In the typesetting process of newspapers, the processing of color pictures is often encountered. When a certain color picture is opened, it may be RGB mode or CMYK mode. So when using Photoshop, do you use RGB mode or use CMYK mode for color image processing? This article talks about the author's opinion on this issue.
In the process of using Photoshop to process pictures, the first thing to note is that for an opened picture, whether it is a picture of CMYK mode or an image of RGB mode, do not transfer between these two modes. Both modes are turning around. Because, in the dot matrix picture editing software, every time the picture color space is converted, the original picture details will be lost. If you turn a picture into an RGB mode for a while and turn it into a CMYK mode, the loss of information in the picture will be great. It should be noted here that the picture mode used for platemaking in the publication of color newspapers must be a CMYK mode picture, otherwise it will not be possible to print. However, it does not mean that the printing effect of processing pictures in CMYK mode during image processing will be very good, or it should be determined according to the situation. Its practical Photoshop processing picture selection RGB mode effect is stronger than the use of CMYK mode effect, as long as the RGB mode after processing the good picture, and then convert it to CMYK mode picture output film can be plated and printed.
In the image processing, if the picture opened for processing is an RGB mode picture or the original picture is allowed to select the RGB mode for scanning during the use of the scanner input process, this is a better situation for the color newspaper layout. . When using Photoshop to scan the original picture, just select the RGB setting option in the color setting option in the file menu bar. The color picture input through the scanner is the picture in RGB mode. In short, without first converting the picture mode, it is possible to obtain pictures in the RGB mode and use this mode to process the pictures. In particular, pictures downloaded from the Internet must be printed in order to ensure the printing effect of the pictures. Use RGB mode for processing. From the following aspects of the statement to illustrate this point.
1. RGB mode is the color mode adopted by all devices based on optical principles. For example, monitors work in RGB mode. The color range of the RGB mode is larger than the CMYK mode. Therefore, the RGB mode can express many colors, especially bright and bright colors (of course, the color of the display must be corrected so that no picture color distortion occurs). This color is difficult to print when printing. This is also the main reason why the picture becomes dark when converting the picture color mode from RGB to CMYK. When editing images in RGB mode in Photoshop, you must first select the CMYK Preview command in the View menu (if the Photo shop is in Chinese, select the preview option in the View menu bar, select the CMYK option), and That is, using RGB mode to edit and process pictures, and displaying pictures in CMYK mode, so that the color of the picture on the display screen seen by the operator is actually the color needed during printing. This is very important when applied to printing. This is a good image processing method. When Photoshop is working in CMYK mode, the color channel is one more than RGB. In addition, it uses the RGB display mode to simulate the display effect of CMYK, and the operation mode of CMYK is totally different from the RGB principle based on optics. Therefore, The efficiency of processing pictures with the CMYK mode is lower, and the quality of the processed pictures is also worse.
2. Some filters in Photoshop do not support the CMYK mode when working with photos using Photoshop. In addition, the image editing process often goes through many subtle processes. For example, it may be necessary to combine the contents of several pictures together. Since the original tones of the various components cannot be the same, they need to be adjusted and they may also have to be different. Partially combined in some way, and filter processing and so on. Regardless of the effect of the picture processing, the operator wants to create and retain as many subtle effects as possible, to make the picture as real and rich as possible, since the color range of the RGB mode is much larger than the CMYK mode. Therefore, when processing images in RGB mode, a wider color space and more nuanced editing effects will be obtained throughout the editing process, and these effects, if used well, can be largely retained. Although in the end it still had to be converted to CMYK mode and there was definitely a loss of color, it was much better than letting pictures lose color from the beginning.
3. When converting an RGB mode picture into a CMYK mode picture, the separation parameters will play a decisive role in the effect of the picture conversion. The adjustment of color separation parameters will affect the conversion of pictures to a large extent. Photoshop image processing software has the ability to control color separation parameters. In other words, when it is necessary to convert a picture processed in the RGB mode into the CMYK mode for output, adjustment of the color separation parameter in the conversion process can reduce the color loss when the picture is converted in the mode.
