The difference between RGB and CMYK in school color (below)

Lino Color also introduced most prepress workers to CIE LAB color space - neither RGB nor CMYK. The Lino Color workflow developed by Commission International edel'Eclairage captures RGB image data, performs color correction and corrections in CIE LAB mode, and then decomposes the data in CMYK mode.

The ICC-approved color management workflow promoted by Apple Computer's ColorSync software attributes its roots to the LinoColor'sRGB-CIELAB-CMYK workflow. Apple's software tool for color conversion (theColorSync color management model) is the approved LinoColor adaptation. The significant advantage of the CIELAB color space is that the image can be converted to CIELAB mode and then back to RGB without any significant change in image quality - although the exact size of the input or output CIELAB image is still a contentious issue. CIELAB contains all the colors that are visible to the naked eye, so the hue, saturation, and brightness are adjustable so that the image can adapt to any hue range or reproduction system.

CIELAB can provide numerical positions for any color based on the naked eye based on the three markers (L, A, and B). The value L indicates the brightness of the color from light to dark. The signs A and B are merely the positions along the weft axis (A) and the warp axis (B), which are drawn through a circular color space and have no saturation in the center of the circular color space. Color saturation (also known as chromaticity) increases when the specified point moves away from the center of the circle. Moving around the circle determines the hue being described.

However, in order to use the color correction method of hue, saturation, and brightness (HSL), it is not necessary to convert the image to CIELAB. Professional image editing programs (including Adobe Photoshop and LinoColor) enable RGB mode images to be calibrated by adjusting HSL values, including HSL values ​​from the overall or specific base color or inter-color. Using CMYK's Fixed Photoshop Users can find countermeasures through the Info palette and the View mouse: The CMYK mode value of the image is displayed in real time before the image is color-separated. The palette can be adjusted to show the actual value obtained from the RGB data separation. Similarly, selecting CMYKPreview by the View mouse can colorize the image information used to drive the monitor. Using these two tools, even high-end scanner operators will think that color calibration in RGB mode is feasible, and the results of CMYK value display can be observed at the same time.

Color cast correction

The reason is conceptually very simple: if color casts can be found on an RGB image, the required adjustments are very simple and change the overall tonal range of the image in a balanced manner. However, if you wait until the image is color-separated and the same color correction is performed, the effect of color cast will be distributed among the four colors. In many cases, only the color casts of the two colors in the three primary colors of the additive method (such as the partial cyan due to excessive green and blue colors) are now distributed in all four colors of the CMYK image. It is easy to use Photoshop's Color Balance control to remove the cyan in the RGB image. In order to change the highlights, midtones, and shadows by entering the appropriate values, the entire gray scale becomes neutral. If you try to perform the same bias correction on the image after the CMYK conversion, the residual part of the cyan will remain in the gray scale.

Control the spot size of highlights and shadows

Another important advantage of RGB color correction is that the user can control the size of highlights and dark spots. When the image is color corrected, the desired hue adjustment is performed to remove the hue that extends to the brightest and darkest portions of the image. Pay special attention when adjusting, otherwise the color correction will remove the highlights of the image, or incorporate the unwanted color cast into the dark portion. Some tone correction methods are widely used because they are suitable for controlling a large number of highlights and dark spots (such as Photoshop's Curves function).
No matter what color correction method is used, choosing the correct highlight or dark spot depends on the copying system used. It requires that these dot sizes must be adjusted correctly to reflect the characteristics of the printer, proofing device, or computer monitor used for output. .

Today's system-level color management makes the following two points easy: one is to obtain the appropriate minimum and maximum dots on the image, and the second is to generate CMYK images that are particularly suited to the output device for gray balance. The ColorSync user workflow is simple: Make a special profile file for each output device and provide a color balanced RGB image as input. Each RGB image should have a consistent minimum and maximum density (ie, RGB value). The ColorSync software then performs color separations on the images while performing appropriate color adjustments, including arranging appropriate highlights and shadows, device-specific gray balance, and the type of blackplate that is required.

The flexibility of the situation just described is compared with the workflow of determining the minimum and maximum dots of the CMYK image during the color calibration process, and then a device-specific image is generated. If the image is definitely printed on a coldset web press and this process is used, then if the on-line dry sheetfed press is reintroduced, the image cannot reach its highest quality. Adjusting the highlights and shadows of the image to cover the increased tonal range does not increase the number of gray levels captured by the image itself. Of course, when CMYK images are used for electronic delivery (Web pages, CD-ROMs, FDF files), this problem is exaggerated because the range of colors obtained from the RGB monitor greatly exceeds the tonal range of the three primary colors.

Tone range adjustment

The same argument also applies to compensated dot gain (a combination of mechanical and optical effects that darken the image during print copy). The brightness of images reproduced on non-coated paper or white paper is increased, and the use of coated paper requires the image to be darkened in order to achieve the same effect. Unfortunately, brightening the image compresses the tonal range. Adding a weighted value to a scanned or digital image (darkening the image) not only restores the original midtone dot value but can also result in subtle levels of loss.

in conclusion

With a desktop publishing system, do not use coated cardboard and make-ups? No, not exactly. Similarly, there are always a few professionals who convert an image to CMYK before it is color corrected and then archive the result.

More and more color separation departments recognize the main advantage of RGB - flexibility. By color balancing and archiving RGB image data, users can freely create multiple CMYK images with their own gray balance characteristics, special blackplates, and a specified tonal range (including appropriate highlights and dark tones and dot gain compensation). Archived saved RGB images can also be used for new media, including monitor-based content delivery.

Source: Ke Yin Network