Which mode of prepress to handle color pictures is the best

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?

In the process of using Photoshop, the first thing to note is that for open pictures, whether in CMYK mode or RGB mode, do not switch between the two modes, and do not transfer the two modes. . Because, in the dot 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 information loss of the picture is very large. It is worth reminding that the picture mode used for platemaking in the publication of color newspapers must be in CMYK mode, otherwise it will be impossible to print. However, it does not mean that the printing effect of processing pictures in CMYK mode during image processing will be very good, and it should be determined according to the situation. Use Photoshop to process pictures Select RGB mode is more effective than CMYK mode, as long as the picture is processed in RGB mode, and then converted to CMYK mode, the output film can be plate-printed.

In the picture processing, if the picture opened and processed by itself is RGB mode or the original picture is allowed to select the RGB mode for scanning during the use of the scanner input process, this situation is even better for the layout of the color picture. 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.

1. RGB mode is the color mode adopted by all devices based on optical principles. The monitor works 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 (the color of the display must be corrected). This also changes the picture color mode from RGB to CMYK. The main reason for darkness. When editing an RGB mode picture in Photoshop, you must select the CMYK Preview command in the View menu (if you are using the Photoshop Chinese version, select the preview option in the view menu bar, select the CMYK option), that is, use the RGB mode Edit the picture and display the picture in CMYK mode so that the color of the picture on the screen seen by the operator is the color needed for printing. When Photoshop is working in CMYK mode, the color channel is one more than RGB. It uses the RGB display mode to simulate the display effect of CMYK, but the operation mode of CMYK is completely different from the RGB principle based on optics. Therefore, the CMYK mode is used. The efficiency of processing pictures is lower, and the quality of processing pictures is also worse.

2. Some filters in Photoshop do not support the CMYK mode when working with photos using Photoshop. In addition, the photo editing process often goes through many subtle processes, such as combining the contents of several pictures together. Since the original hue of each component part cannot be the same, it is necessary to adjust them so that each part is synthesized in some manner. Filter processing and so on. Because the color range of the RGB mode is much larger than the CMYK mode, when the picture is processed in the RGB mode, a wider color space and more subtle editing effects will be obtained during the entire editing process. The effect, if used well, can be retained for the most part. 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 parameters will play a decisive role in the effect of the picture conversion. The adjustment of color separation parameters greatly affects the conversion of pictures. Photoshop image processing software has the ability to control color separation parameters. 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 used for mode conversion.

4. For newspaper publishing, the pictures used need to be retained for a long time. It is ideal to keep pictures in RGB mode. The corrected and corrected RGB mode picture data information can become a long-term stored effective document, so that the RGB mode pictures retrieved 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 blue, magenta, and yellow is called the gray balance of the replication system. If the picture is converted to CMYK mode, when the different output devices are used again, the picture needs to adjust the highlight, midtone, and dark tone points of the CMYK picture to 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, poor printing results will be produced. For example, the original CMYK mode image for a high-quality sheet-fed press would be smeared if it was printed on a web press. The amount of black in the picture is a bit larger and the processing method can only be modified CMYK mode. image. The RGB mode picture can use a larger RGB tone range to reproduce brighter, more saturated colors.

As can be seen from the above discussion, using Photoshop to process color pictures should be done using RGB mode as much as possible. During operation, it should be noted that the pictures processed using the RGB mode must ensure the authenticity of the colors of the pictures when outputting in the CMYK mode; the pictures processed using the RGB mode must be sure that the pictures are completely processed and then converted into the CMYK mode pictures. It is better to leave a picture of RGB mode for your spare.

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 gray image). 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.

Source: Shenzhen Huaxu Design Co., Ltd.