Several Color Modes of Digital Images (5)

These additional bits of data are very useful when the image is enlarged after scanning or capturing because the additional bit depth is suitable for better interpolation.

Color separation

The so-called color separation refers to a process in which RGB image data is converted to the closest equivalent cyan, magenta, yellow, and black (CMYK) values. This is very necessary for a general print reproduction process because most printing apparatuses use cyan, magenta, yellow subtractive primary colors and black (it is not a basic color). Black is used to compensate for less-than-ideal absorption characteristics of printing inks (ie, colorants). The use of black expands the printed tonal range, resulting in a darker, richer dark shade.

Separation depends on how much CMYK is needed for accurate calculations to get close to an RGB scan. Traditionally, this is done by pre-installing an on-board computer attached to a roller scanner. For decades, these "high-end" scanners have captured RGB data during the scan and converted it to "CMYK data" while "running" (while scanning the image). In today's printing industry, this separation method is rapidly being replaced by a workflow that captures RGB data and stores it as RGB on disk. Separation and conversion to CMYK is done at a later time using software or any software program that can connect to a digital camera.

However, both color separation methods severely limit the flexibility of outputting the same color separation data to various different devices because color separation is performed specifically for a particular printing reproduction system. A document that is color-coded for lithographic printing will not look the same when it is output to a color copier, even if both are CMYK output devices.

CMYK color separation is specific to a single device for a variety of reasons: First, each device has its unique gray balance and tone reproduction (including dot gain) characteristics. In addition, the operator setting the color separation control can change the amount of black during the conversion from RGB to CMYK.

Black version information

As previously mentioned, the amount of black required to produce an approximate tonal range depends primarily on the light absorption characteristics of the printing ink used. The user's choice of substrate is also part of this factor. However, skilled printer operators can also change the thickness of the ink they choose. The thicker the ink layer, the higher the density and generally results in a more saturated appearance of the printed image. Increasing the thickness of the ink layer can make it difficult to maintain the desired ink-water balance. Some printers therefore prefer the separation of thinner ink prints to ensure consistent print quality throughout the print process.

The effect of all this on color separation is that an image prepared for thick ink layer printing will require a reduction in black in the dark tone area because dark shades can be produced by printing a high percentage of cyan, magenta, and yellow inks. The color separation process for determining the amount of black information in the color separation includes UCR (Background Removal) and GCR (Grey Component Substitution).

Increased tone value

When considering the increase in the hue value (the dot gain), the difference between the CMYK images prepared for various print copying systems is increased. Scanners and printer operators understand that ink dots printed on substrates produce images that are much darker than the original digital data - an effect known as dot gain.

In addition to factors such as the surface of the paper and the viscosity of the ink, each printer also plays a role in determining the dot enlargement of the printed image. Compensating for dot gains during color separation means that the darkening that occurs during printing can be offset, making the image brighter when converted to CMYK.

Moving an image from one printing state to another without compensating for changes in the tonal value will make the image too dark or too bright, which will cause color shifts because the gray balance of highlights, midtones, and shadows increases the dot gain. Big plays a different role.

Use RGB and CMYK image data

Few modern prepress departments are aware of the importance of RGB image data. These imaging professionals recognized that scanning and digital photography should be saved in RGB mode throughout the color correction and revision process, and after all adjustments were completed, the conversion to CMYK. Because of these corrected and corrected RGB data, the professional prepress department can store files for a long period of time. This allows images retrieved from archive storage to be used on a printer (or other replication system) different from the original output device. This emphasis on RGB image data has had a good effect in many publishing workflows, whether the color separation method is a system-level color management method or an image batch conversion method in Photoshop using predetermined Actions.

The most important thing is that the effect of copying the same image on a variety of printers, digital proofing devices, or computer monitors should be exactly the same. This is possible when separate separations are made for each device. Because each replication system requires slightly different mixes of cyan, magenta, yellow, and black to produce a similar appearance, separate color separations make the images look the same on different devices.

The way to observe (and measure) the color differences replicated by these devices is to measure the amount of cyan, magenta, and yellow needed to produce neutral ash—a kind of gray balance we call the replication system.

If the image has been color corrected or corrected after conversion to CMYK, reusing the last image on a different output device requires adjusting the highlight, midtone, and dark tone points of the CMYK image and changing the overall gray balance and color saturation. It is difficult to change the amount of black in the image without impairing the image quality, but printing the image without correcting the black data may cause undesirable results.

For example, CMYK images originally color-separated for high-quality, on-line, dry sheet-fed presses can cause smearing if printed on a coldset web press. The compromise is to correct any CMYK image used in web pages or CD-ROM electronic publications. RGB images can use a larger RGB tone range to reproduce brighter, more saturated colors. However, after the image is separated into CMYK, all the pixels in the image are within the CMYK tone range.

The development trend of the entire printing industry filing RGB images has encountered some resistance from experienced scanner operators and color separation specialists. These veteran professionals learned color separation techniques when scanners using arranging rows of knobs and RGB image data were only able to drive the output drum's laser beam. But they did not hear RGB image files used for prepress until the customer began scanning on their cheap desktop CCD scanner. For departments with high-end color devices, RGB images begin to symbolize desktop scanners as a threat. As a result, some prepress technicians linked RGB color correction to low quality image capture.

Almost a decade ago, Linotype-Hell (now HeidelbergPrepress) published its first LinoColor. This software program supports the color correction of image data before the image data is converted to CMYK.

CIE LAB mode

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 through 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 the image quality - although the accuracy of the input or output CIELAB image is still a matter of debate. 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 by color separation of RGB data. 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 highlight, midtone, and shadow values ​​and enter the appropriate value, 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 users 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 right highlight or dark spot depends on the copying system used. It requires that the dot size 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 it easy to get the following two points: First, to obtain appropriate minimum and maximum dots on the image; and second, to produce CMYK images that are particularly suited to output devices 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 for 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 color of the three primary colors.