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Color SystemsFor color reproduction in a DTP environment, three different systems are used to describe color. The additive and subtractive systems describe a color's relative composition of three primary colors. The third system, the CIE system, is an absolute system in which a particular color is assigned three numerical coordinates in a color model.
The additive color system uses combinations of red, green and blue to produce all the colors of the spectrum. (Equal parts of red, green and blue light create white light.) This is the system that is used for scanning an image and reproducing it on a monitor. The intensity of minute red, green and blue dots on the monitor is varied electronically to build up an image. The additive color system also represents a good method for measuring how the eye sees colors. It is usually called the RGB model, taken from the first letters of each primary color.
The subtractive color system uses cyan, magenta and yellow. Mixing these colors in near-equal parts creates black. This system is the foundation of the art of printing. Cyan, magenta and yellow are called process colors. In Europe, they are defined as the Euroscale. In the U.S.A., they are defined according to SWOP standards. These two definitions are somewhat different, especially regarding cyan.
The most serious fault of the subtractive system is that 100% of cyan, magenta and yellow do not produce a true, solid black but rather a dark brown. For this reason, black is added in practice, and the system is thus called CMYK (K stands for Key Color).
In both the additive and the subtractive color systems, the numerical definitions of the colors depend on the properties and appearances of the primary colors used. For example, a particular color that is composed of 20% red, 30% green and 50% blue will appear somewhat different depending on how the red, green and blue primaries actually look. Such a difference can be found, for instance, between two different monitor screens.
It is advantageous to have a color system with primaries that are standardized and fixed. Such a standardized descriptive color system is the CIEXYZ system. This is a system in which the three basic colors (red, green and blue) have been systematically chosen and assigned special coordinates. They therefore constitute fixed points in the system.
In a CIE system, all pure hues are plotted along a curve in the x-y plane. Perpendicular to this surface is a Y-axis representing lightness of color, with greater lightness toward the top.
A variation of the CIEXYZ system is called CIELAB. It is designed to be perceptually uniform-that is, equal movements within the color space are perceived by the eye as equal differences in color. This is not the case with the original CIEXYZ system.
In both the RGB system and the CMYK system, there are limitations as to what colors can be reproduced. Unfortunately, the reproducible colors vary for the two systems. This causes difficulties when a color image is to be converted from one system to the other. Some colors simply cannot be reproduced in both systems and must therefore be approximated by some adjacent color.
If a CIE system is used as a basis for all conversion calculations, it is possible to take the various properties of (for example) the scanner, monitor, color printer and process inks into consideration. Examples of such properties are the type of phosphor used in the monitor, the hue error (or contaminant color) of the process inks and the density of the printer's colors.
In order for the colors displayed on the screen to match the printed colors as closely as possible, it is important that all the components of the system be calibrated and finely tuned....
Table of Contents
- Color Systems
- Tonal Range
- Original Classification
- File Formats
- Tone Correction
- Color Correction
- Image Manipulation
- Tonal Value Changes
- Sharpening Filter
- Image Compression
- Questions to Ask the Printer
- Output Specifications
- Calibration, Step by Step
- Scanning, Step by Step
- Photo CD, Step by Step
- Tone Correction, Step by Step
- Color Correction, Step by Step
- Sharpening, Step by Step
- Separation, Step by Step
- Background Stripping with Pen
- Background Stripping with Mask
- Wirephoto, Step by Step
- Photoshop Memory Tips
Whether or not you have read the earlier version of Photoshop in 4 Colors (previously entitled Four Colors/One Image), you will find the current volume full of useful information about modern image processing.
The focus of this book is on producing highquality color output in a desktop publishing environment. I describe the basics of color theory and show how to utilize them in Photoshop, QuarkXPress and PageMaker. I also include directions for proceeding step-by-step through everyday processing tasks.
I hope that all who seek knowledge of electronic photo reproduction will find it here.