Summary of Topic 2
 Human visual system
 Cones
 Photopic or bright-light vision
 Highly sensitive to color
 Rods
 Not involved in color vision
 Sensitive to low level of illumination (scotopic or dim-light
vision)
G52IIP, School of Computer Science, University of Nottingham
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Summary of Topic 2
 Human visual system
 Brightness adaptation
 Subjective brightness is a logarithmic function of the light
intensity incident on the eye
 The HVS cannot operate on the full perceptible brightness
range (~10 orders of magnitude) simultaneously
 The total brightness range the HVS can discriminate
simultaneously is rather small in comparison (about 4 orders
of magnitude)
 It accomplishes this through (brightness) adaptation
G52IIP, School of Computer Science, University of Nottingham
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Summary of Topic 2
 Human visual system
 Brightness discrimination
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Perceivable changes at a given adaptation level
Weber ratio, DI/I, where I is background, DI intensity change
Small Weber ratio - good discrimination
Larger Weber ratio - poor discrimination
 Perceived brightness is not a simple function of intensity
 Mach band pattern
 Simultaneous contrast
G52IIP, School of Computer Science, University of Nottingham
3
Summary of Topic 2
 A simple image model
 Sampling
 Quantization
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4
Summary of Topic 2
 Colour image
 The RGB Color Model
 R, G, B at 3 axis ranging in [0 1] each
 Gray scale along the diagonal
 If each component is quantized into 256 levels [0:255], the total
number of different colors that can be produced is (28)3 = 224
=16,777,216 colors.
 The YIQ Color Model
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Video (NTSC) standard
Y encodes luminance; I and Q encode chrominance (“color”)
Black and white TV shows only the Y channel
Backward compatibility; efficiency
 YCbCr
G52IIP, School of Computer Science, University of Nottingham
5
Summary of Topic 2
 Colour image representation
G52IIP, School of Computer Science, University of Nottingham
6