The difference between RGB and CMYK in color correction
Advances in color management, digital photography, and color scanning have prompted operators of new and old scanners to carefully consider when to perform color correction and when to perform color separation. Drum scanner operators use traditional methods to produce scanned images composed of yellow, magenta, cyan, and black, but today ’s new tools have led to widespread adoption of new workflows—that is, scanning and color separation before CMYK. School color. This article describes the advantages of this method and some background knowledge about scanning, color correction and color separation.
Both scanning and digital photography capture red, green, and blue information about an image, but various image capture methods produce different amounts of information depending on their bit depth.
Although most scanners use 1 byte (8 bits) of information in each color channel, it has become increasingly common for scanners and digital cameras to use more than 8 bits of bytes to describe each basic color. These additional bits are used to capture the large number of dark tones of each pixel, resulting in a subtle description between the multi-color and the maximum color of each channel (mostly gray tones). The number of bits used in each channel is what we call the bit depth of the digital image.
For example, in an RGB mode with an 8-bit bit depth per channel, scans or digital photos use a total of 24 bits to describe the color of each pixel, which is called 24-bit color, because each channel counts 8 The channels (red, green, and blue) are 24 bits per pixel. Other common configurations for capturing RGB data include:
10 bits per channel (also known as 30-bit color, because there are 3 channels in total according to 10 bits);
12 bits per channel (36-bit color);
16 bits per channel (48-bit color).
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 the process of converting RGB image data to the nearest equivalent of cyan, magenta, yellow, and black (CMYK) values. This is very necessary for the general printing and copying process, because most printing equipment uses the cyan, magenta, and yellow subtractive primary colors and black (it is not a basic color). Use black to compensate for the less ideal absorption characteristics of printing inks (ie toners). The use of black expands the tonal range of printing, resulting in deeper and richer dark tones.
Color separation depends on accurate calculation of how much CMYK is required to approach RGB scanning. Traditionally, this is done by presetting the on-board computer attached to the drum scanner. For decades, these "high-end" scanners captured RGB data during the scanning process and converted it to CMYK data in the "running state" (simultaneously scanning images). In today's printing field, this color separation method is rapidly being replaced by a workflow that captures RGB data and stores it as RGB on disk. Color separation and conversion to CMYK are 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 devices because color separation is specifically designed for specific printing and copying systems. When a color-separated document is copied to a color copier when it is copied for a lithographic printing machine, even if both are CMYK output devices, it will not look the same.
CMYK color separation is specific to a single device for a variety of reasons: First, each device has its unique characteristics of gray balance and color reproduction (including dot gain). In addition, the operator who sets the color separation control can change the amount of black during the conversion from RGB to CMYK.
Black version information
As mentioned earlier, the amount of black required to produce an approximate tonal range depends mainly 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 ink layer thickness they choose. The thicker the ink layer, the higher the density, and generally the printed image will have a more saturated appearance. Increasing the thickness of the ink layer will make it difficult to maintain the ideal water-ink balance. Some printing houses therefore prefer the separation of printed matter with thinner ink layers to ensure consistent print quality throughout the printing process.
The effect of all this on color separation is that images prepared for thick ink layer printing will require reducing black in dark areas, because the darkness of dark tones can be produced by printing a high percentage of cyan, magenta, and yellow inks. The color separation process that determines the amount of black version information in color separation includes UCR (Underground Color Removal) and GCR (Gray Component Replacement).
Tonal value increases
When considering the increase in tone value (increase in dots), the difference between CMYK images prepared for various printing and copying systems is increased. Operators of scanners and printers understand that the ink dots printed on the substrate produce images that are much darker than the original digital data-an effect called "dot increase".
In addition to factors such as paper surface and ink viscosity, each printing machine also plays a role in determining the amount of dot increase in the printed image. During the color separation process, the compensation dots increase, which means that the darkening phenomenon that occurs during printing can be offset, and the image becomes brighter when converted to CMYK.
Moving an image from one printing state to another without compensating for changes in the increase in tone value will make the image too dark or too bright, which will cause color shifts because the gray balance of highlights, midtones, and dark tones increases It plays a different role.
Use RGB and CMYK image data
Few modern prepress departments realize the importance of RGB image data. These imaging professional organizations recognize that scanning and digital photography should be saved in RGB mode throughout the color correction and revision process, and after all adjustments are completed, convert to CMYK. Because of these RGB data after color correction and correction, professional pre-press department can archive and store for a long time. This allows images retrieved from the archive memory to be used on printers (or other copy systems) that are different from the original output device. This emphasis on RGB image data has had a good impact on many publishing workflows, regardless of whether the color separation method uses the system-level color management method or the image batch conversion method in Photoshop using predetermined Actions.
The most important thing is that the effects of copying the same image on various printing machines, digital proofing equipment or computer monitors should be strictly the same. This is possible when performing separate color separations for each device. Because each replication system requires a slightly different blend of cyan, magenta, yellow, and black to produce a similar appearance, separate color separations make the image look the same on different devices.
The way to observe (and measure) the difference in color reproduced by these devices is to measure the amount of cyan, magenta, and yellow needed to produce neutral gray—a gray balance that we call the replication system.
If the image has been corrected or corrected after conversion to CMYK, then reusing the final image on a different output device requires adjusting the highlight, midtone, and dark dots of the CMYK image and changing the overall gray balance and color saturation. It is difficult to change the amount of black in an image without compromising the image quality, but printing the image without correcting the black data will produce undesirable results.
For example, a CMYK image originally separated for a high-quality on-line drying sheet-fed printing press would cause smudging if printed on a cold-set web printing press. The compromise is to correct any CMYK images used in web pages or CD-ROM electronic publications. RGB images can use a larger range of RGB tones to reproduce brighter, more saturated colors. However, after the image is separated into CMYK, all pixels in the image are within the CMYK tone range.
