The difference between RGB and CMYK in school color
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Advances in color management, digital photography, and color scanning have prompted new and old scanner operators to carefully consider when to color and when to color. Roller scanner operators use traditional methods to produce scanned images of yellow, magenta, cyan, and black, but today's new tools lead to the widespread adoption of new workflows—that is, scanning before color separation into CMYK. School color. This article describes the advantages of this approach and some background information on scanning, color correction, and color separation.
Both scanning and digital photography capture red, green, and blue information about the image, but various methods of image capture produce different amounts of information depending on their bit depth.
Although most scanners use 1-byte (8-bit) 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 amount of dark tones of individual pixels, resulting in a subtle description (mostly gray tones) between multiple colors and the maximum color of each channel. The number of bits used per channel is what we call the bit depth of a digital image.
For example, in an RGB mode with an 8-bit depth per channel, the scan or digital photo uses a total of 24 bits to describe the color of each pixel, which is called a 24-bit color, because 8 bits per channel, 3 The channel (red, green, blue) is a total of 24 bits per pixel location. 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 by 10 bits);
12 bits per channel (36-bit color);
16 bits per channel (48-bit color).
These additional bits of data when the image is enlarged after scanning or capturing are very useful 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 nearest equivalent amount of cyan, magenta, yellow, and black (CMYK) values. This is necessary for the general printing reproduction process because most printing equipment uses cyan, magenta, yellow subtractive primary colors and black (it is not a basic color). Black is used to compensate for the less desirable absorption characteristics of the printing ink (i.e., toner). The use of black expands the tonal range of the print, resulting in deeper, richer shades of darkness.
Separation depends on how much CMYK is required to accurately calculate the RGB scan. Traditionally, this is done by prepending an onboard computer attached to the drum scanner. For decades, these "high-end" scanners captured RGB data during the scan and converted it to CMYK data in "running state" (simultaneously scanning the image). In today's printing world, this color 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 a variety of different devices because color separation is performed for a particular print copy system. A document that is copied and copied for a lithographic press will not look the same when it is output to a color copier, even if both are CMYK output devices.
CMYK separations are specific to a single device for a number of reasons: First, each device has its unique gray balance and tone reproduction (including dot gain) characteristics. 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 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 layer they choose. The thicker the ink layer, the higher the density, which generally results in a more saturated appearance of the printed image. Increasing the thickness of the ink layer makes it difficult to maintain the desired ink balance. Some printers therefore prefer the separation of thinner ink layers to ensure consistent print quality throughout the printing process.
All of this effect on color separation is that images prepared for thick ink layer printing will require black reduction in dark areas, as the darkness of dark tones 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 (Gray Component Substitution).
Increased tonal value
The difference between CMYK images prepared for various print copy systems is increased in consideration of an increase in tone value (dot increase). Both the scanner and the press operator understand that the ink dots printed on the substrate produce an image that is much darker than the original digital data - an effect known as "dot increase."
In addition to factors such as paper surface and ink stickiness, each printer also plays a role in determining the dot gain of the printed image. Compensating the dot gain during the color separation process 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 increase in tonal value will make the image too dark or too bright, which will result in a color shift because the gray balance of highlights, midtones, and dark tones increases 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 specialists recognize that scanning and digital photography should be saved in RGB mode throughout the color correction and revision process, and converted to CMYK after all adjustments have been made. It is precisely because of these RGB data that has been corrected and corrected that the professional prepress department can record storage for a long time. This allows images retrieved from the archive memory to be used on a printer (or other copying system) that is different from the original output device. This emphasis on RGB image data has had a good impact 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.
Most importantly, the effects of copying the same image on a variety of presses, digital proofing devices, or computer monitors should be exactly the same. This is possible when performing separate color separation for each device. Because each copying 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 method of observing (and measuring) the difference in color reproduced by these devices is to measure the amount of cyan, magenta, and yellow required to produce neutral ash - a gray balance we call a replication system.
If the image has been color corrected or corrected after conversion to CMYK, then reusing the last image on a different output device requires adjusting the highlights, midtones, and shadows of the CMYK image and changing the overall gray balance and color saturation. The amount of black in the image is difficult to change without compromising the image quality, but if the black data is not corrected, the printed image will produce undesirable results.
For example, CMYK images that were originally color-separated for high-quality in-line dry sheet-fed presses can cause smudging if printed on a cold-set web press. The compromise is to fix any CMYK image used in a web page or CD-ROM electronic publication. RGB images can use a larger RGB tonal range to reproduce brighter, more saturated colors. However, after the image is color separated into CMYK, all pixels in the image are within the CMYK tonal range.
The trend in the entire printing industry to maintain RGB images has met some resistance from experienced scanner operators and color separation specialists. These old professionals learned the technique of color separation when using a row of knobs decorated with a scanner and the length of RGB image data can only drive the laser beam of the output roller. But they didn't hear about RGB image files for prepress until the customer started scanning on their cheap desktop CCD scanner. For departments with high-end color devices, RGB images are beginning to symbolize the threat of desktop scanners. As a result, some prepress technicians have linked RGB color correction to low quality image capture.