4. For the current newspaper publication, the pictures used need to be retained for a long time. It is ideal to retain the picture data in RGB mode. After the correction and correction of the RGB mode picture data information can be a long-term storage of valid documents, so that future retrieval of RGB mode pictures from the archive can be used on different output devices. When RGB mode picture data information needs to be reused in many workflows in the future, it is very convenient whether the color separation method uses the system-level color management method or the image conversion method in Photoshop.
5. When printing, proofing, and outputting pictures using various printers, digital proofing devices, or computer monitors, the main method of observing (and measuring) the color difference of pictures reproduced by the above printing output device is to measure the amount of neutral ash needed to produce the picture. The amount of cyan, magenta and yellow is called the gray balance of the replication system in printing. If the picture is converted to CMYK mode, when the different output devices are reused, the picture needs to adjust the highlight, midtone, and dark tone points of the CMYK picture and change the total gray balance and color saturation. In order not to affect the print quality of the picture, the amount of black in the picture has to be changed, but if the picture is printed without correcting the black data, a bad print result will be produced. For example, a CMYK model image originally color-separated by a high-quality sheet-fed printing press may cause smearing if it is printed on a web printing press. The amount of black in the picture is large, and the processing method can only be modified CMYK. Pattern picture. The RGB mode picture can use a larger RGB tone range to reproduce brighter, more saturated colors. However, after the picture is separated into CMYK, all the pixels in the picture are within the CMYK tone range.
As can be seen from the above discussion, using Photoshop to process color pictures should be done using RGB mode as much as possible. However, during operation, it should be noted that the pictures processed using the RGB mode must ensure the authenticity of the picture colors when outputting in the CMYK mode, and the pictures processed using the RGB mode must be sure that the pictures are completely processed and then converted into CMYK mode pictures. It is best to leave a picture in RGB mode for backup.
In addition to processing images in RGB mode, Photoshop's Lab color mode also has good features. The RGB mode is based on the optical principle, while the CMYK mode is the color mode in which the pigment reflects light. The advantage of the Lab mode is that it compensates for the lack of the previous two color modes. RGB has too many transitional colors between blue and green, too little transition between green and red, and CMYK mode loses more color in the process of editing and processing images. Lab mode has these aspects. Compensated. The Lab mode consists of three channels. The L channel represents the brightness, which controls the brightness and contrast of the picture. The colors included in the a channel range from dark green (low brightness) to gray (middle brightness) to bright red (high brightness) The b channel includes colors ranging from bright blue (low brightness) to gray to yellow (high brightness). Lab mode is similar to RGB mode, and a mixture of colors will produce brighter colors. Only the value of the luminance channel affects the light and dark changes in color. The Lab mode can be thought of as the RGB mode of two channels plus a luminance channel mode. The Lab mode is device-independent and can be used to edit and process any picture (including grayscale pictures), and is as fast as the RGB mode and several times faster than the CMYK mode. Lab mode guarantees no loss of color in the CMYK range when performing color mode conversion. If you convert an RGB mode picture to a CMYK mode, an intermediate step should be added to the operation step, that is, it is first converted to Lab mode. In the typesetting process of non-colored newspapers, it is often used to convert images into grayscale images using Lab mode. For some RGB mode pictures downloaded on the Internet, if you do not use the Lab mode transition and convert it to a grayscale image, you may sometimes not be able to typeset the picture when you use the Founder Feiteng or Wysstek typesetting software to schedule the version.
It can be seen that when editing and processing pictures, try to use Lab mode or RGB mode as much as possible, and change to CMYK mode when it is a last resort. Once it is converted into a CMYK mode picture, it will not be easily transferred back. If it is really needed, it will be converted into Lab mode to process the picture. If the original picture used for the scan input is a color picture, but the picture is used in the grayscale layout, when the picture is input with the scanner, the original picture should not be directly input as the grayscale mode, and the picture should be input with the RGB mode. After the RGB mode has processed the picture, it will be converted into a Lab mode picture first, and then the channel separation command will be used to select the L channel picture as the grayscale picture for printing.
Most operators are relatively straightforward in the process of actually processing pictures. What kinds of pictures are needed are directly scanned into the desired picture mode by a scanner. After a little processing, they are used for typesetting. They also look like they are printed. It's still the case. In fact, if you want to really get a good picture printing effect, do not be afraid of trouble, according to the standard operating procedures.
Source: Hubei Jingmen Daily Printing Factory
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