Basic Color Concepts in the Field of Computer Graphics   

Understanding color on the computer can be a bit tricky since the computer makes use of more than one type of color model. This article will address two types of color spaces - additive and subtractive and explain how they interact and function.

Additive colors are the colors that are inherent in light. They are the colors indigenous to monitors, digital cameras, and scanners. The human eyes, also, sees these colors. They are red, green, and blue (RGB). They are called primary additive colors because when added together, they form white.

Subtractive colors are those used in the printing trades such as dyes, inks, and pigments. I am sure the term four color separation is familiar to many people. These colors are cyan, magenta, and yellow. These colors are called absorbing or subtractive colors since when light is absorbed by all of them, they produce black. These are the colors used in printers. They are commonly referred to as CMY colors. However, usually one sees CMYK. K stands for a truer black since when 100% of C, M, and Y are combined, the resulting color is a muddy dark brown. All of these colors are related. For example:

     white = red + green + blue        black = cyan + magenta + yellow 
     cyan = green + blue               red = yellow + magenta 
     magenta = blue + red              green = yellow + cyan
     yellow = red + green              blue = cyan + magenta

I hope that the above lists show an emerging pattern - one of  interrelationships. RGB and CMY colors form a complementary relationship.


Thus, if  I have an image that contains yellow and I want to intensify the yellow, I can add more yellow or I can subtract blue. The best way to understand complementary colors is to set up an image and manipulate the individual colors.

Computers are capable of generating images of varying numbers of colors. These can range from 16 colors to 16 million colors. If an image is created with only 16 colors and this image is turned into a grayscale image, there are only a certain number of shades of gray possible. Likewise, if an image that has 16 million colors is turned into a grayscale image, there are many more shades of gray even though the human eye cannot discern them all. How can the computer vary the number of colors or shades of gray? The computer monitor is made up of red, green, and blue phosphors or light producing elements. Each of the colors associated with a pixel (picture element) can have attributed to it a certain number of colors. Bit-depth determines how many colors or levels of gray each pixel carries. In scanning for example, a bit-depth of 24 means that the red, green, and blue sources of light each have 8 bits of color assigned to them. The more color bits assigned to a pixel, the more colors can be displayed and, theoretically, the more shades of gray. However, the human eye can only see a certain number of shades of gray. Also, not all visual colors can be transmitted to the printed medium. Thus, for example, when a scanner advertises that it can produce a bit-depth of 36, this bit-depth will not necessarily produce a better image than one with a bit-depth of 30 or even 24.

There are other models for assigning colors to an image:

               HSB = Hue, Saturation, & Brightness 
               HSL=Hue Saturation, & Lightness 
               HSV=Hue, Saturation, & Value Color Space

               Hue = The color of something
               Saturation = The strength of a color 
               Value = How dark or light is a color

The last term to be discussed is that of monitor gamma. Monitor gamma is basically the numerical representation of amonitor's contrast and brightness level. 2.2 is the gamma of Windows monitors. An image on a monitor set for a gamma of 1.8 appears lighter than an image on a monitor with a gamma of 2.2.

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