Almost ten years ago, Linotype-Hell (now Heidelberg Prepress) published its first LinoColor. The software program supports color correction of image data before image data is converted to CMYK.
CIE LAB mode
Lino Color also introduces most prepress workers to the 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 correction in CIE LAB mode, and then decomposes the data in CMYK mode.
The ICC-accepted color management workflow, promoted by Apple Computer's ColorSync software, attributes its roots to the LinoColor's RGB-CIELAB-CMYK workflow. Apple's software tool for color conversion (theColorSync color management model) is an approved LinoColor adaptation. A significant advantage of the CIELAB color space is that the image can be converted to CIELAB mode and then back to RGB without significant changes in image quality - although it is still a matter of debate as to how accurate the input or output CIELAB transforms the image. CIELAB contains all the colors visible to the naked eye, so hue, saturation and brightness can be adjusted to adapt the image to any tonal range or copy system.
CIELAB provides numerical position for any color that is visible to the naked eye based on three markers (L, A, and B). The value L represents the brightness of the color from light to dark. The marks A and B are simply the positions along the latitude (A) and the warp beam (B), drawn through a circular color space, and have no saturation at the center of the circular color space. Color saturation (also known as chromaticity) increases as the specified point moves away from the center of the circle. The described hue can be determined by moving around the circumference.
However, in order to utilize the color correction, saturation, and brightness (HSL) color correction methods, 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 based on the overall or specific base or inter-color. Fixed CMYK usage Photoshop users can find countermeasures through the Info palette and the View mouse: display the CMYK mode value of the image in real time before the image is separated. The palette can be adjusted to display the actual values obtained by RGB data separation. Similarly, CMYKPreview can be selected by the View mouse to separate the image information used to drive the monitor. Using these two tools, even high-end scanner operators will consider color correction in RGB mode and can simultaneously observe the results of CMYK values.
Color correction
The reason is conceptually simple: if a color cast can be found on an RGB image, the required adjustment is 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 correction is performed, the effect of the color cast is distributed among the four colors. In many cases, only the color casts of the two colors in the additive primary colors (such as the cyan color produced by a large amount of green and blue) are now distributed among all four colors of the CMYK image. It is easy to use Photoshop's Color Balance control to remove cyan from RGB images. In the case of entering the appropriate values for changing the highlight, midtone, and shadow values, the entire gray scale becomes neutral. If an attempt is made to perform the same cyan correction on the image after CMYK conversion, the remaining cyan color will remain in the gray scale.
Control dot size for highlights and shadows
Another important advantage of RGB color correction is that the user can control the size of the highlights and shadows. When the image is color corrected, the desired tone adjustment is performed to remove the hue that extends to the brightest and darkest portions of the image. Pay special attention to adjustments, otherwise the color correction will remove the highlights of the image or incorporate unwanted color casts into the dark portion. Some tonal correction methods are widely used because they are suitable for controlling a large number of highlights and shadows (such as Photoshop's Curves function).
Regardless of the color correction method used, choosing the right highlight or shadow point depends on the replication system used—it requires these dots to be properly sized to reflect the characteristics of the press, proofing device, or computer monitor used in the output. .
Today's system-level color management makes it easy to get the following two points: one is to get the appropriate minimum and maximum dots on the image; the other is to create a CMYK image that is especially suitable for the output device. The ColorSync user workflow is simple: create a dedicated 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 values). The ColorSync software then color-separates the image while making appropriate color adjustments, including arranging the appropriate highlight and shadow points, the device-specific gray balance, and the desired black version.
The flexibility of the situation just described is compared to the workflow for determining the minimum and maximum dot points of the CMYK image during the color correction process, and then the device-specific image is generated. If the image is definitely printed on a cold-set web press and this process is used, then if the on-line dry sheetfed press is re-intended, the image will not 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 overstated 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 applies to the compensation of dot gain (a combination of mechanical and optical effects that darken the image during print reproduction). The image reproduced on uncoated paper or white paper should be brighter, and the use of coated paper requires the image to be darkened to achieve the same effect. Unfortunately, making the image brighter compresses the tonal range. Adding a weighted value to a scanned or digital image (making the image darker) not only restores the original midtone dot value but also causes a subtle level of loss.
In conclusion
Do you not use paintboard and imposition with the desktop publishing system? No, not exactly. Similarly, there are always a few professionals who convert images to CMYK before performing color correction and then save the results.
More and more color separation departments recognize the main advantage of RGB - flexibility. By color balancing and saving RGB image data, users can create multiple CMYK images with their own gray balance characteristics, special black version and specified tonal range (including appropriate highlight and dark tones and dot gain compensation). RGB images saved by the archive can also be used for new media, including monitor-based content delivery.