Prepress color picture processing mode selection

In the typesetting process of newspapers or magazines, 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.

1. RGB, CMYK mode can not be ordinary interchange

In the process of using Photoshop to process the image, the first thing to note is that for an open image, whether it is the CMYK mode picture or the RGB mode picture, do not switch between the two modes. Turn both modes around. Because, in the bit picture editing software, every time the conversion of the picture color space is performed, some of 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 information loss of the picture will be great. It should be said here that the picture mode used for platemaking during the publication of color newspapers must be a picture of CMYK mode, 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.

2. In order to ensure the printing effect of the picture, you must use RGB mode for processing

During image processing, if the picture opened for processing is an RGB mode picture itself, or the original picture is allowed to select the RGB mode for scanning using the scanner input process, this is a good situation for the color newspaper layout. Now. When you use Photoshop to scan the original image, 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, you can get pictures in RGB mode and use this mode to process the pictures. Especially for pictures downloaded from the Internet, you must ensure the printing effect of pictures. Use RGB mode for processing. The discussion of the following aspects can explain this point of view.

(1) RGB mode is the color mode used 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 RGB mode pictures in Photoshop, you must first select the CMYKPreview command in the View menu (if you use Photoshop for the Chinese version, select the preview option in the View menu bar and select the CMYK option), which means It is very important to use RGB mode to edit and process pictures, and to display pictures in CMYK mode so that the picture color on the screen seen by the operator is actually the color needed during printing. A very 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 completely different from the principle of RGB 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 slow down the CMYK mode when using Photoshop to process images. 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 color separation parameter 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) At present, for newspaper publishing, 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. For RGB mode image data information to be reused in many future publishing workflows, 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 calculator monitors, the main method for observing (and measuring) the color difference of pictures reproduced by the above printing output device is measurement generation. The amount of cyan, magenta, and yellow required for sex ash is printed on the grey balance of the replication system. 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-mode image originally color-separated by a high-quality sheet-fed press would have a larger amount of black in the picture if it was printed on a web-fed press. The processing method could only be to modify the CMYK mode 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.

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