Microsoft KB Archive/137693

= PUB: Definitions of Color Terminology =

PSS ID Number: 137693

Article Last Modified on 9/26/2003

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The information in this article applies to:


 * Microsoft Publisher for Windows 95 3.0

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This article was previously published under Q137693



SUMMARY
This article describes the major ways you can describe the colors in a graphic and provides definitions for some of the major terms.



MORE INFORMATION
When you define a color in a computer graphic, you have to come up with a numerical value to describe how to display that color on a computer monitor, a color printer, or a television set. There are several major ways to do this. Because computers work in units of bytes (a series of 8 bits), most of the ways to define colors use units of 8-bits. In some graphics, the 8-bit units are called Channels. In others, they are called Color Planes. It doesn't matter whether your graphic uses channels or color planes; the final colors are identical.

RGB
This acronym stands for reg-green-blue. This is the way that most computer graphics files describe their colors.

In most cases, each of the three values is a number between 0 and 255. This provides up to 16,777,216 (24-bit) discrete colors.

Palettized Color (256-color)
This is a method of reducing the file size of an RGB graphic by using up to 256 discrete colors. Each of these 256 colors can be any one of 16,777,216 possible colors. A graphic that uses palettized color contains a palette, which consists of the definitions of the colors the graphic uses.

Each entry on the palette is described using a 24-bit RGB color definition. However, after the palette is defined, the graphic itself only has to define each color by storing the single 8-bit number that represents the position on the palette of each color that it uses.

NOTE: A grayscale image is a palettized image where the palette consists of up to 255 shades of gray.

Some graphics programs can extract only the palette and save it as a palette (.pal) file. These palette files can then be used in other graphics.

CMYK
This acronym stands for cyan, magenta, yellow, and black. This is a method of describing colors to make it easier to do process-color separations.

In most cases, each of the four channels is a value between 0 and 255. While this provides up to 4,294,967,296 different color combinations (32- bit), you actually only get the same number (16,777,216) of discrete colors as if you are using 24-bit colors because many of the possible color combinations duplicate each other. For example, if the black channel is 255 (the maximum value), it doesn't matter what values the other three channels have; the color is black. In addition, you could describe a middle gray color by either of the following sets of values:

Cyan=128, Magenta=128, Yellow=128, Black=0

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Cyan=0, Magenta=0, Yellow=0, Black=128.

When most computer programs display a graphic that use CMYK, they first convert the image to CMY by adding the value of the black channel to each of the other three and then removing the black channel. This CMY can easily be converted to RGB.

Hi-Color
This method of storing colors in a graphic is similar to RGB. The only difference is that instead of each of the red, green, and blue channels being defined with an 8-bit number (0-255), each of the channels is defined with a 5-bit number (0-31). Alternatively, the red and blue channels can be defined by a 5-bit number, and the green channel can be defined by a 6-bit (0-63) number. Thus, this method of color storage is also called 15-bit or 16-bit.

While this method of storing colors lets you have more discrete colors within a graphic than you can have with a palettized graphic (32,768 or 65,536 instead of 256), there are fewer possible color choices available.

Computer graphics formats that use this color storage method are typically animation formats, such as the Microsoft Video for Windows (.avi) format.

HSL
This acronym stands for hue, saturation, and luminance. This method of describing colors is also known as HSB (hue, saturation, and brightness), HSI (hue, saturation, and intensity), or HSV (hue, saturation, and value).

The hue value describes the position on the spectrum where the color is located, with red at the low end of the spectrum and violet at the high end of the spectrum. This number can be either an 8-bit value (a number between 0-255), a percentage (0-100 percent), or a number between 0-359 (representing the degrees on a color wheel).

The saturation value describes how bright the color is, between gray at the low end and very bright at the high end. This number can be either an 8-bit value or a percentage.

The luminance (intensity or brightness) value describes where on the scale between black and white the color falls. This method of describing color is easy for many artists to use, and it is usually used only in the interface of a graphics program. Once the graphic is saved, it is converted to RGB, Palettized, or CMYK color.

The only time this color definition method is used natively is by color television, where it is referred to as YUV (Y-signal, U-signal, and V- signal.) The Y-signal represents the intensity, and is the only part of the signal a black-and-white television set uses. The U- and V-signals define a color spectrum that a color television uses to choose which color to display each pixel.

RGBA
This acronym stands for red, green, blue, alpha. This method of describing colors is similar to RGB, except that it provides an extra channel to indicate how transparent a color is.

In most cases, each of the four channels is a value between 0 and 255. While this provides up to 4,294,967,296 different possible color combinations (32-bit), you actually get only the same number (16,777,216) of discrete colors as if you are using 24-bit colors. This is because a semi-transparent pixel will show up as a combination of the foreground color of the graphic and its background color. This pixel will be displayed using the 24-bit color addressing scheme.

