(1) Adobe (RGB): In all RGB work color spaces supported by Photoshop, Adobe RGB (1998) is the largest color space in the color gamut and is suitable for color image reproduction processes that require a large range of color expressions. For color images that need to be converted to CMYK data;
(2) Apple RGB reflects the average characteristics of Macintosh computer monitors. A typical application example is an old Macintosh monitor with a diagonal size of 13 inches. It is used for the early development of Macintosh desktop publishing applications (including Photoshop 4.0 or earlier). Requires that the Apple RGB color space be selected when the processing results in the Windows environment are displayed on an Apple computer or used in the desktop publishing application environment of the earlier Apple computer;
(3) ColorMatch RGB: Early desktop publishing systems used a radiation-type display to simulate the effects of the four-color overprinting process on computer screens. The ColorMatch RGB color space was chosen to match the native color space of such displays. ColorMatch RGB has a smaller color representation than Adobe RGB (1998), but it is also suitable for printing workflows.
(4) sRGB: Called the standard RGB color space, which reflects the average characteristics of PC monitors. Supported by many hardware manufacturers and software developers, it is gradually becoming the default working color space for many scanners, low-end printers, and application software. It is recommended not to apply to prepress production workflows, but it is suitable for preparing color images for Web product design;
(5) ProPhoto RGB: equivalent to ROMM RGB sample files provided by Adobe, can provide a wide range of RGB color gamut, covering the entire color representation of photographic materials, such as photographic paper and photo quality printing paper, etc., especially suitable for Sublimation printers, photo-quality inkjet printers, and digital printing systems based on photographic imaging technology are also suitable for high-fidelity color printing applications.
Many people think that the gamut of sRGB color space is the largest among all RGB working color spaces. In fact, this is completely misunderstood. Figure 1 compares the Adobe RGB (1998), sRGB, and Lab color spaces. The gamut ranges of the two RGB working color spaces are clearly smaller than the Lab color space, and the sRGB space is smaller than the Adobe RGB (1998) space, and thus Adobe RGB (1998) uses The working color space for image processing and hard copy output is more appropriate.
3 Photoshop's built-in sample files
On many occasions, image processing workers faced with how to reasonably use the built-in sample files provided by Photoshop, or based on the built-in sample files, to create sample files that meet the goals and requirements of the copy, and then decide whether to embed the sample files or immediately switch to the target. Working color space. The built-in sample files are divided into input devices, display devices, and output devices according to the purpose of the device. The work color space is divided into two major categories of RGB and CMYK. Although grayscale and spot color work space can be set independently, most of the cases The next is associated with the CMYK space.
Scanners and digital cameras: Scanners and digital cameras all belong to the color-adding device, but Photoshop does not have a built-in digital camera sample file. The provided scanner sample file is quite limited, only a Flatbed Scanner, and has already been used by Photoshop. The designer has made a simplification process and the default gamma value is equal to 0.87. Therefore, if you want to use a scanner or digital camera sample file, you should create it yourself.
Device-independent RGB working color spaces: As already mentioned above, they are Adobe RGB (1998), Apple RGB, ColorMatch RGB, and sRGB IEC61966 2.1.
Other RGB sample files: Embedding the sample file is equivalent to selecting or setting the RGB working color space, where selecting the RGB working color space is to select Photoshop's built-in sample file, and setting the RGB working color space should be measured in advance with the display color description feature. The relevant parameters such as the gamma value, the white point (corresponding to the color temperature of the standard illuminant), and the chromaticity values ​​corresponding to white, red, green, and blue, and the like. The color-adding device sample files provided by Photoshop include a variety of displays in addition to scanners.
CMYK sample files: Photoshop offers not only standard sample files for sheetfed and web printing in North America, Europe, and Japan, but also the default CMYK parameters used in versions 4.0 and 5.0. It also allows setting of four-color overprints through the Custom CMYK dialog box. Parameters to generate user-defined CMYK sample files.
Another notable concept is the built-in sample file parameter set, which means that each type of Photoshop built-in sample file contains a set of parameters that are the basis for modifying the color data when the color image is displayed or exported. Take Photoshop 4 Default CMYK (which means the CMYK setting parameter set used by Photoshop version 4.0 by default) as an example. The printing standard used is SWOP (Coated), that is, North American web offset standard and coated paper printing, and the cyan dot enlargement value is 16%. The other three-color dot gain values ​​are equal to 12%. The color separation type is gray component substitution. The medium black color generation function limits the total amount of black ink and four-color ink to 100% and 300%, respectively.
4 Sample File Embedding and Validation
In the image processing phase, the method of validating the sample file depends on whether the current image contains the sample file and whether the included sample file can reflect the color description characteristics of the target device, thereby generating two methods for validating the sample file, ie, When an image does not contain a sample file, you need to specify the ICC sample file or directly convert to the target color space and save it as an image format that supports embedded sample files, such as TIFF, JPEG, EPS, and PDF; if the included sample file does not reflect the color of the device Describe the characteristics, you must use the method of converting the sample file.
(1) Designation
The Assign Profile subcommand in the Mode command group of the Image menu is used to implement the Assign Profile dialog box. There are three options after opening the Assign Profile dialog box. The first item is to not use color management technology so that no sample files need to be embedded; the second option is per user. The current RGB working color space specified by Photoshop is embedded in the sample file, so the current RGB working color space appears behind Working RGB; the third is to specify a new RGB working color space for image processing. To do this, select one from the Profile list. Suitable sample documents.
Specifying a sample file for an image does not mean that the sample file is actually embedded. For this purpose, the current image must be saved to support the image format that contains the sample file, and the ICC Profile entry in the Save As dialog box must be approved. As long as the selected image format supports embedded sample files, the ICC Profile item will be activated.
(2) Conversion
If the current working color space does not match the color description feature of the target display, or if a CMYK sample file needs to be specified for a four-color overprint process (including digital proofing and digital printing), the Convert to Profile command in the Mode command group of the Image menu can be used. The command is implemented. An interface different from the Assign Profilie dialog will appear after opening the Convert to Profile dialog, where Source Space indicates the current working color space of the color image, and the Distination Space is used to set the color conversion target. Color space is divided into two kinds of work goals.
1RGB target color space
The Generic Monitor D93 WP 1.8 Gamma sample file can be selected in accordance with the target display environment and device designation. For example, when the color image is to be displayed on the standard monitor D93 (standard color temperature Kelvin 9300) and gamma = 1.8 general-purpose monitor screens. Modern television communication technology and equipment production has seen a gradual convergence of TV and computer fields. When the color image display environment is
For PAL/SECAM color TV screens, PAL/SECAM sample files should be selected from the list of built-in sample files. At this time, the gamma value is equal to 2.2. The screen display feature is designed according to the D65 standard illumination body.
2CMYK target color space
Suitable for color images using a four-color overprint process, can be selected from Photoshop's built-in North American, European or Japanese standard sample files, or set CMYK copy parameters based on these standard sample files, you can also use your own measured printer, digital Proofing equipment, printing press or digital printing machine sample files.
Source: "Printing World" Author: Yao Hagen
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