Expanding Range for Color Space Representation

The traditional color signal specification defined primary colors and a white point using chromaticities based on red (R), green (G) and blue (B) phosphor emissions in a cathode ray tube (CRT). The ITU-R BT.709 picture signal standard for digital (HDTV) broadcasting was adopted in 1990, while the sRGB (IEC61966-2-1) standard for displays used with personal computers and other equipment was adopted by the International Electrotechnical Commission in 1999.

Except for the fact that there is no overwhite in picture signals based on the sRGB standard, the two standards have very similar color gamut. In CRTs, R, G and B phosphors are used to represent a triangular gamut encompassed within the color points for the primary colors on an x,y chromaticity diagram.

Expanding the Color Space by Increasing Primary Color Purity and Enlarging the Triangle

A wide range of colors can be represented in television and video picture signals by mixing the intensities of the three primary colors (RGB) as color signals. Because these composite colors can be used to produce every color contained within a triangle defined by the primary colors, the color space that can be represented can be expanded by raising the purity of the primary colors and enlarging the triangle (Figure 1.1).

Expanding the Color Space Using Negative Color Signal Values

By using negative values for the RGB color signals, colors outside the triangle can be represented without changing primary color chromaticities. Figure 1.2 shows how cyan, which is outside the triangle, can be represented using a negative value for R and positive values for G and B. In an color system of light (additive color mixing), a color that is exactly the same as one represented by mixing G and B can be obtained by mixing cyan, which is outside of the triangle, and R. Other colors outside of the triangle can be represented by mixing negative G and B signals. This characteristic of the xvYCC standard is used to expand the color space for video applications.

Technical Description of xvYCC Color Space

The existing standards for digital video are ITU-R BT.709 for HDTV (1920x1080) and ITU-R BT.601 for SDTV (720x480). The xvYCC standard (IEC 61966-2-4), adopted in January 2006, expands the color gamut by introducing negative color signal values for these video signals. This standard encompasses xvYCC709, which is upwardly compatible with the B7.709 standard, and xvYCC601, which is upwardly compatible with BT.601. The four characteristics of these standards are shown in Figure 2.

1. The chromaticity coordinates of RGB primary colors and the reference white (D65) are the same as for the existing BT.709 standard.
2. The opto-electric transfer characteristics of signals not less than 0 and not greater than 1 are defined using the same formula as for BT.709, and signals above 1 are also defined using the same formula. Signals below 0 are defined to produce a transfer curve with origin symmetry.
3. The conversion matrix from RGB (color signals) to YCC (luminance/color-difference signals) for xvYCC709 is the same as for BT.709, and that for xvYCC601 is the same as for BT.601.
4. With 8-bit quantization of luminance and color-difference signals, definition formula are set and the color gamut is expanded by using values between 1 and 15 and between 241 and 254 as picture signals. Definitions over 8 bits are also used to support precise gradation.