Several Color Modes of Digital Images (3)

(B) Lab Model

Lab mode uses the Lab color model. In Photoshop's Lab mode (with the asterisk removed from the name), the psychological brightness component (L) can range from 0 to 100, and the a component (green-red axis) and b component (blue-yellow axis) can range from -120. To +120. The Lab image is a three-channel image that contains 24 (8×3) bits/pixel.

You can use the Lab mode to process Photo CD images, edit the brightness and color values ​​in images individually, transfer images between systems, and print to Postscript® Level 2 and Level 3 printers. To print Lab images to other color Postscript devices, first convert them to CMYK.

Lab colors are internal color modes used by Photoshop when converting between different color modes.

(c) CMYK model

The CMYK mode uses the CMYK color model. In Photoshop's CMYK mode, each print ink for each pixel is assigned a percentage value. The brightest (highlight) color distribution has a lower percentage of printing ink color, and the darker (dark) color assigns a higher percentage value. For example, bright red may contain 2% cyan, 93% magenta, 90% yellow, and 0% black. In a CMYK image, pure white is produced when the values ​​of all four components are 0%.

To print a created image, use CMYK mode to convert RGB, index color, or Lab image to CMYK image. You can also use the CMYK mode to directly process CMYK images scanned or entered from a high-end system. The CMYK image consists of four colors for printing separations. They are four-channel images containing 32 (8 x 4) bits/pixel.

● Note the following when converting images to CMYK mode:

● Always store a backup of RGB or indexed color images in case you want to reconvert the image.

● When converting from one mode to another, Photoshop uses the Lab color mode, which provides a system for defining color values ​​in all modes. Using Lab will ensure that the color does not change significantly during the conversion process.

For example, when converting an RGB image to CMYK, Photoshop uses the information in the RGB Settings dialog box to first convert the RGB color value to Lab mode. After the image is in CMYK mode, Photoshop converts the CMYK value back to RGB and displays the image on an RGB display.

● The conversion of CMYK to RGB on the screen does not affect the actual data in the file. The conversion is done on the backup of the data.

● Although it is possible to do all the RGB and CMYK modes

Hue and color correction, but still should be carefully selected. Whenever possible, you should avoid multiple conversions between different modes. Because each conversion, the color value requires recalculation, will be lost and lost. If the RGB image is to be used on the screen, do not convert it to CMYK mode. Conversely, if CMYK scans are to be color-separated and printed, do not perform corrections in RGB mode. However, if you have to convert an image from one mode to another, you should perform most of the hue and color corrections in RGB mode and use CMYK mode for fine-tuning.

● In RGB mode, you can use the "CMYK Preview" command to simulate the effect of the change without actually changing the image data.

● For some types of separations, it is still necessary to work in RGB mode. For example, if you use the "Maximum Generation" option in the "CMYK Settings" dialog box to separate an image, even if it is feasible, any correction that requires a large increase in C, M, or Y components will be very difficult . To make these changes, you must reconvert the image to RGB, correct the color, and then re-distinguish the image—otherwise you have to rediscover the image with fewer “black generation” options.

2, overflow color

The color gamut is the range of colors that a color system can display or print. The color spectrum seen by the human eye is wider than the color gamut in any color model. In the color model used by Photoshop, Lab has the widest color gamut and includes all colors in the RGB and CMYK color spaces. Typically, the RGB gamut contains all the colors that can be displayed on a computer monitor or TV screen (they emit red, green, and blue light), as shown in Figure 5-57. Therefore, some colors such as pure blue or pure yellow cannot be accurately displayed on the display.

CMYK has a narrow color gamut and only includes colors that can be printed using printing (printing) inks. When colors that cannot be printed are displayed on the screen, they are called out-of-gamut—that is, outside the CMYK color gamut.

In the Photoshop Info palette, if you move the pointer over the over color, an exclamation mark appears next to the CMYK value. When an overflow color is selected, a warning triangle appears in both the Color Picker and the Color palette and displays the closest CMYK equivalent.