lor prior to limiting. The constant hue having too much orange in the red, or too much blue in principle corresponds to moving invalid CbCr combina- the green. This can usually be compensated by having the tions directly towards the CbCr origin (128, 128), until display professionally calibrated. they lie on the surface of the valid YCbCr color block. When converting to the RGB color space from a non-RGB color space, care must be taken to include Non-RGB Color space saturation logic to ensure overflow and underflow considerations wrap-around conditions do not occur due to the finite precision of digital circuitry. 8-bit RGB values less than 0 must be set to 0, and values greater than 255 must be When processing information in a non-RGB color space set to 255. (such as YIQ, YUV, or YCbCr), care must be taken that combinations of values are not created that result in the generation of invalid RGB colors. The term invalid refers Gamma correction to RGB components outside the normalized RGB limits of (1,1,1). The transfer function of most CRT displays produces an For example, given that RGB has a normalized value of intensity that is proportional to some power (referred (1, 1, 1), the resulting YCbCr value is (235,128,128). If to as gamma) of the signal amplitude. As a result, Cb and Cr are manipulated to generate a YCbCr value of high-intensity ranges are expanded and low-intensity (235, 64, 73), the corresponding RGB normalized value ranges are compressed (see Figure 7.2-17). This is an becomes (0.6, 1.29, 0.56)-note that the green value advantage in combatting noise, as the eye is approxi- exceeds the normalized value of 1. mately equally sensitive to equally relative intensity Colour spaces CHAPTER 7.2 To compensate for the nonlinear display, linear RGB data was "gamma-corrected" prior to transmission by the inverse transform. RGB values are normalized to have a range of 0 to 1: R' = R1/2.2 TRANSMITTED G' G 1/2.2 PRE-CORRECTION B' = B1/2.2 Early PAL and SECAM Systems Most early PAL and SECAM systems assumed a sim