CIE L*u*v* and CIE L*a*b* summary

Colour specification and colour image coding

A colour specification system needs to be able to represent any colour with high precision. Since few colours are handled at a time, a specification system can be computationally complex. A system for colour specification must be intimately related to the CIE system. The systems useful for colour specification are CIE XYZ and its derivatives xyY, u’v’, L*u*v*, and L*a*b*.

A colour image is represented as an array of pixels, where each pixel contains three values that define

a colour. As you have learned in this chapter, three components are necessary and sufficient to define any colour. (In printing it is convenient to add a fourth, black, component, giving CMYK.)

In theory, the three numerical values for image coding could be provided by a colour specification system; however, a practical image coding system needs to be computationally efficient, cannot afford unlimited precision, need not be intimately related to the CIE system, and generally needs to cover only a reasonably wide range of colours and not all possible colours. So image coding uses different systems than colour specification.

The systems useful for image coding are linear RGB; nonlinear RGB (usually denoted R’G’B’, with sRGB as one variant); nonlinear CMY; nonlinear CMYK; and derivatives of R’G’B’, such as Y’CBCR and Y’PBPR. These systems are summarized in Figure 25.12.

If you manufacture cars, you have to match the paint on the door with the paint on the fender; colour specification will be necessary. You can afford quite a bit of computation, because there are only two coloured elements, the door and the fender. To convey a picture of the car, you may have a million coloured elements or more: Computation must be quite efficient, and an image coding system is called for.

Further reading

The bible of colorimetry is Color Science, by Wyszecki and Stiles. But it’s daunting; it covers colour very generally, and contains no material specific to imaging.

For an approachable introduction to colour theory, accompanied by practical descriptions of image reproduction, consult Hunt’s classic work.

Figure 25.12 Colour systems are classified into four groups that are related by different kinds of transformations. Tristimulus systems, and perceptually uniform systems, are useful for image coding. (I flag HSB, HSI, HSL, HSV, and IHS with a question mark: These systems lack objective definition of colour.)

Berns’ revision of the classic work by Billmeyer and Saltzman provides an excellent introduction to colour science. For an approachable, nonmathematical introduction to colour physics and perception, see Rossotti’s book.

Wyszecki, Günter, and Stiles, W.Stanley (1982), Color Science:

Concepts and Methods, Quantitative Data and Formulæ,

Second Edition (New York: Wiley).

Hunt, Robert W.G.,The Reproduction of Colour, Sixth Edition

(Chichester, U.K.: Wiley, 2004).

Hunt, Robert W.G.and Pointer, Michael R. (2011),

Measuring Colour, Fourth Edition (Chichester, U.K.: Wiley).

Berns, Roy S., (2000), Billmeyer and Saltzman’s Principles of

Color Technology, Third Edition (New York: Wiley).

Rossotti, Hazel (1983), Colour: Why the World Isn’t Grey

(Princeton, N.J.: Princeton Univ. Press).

Colour

Classical colour science, explained in the previous chapter, establishes the basis for numerical description of colour. However, colour science is intended for the specification of colour, not for image coding. Although an understanding of colour science is necessary to achieve good colour performance in video, its strict application is impractical. This chapter explains the engineering compromises necessary to make practical cameras and practical coding systems.

Video processing is generally concerned with colour represented in three components derived from the scene, usually red, green, and blue, or components computed from these. Accurate colour reproduction depends on knowing exactly how the physical spectra of the original scene are transformed into these components, and exactly how the components are transformed to physical spectra at the display. These issues are the subject of this chapter.

Once red, green, and blue components of a scene are obtained, these components are transformed into other forms optimized for processing, recording, and transmission. This will be discussed in Component video colour coding for SD, on page 357, and Component video colour coding for HD, on page 369. (Although the BT.709 primaries are now used in both SD and HD, unfortunately, other colour coding aspects differ.)

The previous chapter explained how to analyze SPDs of scene elements into XYZ tristimulus values representing colour. The obvious way to present those colours is to arrange for the display system to reproduce those XYZ values. That approach works in many

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