The development trend of archiving and storing RGB images throughout the printing industry has encountered some resistance from experienced scanner operators and color separation professionals. These old professionals learned the technique of color separation when using scanners decorated with rows of knobs and the length of RGB image data can only drive the laser beam of the output drum. But they didn't hear that RGB image files were used for pre-press until customers started scanning on their cheap desktop CCD scanners. For departments with high-end color equipment, RGB images are beginning to symbolize that desktop scanners are a threat. As a result, some prepress technicians associate RGB color correction with low-quality image capture.
Almost a decade ago, Linotype? Hell (now Heidelberg Prepress) published its first LinoColor. The software program supports color correction of image data before the image data is converted to CMYK.
CIE LAB mode
Lino Color also introduced most prepress workers to the CIE LAB color space-neither RGB nor CMYK. The Lino Color workflow developed by Commission International edel'Eclairage is to capture RGB image data, perform color correction and correction in CIE LAB mode, and then decompose the data in CMYK mode.
The color management workflow approved by ICC through Apple Computer's ColorSync software attributed its roots to the LinoColor'sRGB-CIELAB-CMYK workflow. Apple's software tool for color transformation (theColorSync color management model) is an approved adaptation of LinoColor. The significant advantage of the CIELAB color space is that the image can be converted to CIELAB mode and then back to RGB without significant change in image quality—although the accuracy of input or output CIELAB transformed images is still a matter of debate. CIELAB contains all colors visible to the naked eye, so hue, saturation, and brightness can be adjusted to adapt the image to any tonal range or replication system.
CIELAB can provide numerical positions for any color that is visible to the naked eye based on the three signs (L, A, and B). The value L indicates the brightness of the color from light to dark. The signs A and B are just drawn along a latitude axis (A) and a longitude axis (B) through a circular color space, and there is no saturation in the center of the circular color space. When the specified point moves away from the center of the circle, the color saturation (also called chromaticity) increases. Move around the circle to determine the hue described.
However, in order to utilize 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 adjust colors by adjusting HSL values, including HSL values ​​based on the overall or specific basic colors or intermediate colors. The fixed Photoshop users of the used CMYK can find a countermeasure through the Info palette and View mouse: display the CMYK mode value of the image in real time before separating the image. The color palette can be adjusted to display the actual value obtained by RGB color separation. Similarly, selecting CMYKPreview by the View mouse can separate the image information used to drive the monitor. Using these two tools, even high-end scanner operators will think that it is feasible to perform color calibration in RGB mode, and can observe the results of CMYK value display at the same time.
Correction of color cast
Conceptually, the reason is very simple: if a color cast can be found on an RGB image, the required adjustment is very simple and changes the entire tonal range of the image in a balanced manner. However, if you wait until the image is color-separated and the same color is corrected, the effect of color cast will be distributed among the four colors. In many cases, only the color shift of the two colors of the additive primary colors (such as the cyan color due to too much green and blue) is now distributed among all four colors of the CMYK image. It is easy to use Photoshop's Color Balance control to remove the cyan color in the RGB image. When you enter appropriate values ​​to change the highlight, midtone, and dark tone values, the entire gray scale becomes neutral. If you try to perform the same cyan correction on the image after CMYK conversion, the cyan residue will remain in the gray scale.
Control the dot size of highlights and darks
Another important advantage of RGB color correction is that users can control the size of highlights and dark dots. When the image is color corrected, the required tone adjustment is performed to remove the tone that extends to the brightest and darkest parts of the image. Pay special attention when adjusting, otherwise the color correction will remove the highlights of the image, or add unwanted color casts to the dark parts. Some color tone correction methods are widely used because they are suitable for mass control of highlights and dark dots (such as Photoshop's Curves function).
Regardless of the color correction method used, the selection of the correct highlight or dark tone dots depends on the replication system used-it requires that these dot sizes must be adjusted correctly to reflect the characteristics of the printing press, proofing equipment 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; the second is to generate a CMYK image with gray balance that is particularly suitable for the output device. The ColorSync user workflow is very 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). Then, ColorSync software performs color separation on the image while making appropriate color adjustments, including arranging the appropriate highlight and dark tone dots, the device-specific gray balance, and the type of black plate required.
Compare the flexibility of the situation just described with the workflow of determining the minimum and maximum dots of the CMYK image during color correction, and then generate a device-specific image. If the image must be printed on a cold-set web press and this process is used, then if the on-line drying sheet-fed printing press is revisited, the image cannot reach its highest quality. Adjusting the highlight and dark dots of the image to cover the increased tonal range still 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 color range obtained from the RGB monitor greatly exceeds the tonal range of the three primary colors.
Tonal range adjustment
The same argument applies to compensate for dot gain (the combination of mechanical and optical effects that darken the image during the printing and copying process). The brightness of the image copied on uncoated paper or white newspaper should be increased, and the use of coated paper requires that the image be darkened to achieve the same effect. Unfortunately, brightening the image compresses the tonal range. Adding the weighted value to the scanned or digital image (to darken the image) can not only restore the original midtone dot value but also cause the loss of subtle levels.
in conclusion
With the desktop publishing system, wouldn't paint board and imposition be used? No, not exactly. Similarly, there are always a few professionals who convert images to CMYK before performing color correction and then archive the results.
More and more color separation departments recognize the main advantage of RGB-flexibility. Through color balance and file storage of RGB image data, users can freely create multiple CMYK images with their respective gray balance characteristics, special black version and specified tonal range (including appropriate highlight and dark tones and dot gain compensation). RGB images saved in archives can also be used for new media, including content delivery based on monitors.
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