Why do we use color image
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In automated image analysis, color is a powerful descriptorÆ simplify object identification and extraction from a scene
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For human visionÆ human eye can discern thousands of color shades and intensities (only two dozen shades of gray)
Color Image Processing Techniques
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Full color – image is acquired with a full-color sensor like TV camera or color scanner
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Pseudo color – Assign a color to a range of monochrome intensities
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The availability of inexpensive and powerful hardware has resulted in the proliferation of applications based on full color processing
Color fundamentals
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Color spectrum/prism
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Color
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Characterization of light
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Chromatic light
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Light source characterized by three quantities
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Radiance Total amount of energy emitted by light source
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Physical power of light energy, measured in watts
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Directional quantity
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Measures the quantity of radiation that passes through or emitted from a surface and falls within a given solid angle in a specified direction
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Expressed in a spectral power distribution, often in 31 components, each representing a 10 nm band
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Historically, also called intensity
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Brightness Achromatic notion of intensity to describe color sensation
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Attribute of a visual sensation according to which an area appears to emit more or less light
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Subjective attribute of an object being observed
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Cannot be measured quantitatively
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Luminance Measure of amount of energy as perceived by an observer, measured in lumens or candelas per square meter
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Light may contain a lot of energy in IR bands but that is not perceptible to the observer
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More tractable version of brightness, defined by CIE
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Radiant power weighted by a spectral sensitivity function that is characteristic of vision
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Luminous efficiency peaks at 555nm
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CIE luminance, denoted by Y , is the integral of spectral power distribution, using spectral sensitivity curve as a weighting function
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Magnitude of luminance is proportional to physical power, but spectral composition is related to brightness sensitivity of human vision
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Units of measurement for image processing
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