1. multimedia systems Lecture 5: Color in Image and Video

2. Fundamentals of Multimedia, Chapter 4  Light and Spectra  Light is an electromagnetic wave. Its color is characterized by the wavelength content of the light. ◦ (a) Laser light consists of a single wavelength: e.g., a ruby laser produces a bright, scarlet-red beam. ◦ (b) Most light sources produce contributions over many wavelengths. ◦ (c) However, humans cannot detect all light, just contributions that fall in the “visible wavelengths”. ◦ (d) Short wavelengths produce a blue sensation, long wavelengths produce a red one. 2

3. Fundamentals of Multimedia, Chapter 4  Figure1 shows the phenomenon that white light contains all the colors of a rainbow.  Visible light is an electromagnetic wave in the range 400 nm to 700 nm (where nm stands for nanometer, 10 −9 meters). 3 Fig1: Sir Isaac Newton’s experiments.

4. Fundamentals of Multimedia, Chapter 4  The eye works like a camera, with the lens focusing an image onto the retina.  The human eye can distinguish about 10 million different colors  The retina consists of an array of rods and three kinds of cones.  The rods come into play when light levels are low and produce an image in shades of gray (“all cats are gray at night!”).  For higher light levels, the cones each produce a signal. Because of their differing pigments, the three kinds of cones are most sensitive to red ( R ), green ( G ), and blue ( B ) light. 4

5. Fundamentals of Multimedia, Chapter 4  The eye is most sensitive to light in the middle of the visible spectrum.  The sensitivity of our receptors is also a function of wavelength  The Blue receptor sensitivity is not shown to scale because it is much smaller than the curves for Red or Green.  – Statistically, Blue is the favourite color of humans, regardless of nationality ◦ Fig. 2 shows the overall sensitivity as a dashed line — this important curve is called the luminous-efficiency function. ◦ – It is usually denoted V (λ) and is formed as the sum of the response ◦ curves for Red, Green, and Blue. 5

6. Fundamentals of Multimedia, Chapter 4  The red sensitivity curve looks like the luminous-efficiency function V (λ) but is shifted to the red end of the spectrum.  Fig. 2: R,G, and B cones, and Luminous Efficiency curve V(λ). 6

7. Fundamentals of Multimedia, Chapter 4  Surfaces reflect different amounts of light at different wavelengths, and dark surfaces reflect less energy than light surfaces. 7

8. Fundamentals of Multimedia, Chapter 4  Camera systems are made in a similar fashion;  Analog signals are converted to digital, truncated to integers, and stored. If the precision used is 8-bit, then the maximum value for any of R,G,B is 255, and the minimum is 0.  However, the light entering the eye of the computer user is that which is emitted by the screen—the screen is essentially a self-luminous source. Therefore we need to know the light E (λ) entering the eye. Li & Drew8

9. Fundamentals of Multimedia, Chapter 4  Schemes include: ◦ a) CMY — Cyan ( C ), Magenta ( M ) and Yellow ( Y ) color model; ◦ b) HSL — Hue, Saturation and Lightness; ◦ c) HSV — Hue, Saturation and Value; ◦ d) HSI — Hue, Saturation and Intensity; ◦ e) HCI — C=Chroma; ◦ f) HVC — V=Value; ◦ g) HSD — D=Darkness. Li & Drew9

10. Fundamentals of Multimedia, Chapter 4  Hue is the term for the pure spectrum colors commonly referred to by the "color names" - red, orange, yellow, blue, green violet - which appear in the hue circle or rainbow Li & Drew10

11. Fundamentals of Multimedia, Chapter 4  Saturation is a color term commonly used by (digital / analog) imaging experts. -Saturation is usually one property of three when used to determine a certain color and measured as percentage value. -Saturation defines a range from pure color (100%) to gray (0%) at a constant lightness level. A pure color is fully saturated. Li & Drew11

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13. Fundamentals of Multimedia, Chapter 4  Value is defined as the relative lightness or darkness of a color. Li & Drew13

14. Fundamentals of Multimedia, Chapter 4  Lightness is a color term commonly used by (digital / analog) imaging experts. Lightness is usually one property of three when used to determine a certain color and measured as percentage value. Lightness defines a range from dark (0%) to fully illuminated (100%). Any original hue has the average lightness level of 50%.  Darkness, the polar opposite to brightness, is understood to be an absence of visible light. It is also the appearance of black in a colored space. Li & Drew14

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16. Fundamentals of Multimedia, Chapter 4  Intensity: In general, intensity is a synonym for magnitude, degree or strength. It can therefore be used in conjunction with any color property. Nevertheless, it carries special meaning in certain contexts. -For painters the meaning of intensity is equivalent to the meaning of saturation. -For physicists intensity refers to different aspects of radiation. When speaking of light, the intensity can mean the number of photons a light source emits Li & Drew16

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18. Fundamentals of Multimedia, Chapter 4  Chroma is a component of a color model. There's a blue-yellow and a red-green chroma component. Li & Drew18

19. Fundamentals of Multimedia, Chapter 4 RGB color model: The RGB color model is an additive color model in which red, green, and blue light are added together in various ways to reproduce a broad array of colors. Li & Drew19

20. Fundamentals of Multimedia, Chapter 4  CMY Color Model: This stands for cyan- magenta-yellow and is used for hardcopy devices. In contrast to color on the monitor, the color in printing acts subtractive and not additive., where the color pigments on the paper absorb certain colors. Li & Drew20

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22. Fundamentals of Multimedia, Chapter 4  YIQ and YUV are the two commonly used color models in video  1-YUV Color Model: Initially is used for PAL analog video, it is now also used in CCIR 601 standard for digital video.  Phase Alternating Line (PAL) is a colour encoding system for analogue television.  Y = 0.299R + 0.587G + 0.114B.  U = 0.492 (B - Y) V = 0.877 (R - Y) Li & Drew22

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24. Fundamentals of Multimedia, Chapter 4  YIQ is the color space used by the NTSC color TV system.  The YIQ transform:  Li & Drew24

25. Fundamentals of Multimedia, Chapter 4  YCbCr Color Model: The YCbCr model is closely related to the YUV, it is a scaled and shifted YUV.  Cb = (B - Y) / 1.772 + 0.5 Cr = (R - Y) / 1.402 + 0.5  YCbCr is used in JPEG and MPEG Li & Drew25