Game Development Reference
In-Depth Information
Fig. 2.2
Color sensitivity. Attribution: ByOpenStaxCollege [CC-BY-3.0
(
http://creativecommons.
org/licenses/by/3.0
)
] via Wikimedia Commons
2.1.2 RGB, XYZ, and YUV Color Spaces
RGB color space is based on an additive RGB color model, which describes how
to mix red, green, and blue three colors to produce a given color (Fig.
2.3
). As the
RGB color model is simple for implementation, it has been widely used for sens-
ing, representation, and display of images in electronic systems, such as televisions
and computers. However, the RGB color model is device dependent, and different
manufacturers may define different RGB color spaces, such as sRGB, created coop-
eratively by HP and Microsoft, and Adobe RGB, etc. So color conversion among
different devices is necessary and one unified color space is needed for reference.
Moreover, the additive RGB color model cannot encompass all the colors perceived
by the human eyes. The reason is that the spectral sensitivity curves of the three
types of cone cells overlap and the perceived light will not stimulate only one type
of cone cell. Thus for pure spectral colors, at least one of the three primaries values
would be negative in an additive color space, e.g., RGB color space, to match the
corresponding true LMS tristimulus values. To avoid these negative RGB values, the
International Commission on Illumination (CIE), which is a professional organiza-
tion working on the science, technology and art in the fields of light and lighting,
defined an “imaginary” primary colors-based color space in 1931, also called CIE
1931 XYZ color space. CIE 1931 XYZ color space encompasses all the colors per-
ceived by the human eyes and is usually used as a reference for other color spaces.
XYZ is analogous, but not equal to the LMS cone responses of the human eye. They
are not true colors and can be divided into two parts, luminance or brightness (Y)
and chromaticity (X, Z). Z is quasi-equal to blue stimulation, or the S cone response,
and X is nonnegative as a linear combination of cone response curves. Defining Y as
luminance has the useful result that for any given Y value, the XZ plane will contain
