Color Image Processing

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  • Topic: Color, Color space, RGB color model
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Digital Image Processing
Chapter 6: Color Image Processing

Spectrum of White Light

1666 Sir Isaac Newton, 24 year old, discovered white light spectrum.

(Images from Rafael C. Gonzalez and Richard E. Wood, Digital Image Processing, 2nd Edition.

Electromagnetic Spectrum

Visible light wavelength: from around 400 to 700 nm 1. For an achromatic (monochrome) light source, there is only 1 attribute to describe the quality: intensity 2. For a chromatic light source, there are 3 attributes to describe the quality: Radiance = total amount of energy flow from a light source (Watts) Luminance = amount of energy received by an observer (lumens) Brightness = intensity (Images from Rafael C. Gonzalez and Richard E. Wood, Digital Image Processing, 2nd Edition.

Cross section illustration

The Eye
UMCP ENEE631 Slides (created by M.Wu © 2004)

Figure is from slides at Gonzalez/ Woods DIP book website (Chapter 2)

Two Types of Photoreceptors at Retina
• Rods
– – – – – – – – – Long and thin Large quantity (~ 100 million) Provide scotopic vision (i.e., dim light vision or at low illumination) Only extract luminance information and provide a general overall picture Short and thick, densely packed in fovea (center of retina) Much fewer (~ 6.5 million) and less sensitive to light than rods Provide photopic vision (i.e., bright light vision or at high illumination) Help resolve fine details as each cone is connected to its own nerve end Responsible for color vision our interest (well-lighted display)

• Cones

– Mesopic vision

• provided at intermediate illumination by both rod and cones

Sensitivity of Cones in the Human Eye
6-7 millions cones in a human eye - 65% sensitive to Red light - 33% sensitive to Green light - 2 % sensitive to Blue light

Primary colors: Defined CIE in 1931 Red = 700 nm Green = 546.1nm Blue = 435.8 nm CIE = Commission Internationale de l’Eclairage (The International Commission on Illumination)

(Images from Rafael C. Gonzalez and Richard E. Wood, Digital Image Processing, 2nd Edition.

Luminance vs. Brightness
Same lum. Different brightness Different lum. Similar brightness

• Luminance (or intensity)
– Independent of the luminance of surroundings

I(x,y,λ) -- spatial light distribution V(λ) -- relative luminous efficiency func. of visual system ~ bell shape (different for scotopic vs. photopic vision; highest for green wavelength, second for red, and least for blue )

• Brightness
– Perceived luminance – Depends on surrounding luminance

Luminance vs. Brightness (cont’d) • Example: visible digital watermark – How to make the watermark appears the same graylevel all over the image?

from IBM Watson web page “Vatican Digital Library”

Look into Simultaneous Contrast Phenomenon • Human perception more sensitive to luminance contrast than absolute luminance • Weber’s Law: | Ls – L0 | / L0 = const – Luminance of an object (L0) is set to be just noticeable from luminance of surround (Ls) – For just-noticeable luminance difference ∆L: ∆L / L ≈ d( log L ) ≈ 0.02 (const)

• equal increments in log luminance are perceived as equally different

• Empirical luminance-to-contrast models
– Assume L ∈ [1, 100], and c ∈ [0, 100]

– c = 50 log10 L (logarithmic law, widely used) – c = 21.9 L1/3 (cubic root law)

Mach Bands
UMCP ENEE631 Slides (created by M.Wu © 2004)

Figure is from slides at Gonzalez/ Woods DIP book website (Chapter 2)

• Visual system tends to undershoot or overshoot around the boundary of regions of different intensities è Demonstrates the perceived brightness is not a simple function of light intensity

Color of Light
UMCP ENEE408G Slides (created by M.Wu & R.Liu © 2002)

• Perceived color depends on spectral content (wavelength
composition) – e.g., 700nm ~ red. – “spectral color” • A light with very narrow bandwidth

“Spectrum” from http://www.physics.sfasu.edu/astro/color.html

• A light with equal energy in all visible bands...
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