So-called RGB+W displays were commercialized in the in the 1990s and early 2000s, mainly in coloursequential DLP projectors. In an RGB+W display, the luminance of white is considerably greater than the sum of the luminances of red, green, and blue: High brightness is claimed; however, such displays do not exhibit additive colour mixture. As I write, virtually all presentations include pictorial imagery; customers demand proper colour portrayal, and RGB+W projectors have consequently fallen out of favour.

If you are unfamiliar with the term luminance, or the symbols Y or Y’, refer to Luminance and lightness, on page 255.

applications of colour reproduction, and it’s the basis for colour in video. However, in image reproduction, direct recreation of the XYZ values is unsuitable for perceptual reasons. Some modifications are necessary to achieve subjectively acceptable results. Those modifications were described in Constant luminance, on

page 107.

Should you wish to skip this chapter, remember that accurate description of colours expressed in terms of RGB coordinates depends on the characterization of the RGB primaries and their power ratios (white reference). If your system is standardized to use a fixed set of primaries throughout, as in SD and HD, you need not be concerned about different “flavours” of RGB. However, if your images have different primary sets in different stages or production – in digital cinema, or in digital still photography – it is a vital issue.

Additive reproduction (RGB)

In the previous chapter, I explained how a physical SPD can be analyzed into three components that represent colour. This section explains how those components can be mixed to present (“reproduce”) colour.

The simplest way to reproduce a range of colours is to mix the beams from three lights of different colours, as sketched in Figure 26.1 opposite. In physical terms, the spectra from each of the lights add together wavelength by wavelength to form the spectrum of the mixture. Physically and mathematically, the spectra add: The process is called additive reproduction.

I described Grassmann’s Third Law on page 272: Colour vision obeys a principle of superposition, whereby the colour produced by any additive mixture of three primary SPDs can be predicted by adding the corresponding fractions of the XYZ tristimulus components of the primaries. The colours that can be formed from a particular set of RGB primaries are completely determined by the colours – tristimulus values, or luminance values and chromaticity coordinates – of the individual primaries. Subtractive reproduction, used in photography, cinema film, and commercial printing, is much more complicated: Colours in subtractive mixtures are not determined by the colours of the individual primaries, but by their spectral properties.

ITU-R Rec. BT.709, Parameter values for the HDTV standard for the studio and for international programme exchange.

Characterization of RGB primaries

An additive RGB system is specified by the chromaticities of its primaries and its white point. If you have an RGB image without information about its primary chromaticities, you cannot accurately reproduce the image. In Figure 26.2 opposite, I plot the primaries of a few RGB systems that I will discuss.

BT.709 specifies the primaries for HD. The BT.709 triangle is shaded in Figure 26.2.

The range of colours – or gamut – that can be formed from a given set of RGB primaries is given in the [x, y] chromaticity diagram by a triangle whose vertices are the chromaticities of the primaries. This two-dimen- sional plot doesn’t tell the whole story, though: The range of [x, y] values that can be covered is a function of luminance. For example, BT.709’s saturated blue colour at [0.15, 0.06] is only accessible at luminance below about 7% of white luminance; no chroma excursion is available at reference white! Gamut should be considered in three dimensions. I’ll discuss gamut further on page 311.

In computing, the sRGB standard is now ubiquitous. The sRGB standard shares the BT.709 primaries. Many applications in desktop computing assume an sRGB interpretation unless other information accompanies the image.

The SMPTE/DCI P3 primaries that are standardized for D-cinema are overlaid on Figure 26.2.

Each of these systems will now be described in detail.

BT.709 primaries

International agreement was obtained in 1990 by the former CCIR – now the ITU-R – on primaries for highdefinition television (HD). The standard is formally denoted Recommendation ITU-R BT.709 (formerly CCIR Rec. 709). I’ll call it BT.709. Implausible though this sounds, the BT.709 chromaticities were agreed upon as the result of a political compromise that culminated in EBU red, EBU blue, and a green which is the average (rounded to 2 digits) of EBU green and SMPTE green! These primaries were adopted into the sRGB standard for computing and computer graphics. The BT.709 primaries are closely representative of contemporary displays in studio video. The chromaticities of the

y 520

0.8

540

0.7

500

0.6

0.5

0.4

0.3

480

0.2

0.1

460

440 400

0.0

560

[.314, .351] CIE D65

SMPTE/DCI P3

Reference Projector

BT.709

580

600

620

640

700

Table 26.1 BT.709 primaries apply to 1280× 720 and 1920× 1080 HD systems; they

are incorporated into the sRGB standard for desktop PCs.

BT.709 primaries and its D65 white point are specified in Table 26.1: