In the typesetting process of newspapers or magazines, we often encounter the processing of color pictures. When a certain color picture is opened, it may be in RGB mode or CMYK mode. So when using Photoshop, do you use RGB mode or CMYK mode for color image processing? This article talks about the author's views on this issue.
In the process of using Photoshop to process pictures, you should first pay attention to that, for an opened picture, whether it is a CMYK mode picture or an RGB mode picture, do not switch between these two modes, let alone Go back and forth between the two modes. Because, in the bitmap image editing software, each time the color space of the image is converted, a part of the details of the original image will be lost. If you turn a picture into RGB mode in a while and CMYK mode in a while, the information loss of the picture will be great. What should be said here is that the picture mode used for plate-making printing in the publishing process of color newspapers must be a CMYK mode picture, otherwise printing will not be possible. But it is not to say that the printing effect of processing pictures in CMYK mode during picture processing must be very good, or it should be determined according to the situation. The effect of using Photoshop to select RGB mode for image processing is stronger than that of using CMYK mode. As long as the image is processed in RGB mode, and then converted into a CMYK mode image, the output film can be processed for printing.
In the process of picture processing, if the picture opened for processing is itself a picture in RGB mode, or the original picture is allowed to be scanned in RGB mode during the input process of the scanner, this situation is no better for the layout of the color newspaper. Too. When using Photoshop to scan the original picture, as long as you select the RGB setting option in the color setting option in the file menu bar, the color picture input through the scanner is the RGB mode picture. In short, if you do n’t need to convert the picture mode first, you can get the RGB mode pictures, and use this mode to process the pictures, especially the pictures downloaded from the Internet. To ensure the printing effect of the pictures, you must Use RGB mode for processing. The following aspects can explain this point.
(1) RGB mode is the color method adopted by all devices based on optical principles. For example, the display works in RGB mode. The color range of the RGB mode is larger than that of the CMYK mode. Therefore, the RGB mode can express many colors, especially bright and bright colors (of course, the colors of the monitor must be corrected so that the color of the picture will not be distorted). This color is difficult to print when printing. This is also the main reason why the picture becomes darker when the color mode of the picture is converted from RGB to CMYK. When editing RGB mode pictures in Photoshop, you must first select the CMYKPreview command in the View menu (if the Photoshop used is the Chinese version, select the viewing option in the bright picture menu bar and select the CMYK option), also That is to say, editing and processing pictures in RGB mode, 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 required during printing. This is very important. When applied to printing This is a good picture processing method. When Photoshop works in CMYK mode, there is one more color channel than RGB. In addition, it also uses RGB display to simulate the display effect of CMYK, and the calculation method of CMYK is completely different from the optical-based TGB principle. Therefore, The efficiency of processing pictures in CMYK mode is lower, and the quality of processing pictures is also worse. [next]
(2) When using Photoshop to process pictures, some filters in Photoshop do not support CMYK mode. In addition, the picture editing process often goes through many subtle processes. For example, it may be necessary to combine the contents of several pictures. Since the original tone of each component cannot be the same, they need to be adjusted. Parts are synthesized in a certain way and processed with filters, etc.
No matter what effect the image processing needs to achieve, the operator wants to produce and retain various subtle effects as much as possible, so that the picture has true and rich details as much as possible, because the color range of RGB mode is much larger than that of CMYK mode Therefore, when processing images in RGB mode, a wider color space and a variety of editing effects will be obtained in the entire editing process. If these effects are used well, most of them can be retained. Although in the end still have to switch to CMYK mode and there will definitely be color loss, it is much better than letting the picture lose color in the first place.
(3) When converting RGB mode pictures into CMYK mode pictures, the color separation parameters will play a decisive role in the effect of the picture conversion. The adjustment of color separation parameters will greatly affect the conversion of pictures. Photoshop image processing software has the ability to control the color separation parameters. That is to say, when it is necessary to convert the pictures processed in the RGB mode into the CMYK mode for output, the color loss during the mode conversion of the pictures can be reduced by adjusting the color separation parameters during the conversion process.
(4) At present, for newspaper publishing, the pictures used need to be retained for a long time, and it is ideal to retain the picture data in RGB mode. The RGB mode picture data information after color correction and correction can become an effective document for long-term storage, so that RGB mode pictures retrieved from the archive in the future can be known on different output devices. It is very convenient for the RGB mode image data information to be reused in many publishing workflows in the future, regardless of whether the color separation method uses the system-level color management method or the image conversion method in Photoshop. [next]
(5) When using various printing machines, digital proofing equipment or computer monitors to print, proof, and output pictures, the main method of observing (and measuring) the color difference of the pictures copied by the above printout equipment is to measure neutrality The amount of cyan, magenta, and yellow required for ash is called the gray balance of the replication system on printing. If the picture is converted to CMYK mode, then when using a different output device, the picture requires adjusting the highlight, midtone and dark tone dots of the CMYK picture, and changing the overall gray balance and color saturation. In order not to affect the printing quality of the picture, the amount of black in the picture should be changed, but if the picture is printed without correcting the black data, it will produce bad printing results. For example, the original CMYK mode pictures that were separated by high-quality sheet-fed printing presses would increase the amount of ink in the picture if printed on a web press, and the processing method could only be to correct the CMYK mode pictures. The RGB mode image can use a larger range of RGB tones to reproduce brighter and more saturated colors. However, after the picture is separated into CMYK, all pixels in the picture are within the CMYK tone range.
As can be seen from the above discussion, the use of Photoshop to process color pictures should use RGB mode as much as possible. However, during the operation, it should be noted that the pictures processed in the RGB mode must ensure the authenticity of the color of the pictures when output in the CMYK mode; when the pictures are processed in the RGB mode, be sure that the pictures have been fully processed and then converted to 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 principle of optics, and the color mode of the CMYK mode pigment reflects light. The advantage of the Lab mode is that it makes up for the deficiencies of the previous two color modes. RGB has too much excessive color between blue and green, and too little between green and red. CMYK mode loses more colors in the process of editing and processing pictures, while Lab mode has these aspects. Compensated. The Lab mode consists of three channels. The L channel represents brightness, which controls the brightness and contrast of the picture. The colors included in the a channel range from dark green (low brightness value) to gray (medium brightness value) to bright pink (high brightness value) , The b channel includes colors from bright blue (low brightness value) to gray to burnt yellow (high brightness value). Lab mode is similar to RGB mode, the mixing of colors will produce brighter colors.
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