RG color space
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The RG or red-green color space is a color space that uses only two colors, red and green. It is an additive format, similar to the RGB color model but without a blue channel. Thus, blue is said to be out of gamut. This format is not in use today, and was only used on two-color Technicolor and other early color processes for films; by comparison to a full spectrum, its poor color reproduction made it undesirable. The system cannot create white naturally, and many colors are distorted.
Any color containing a blue color component can't be replicated accurately in the RG color space. There is a similar color space called RGK which also has a black channel. Outside of a few low-cost high-volume applications, such as packaging and labelling, RG and RGK are no longer in use because devices providing larger gamuts such as RGB and CMYK are in widespread use.
 Original RGB image |
 Converted to additive RG |
 Converted to subtractive RG |
 Converted to RC (red-cyan) |
[edit] Color Graphics Adapter (CGA)
The first color capable cards and monitors for IBM PC family were the Color Graphics Adapter (CGA), which includes two graphic modes: 320×200 pixels with four colors (two bits per pixel) and 640×200 pixels black-and-white (one bit per pixel). The color mode uses two bits to store red and green 1-bit components (that is, colors in the RG color space) to obtain four combinations: black, red, green and yellow, with two possibilities of intensity: low (darker) and high (lighter). This was known as Fixed palette #2. The Fixed palette #1 adds the blue component, giving black, magenta (red+blue), cyan (green+blue) and white (yellow+blue), with two possible intensities, too.
