Rg chromaticity
The rg chromaticity space, two dimensions of the normalized RGB, or rgb, space,is a chromaticity space, a two-dimensional color space in which there is no intensity information. In the RGB color space a pixel is identified by the intensity of red, green, and blue primary colors. Therefore, a bright red can be represented as (R,G,B) (255,0,0), while a dark red may be (40,0,0). In the normalized rgb space or rg space, a color is represented by the proportion of red, green, and blue in the color, rather than by the intensity of each. Since these proportions must always add up to a total of 1, we are able to quote just the red and green proportions of the color, and can calculate the blue value if necessary. (Wikipedia).
