perceived illumination Spatial Frequencies Stimulus: sinusoidal grating of some frequency f and amplitude A Vary A in a 2-AFC and find the JND for each frequency f Plot the sensitivity as a function of frequency f : the contrast sensitivity function (CSF) Implications: – The eye is less sensitive to extremely gradual changes – The eye is fairly sensitive to more rapid changes – The eye is decreasingly sensitive to yet higher spatial frequencies
5 Human Visual System • Photoreceptor cells – rods, cones • Rods Brightness perception only More concentrated on the periphery Æ Peripheral vision Good for seeing motion Low-lighting conditions (scotopic) • Cones Color perception Concentrated at the fovea Æ Central (foveal) vision Bright-lighting conditions (photopic) Color Perception • Three types of cones: “Green” (most sensitive)

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