The color is represented directly as a combination of the R, G, B data.
The digitized picture is initially written in a specialized area of computer memory called frame buffer. The frame buffer contains three values with each value monotonically related to the luminance of each of the three types of phosphers in a pixel. These values is used to alter the voltage of the electron gun should use in emitting electron beams to the screen, and hence the color of each pixel. Look-up tables may be interposed between the frame buffer and the electron guns for compensation purposes, such as gamma correction. [Travis, 1992]
Color images seen on one display may look different on another due to the different gamma values on different monitors, and differences in the red, green, blue phosphers on different displays. To address the problem, gamma correction is used in color monitors. Also, many graphics softwares provide gamma correction function for the graphical images so that the target gamma value can be set at the desired value.
Color on Printer
In constrast to color on display, which uses the additive RGB color model, color on printing uses the subtractive CMYK (cyan, magenta, yellow, and black) model. The black color in the model is not necessary in theory, but the uses of it is described later. Substractive means that color you see on the paper is a result of adding together the three secondary colors, filtering out the unwanted color components, and reflecting only the desired colors. Cyan absorbs the red color component, magenta absorbs the green, and yellow absorbs the blue [Schindler, 1995]. For example, mixing magenta and yellow together results in the red color seen on the paper.
Using the three secondary colors, any color can virtually be produced and printed on the paper. This includes the black color. However, the black color produced by mixing of the three colors does not look really black. In fact, it looks somewhat like brown or grey. Also, it requires more accuracy in printing mixed black because we need to control the three types of inks to be deposited on the same spot on the paper in order to produce clear black letters, for example [Schindler, 1995]. To print true black color, many color printers provide, in addition to the color cartridge, the black ink cartridge for true black color printing.
At some point during the print process, the color image is analysed and the amount of cyan, magenta and yellow to be used for a particular image spot is determined -- color separation. The black amount of the image to be used is also determined -- black generation. In a color printer with both black and color inks, black color would be printed twice because the black color component on the image is analysed twice, once in color separation and once in black generation. The resulting image would appear too dark. To solve this, color printers use the technique of under-color remmoval which removes an equal amount of cyan, magenta, and yellow proportional to the black amount on the image during the color separation process, so that the black color will not be printed twice.
