In which mode is the best way to process color images
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In the process of newspaper layout, you often encounter the processing of color pictures. When you open a color picture, it may be in RGB mode or CMYK mode. So when using Photoshop, is it using 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 the fact that for a picture that is opened, whether it is a CMYK mode picture or an RGB mode picture, do not switch between the two modes, let alone The two modes are turned around. Because, in the dot matrix image editing software, each time the image color space is converted, part of the original image details will be lost. If you turn a picture into RGB mode for a while and then switch to CMYK mode, the information loss of the picture will be very large. It should be noted here that the picture mode used for plate making in the process of color newspaper publishing must be a picture of CMYK mode, otherwise printing will not be possible. However, it does not mean that the printing effect of processing pictures in CMYK mode when performing image processing is very good, or it depends on the situation. Its practical Photoshop processing image selection RGB mode effect is stronger than the use of CMYK mode, as long as the image is processed in RGB mode, and then converted into CMYK mode image, the output film can be plated.
In the picture processing, if the picture that is opened for processing is itself an RGB mode picture or the original picture is allowed to select the RGB mode for scanning using the scanner input process, this situation is better for the typesetting of the color newspaper. . When using Photoshop to scan the original image, just select the RGB setting option in the color setting option in the file menu bar, and the color image input through the scanner is the RGB mode image. In short, in the case that the picture mode of RGB mode can be acquired without first converting the picture mode, the picture is processed by this mode, especially the picture downloaded from the Internet, in order to ensure the printing effect of the picture, it is necessary to Use RGB mode for processing. This view is illustrated by the following aspects.
1. RGB mode is the color scheme used by all optical-based devices. For example, the display works in RGB mode. The RGB mode has a larger color range than the CMYK mode, so the RGB mode can represent many colors, especially bright and bright colors (of course, the color of the display must be corrected so that the color of the picture does not appear). This color is difficult to print when printing. This is also the main reason why the picture will be darkened when the picture color mode is converted from RGB to CMYK. When editing a picture in RGB mode in Photoshop, you must first select CMYK from the View menu.
Preview command (if the Photo shop used is Chinese version, select the preview option in the view menu bar and select the CMYK option), that is, edit the image in RGB mode and display the image in CMYK mode. The color of the picture on the display screen that the operator sees is actually the color required for printing. This is very important. It is a good image processing method when applied to printing. When Photoshop works in CMYK mode, the color channel is one more than RGB. In addition, it uses RGB display mode to simulate the CMYK display effect, and the CMYK operation method is completely different from the optical-based RGB principle. Therefore, The efficiency of processing pictures in CMYK mode is lower, and the quality of processing pictures is also worse.
2. Some filters in Photoshop do not support CMYK mode when working with photos in Photoshop. In addition, the editing process of images often goes through many subtle processes. For example, it may be necessary to combine the contents of several images. Since the original colors of the components cannot be the same, they need to be adjusted, or they may be Parts are synthesized in some way, processed by filters, and so on. Regardless of the effect of the image processing, the operator wants to produce and retain as many subtle effects as possible, and 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 subtle editing effects will be obtained throughout the editing process, and most of these effects, if used well, can be preserved. Although it still has to be converted to CMYK mode and there will definitely be color loss, it is much better than losing the color of the picture 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 the color separation parameters will greatly affect the image conversion. Photoshop image processing software has the ability to control the color separation parameters. That is to say, when it is necessary to convert the image processed in the RGB mode into the CMYK mode for output, the color loss can be alleviated in the image mode conversion by adjusting the color separation parameter 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 that has been corrected and corrected can be a valid document for long-term storage, so that RGB mode pictures retrieved from the archive in the future can be used on different output devices. For RGB mode image data information to be reused in many future workflows, it is very convenient whether the color separation method is system-level color management or image conversion in Photoshop.
5. When using various printing machines, digital proofing equipment or computer monitors for printing, proofing and outputting pictures, the main method of observing (and measuring) the difference in color of pictures reproduced by the above printing output devices is to measure the need to produce neutral gray. The amount of cyan, magenta, and yellow is printed on the gray balance of the replication system. If the image is converted to CMYK mode, then when reusing a different output device, the image requires adjusting the highlights, midtones, and shadows of the CMYK image, and changing the overall gray balance and color saturation. In order not to affect the quality of the picture print, the amount of black in the picture should be changed, but if the picture is printed without correcting the black data, a bad print result will result. For example, CMYK mode images that were originally color-separated for high-quality sheetfed presses can cause smudging if printed on a web press. The amount of black in the image is large, and the processing method can only be to correct CMYK. Pattern picture. RGB mode pictures can use a larger RGB tonal range to reproduce brighter, more saturated colors. However, after the picture is color separated into CMYK, all pixels in the picture are within the CMYK tonal range.
As can be seen from the above discussion, the use of Photoshop to process color images should be done in RGB mode. However, you should pay attention to the operation process: the picture processed by RGB mode must ensure the authenticity of the picture color when outputting in CMYK mode; when processing the picture in RGB mode, make sure that the picture is completely processed and then convert it into CMYK mode picture. It is best to leave an image 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 shortcomings of the first two color modes. RGB has too many transition colors between blue and green, and there are too few transition colors between green and red. CMYK mode loses more colors when editing and processing images, and Lab mode has these aspects. Compensation. The Lab mode consists of three channels. The L channel represents brightness, which controls the brightness and contrast of the picture. The a channel includes colors from dark green (low brightness value) to gray (middle brightness value) to bright red (high brightness value). The b channel includes colors ranging from bright blue (low luminance value) to gray to focal yellow (high luminance value). The Lab mode is similar to the RGB mode, and the blending of colors produces a brighter color. Only the value of the luminance channel affects the light and dark variations of the color. The Lab mode can be thought of as a mode of two channels of RGB mode plus one brightness channel. Lab mode is device-independent and can be used to edit any image (including grayscale images) and is as fast as RGB mode, several times faster than 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 CMYK mode, you should add an intermediate step to the operation step, that is, first convert to Lab mode. In the layout process of non-color newspapers, it is often used to convert pictures into grayscale images using Lab mode. For some pictures of the RGB mode downloaded on the Internet, if you do not use the Lab mode transition and then convert to grayscale, then sometimes you can't typeset the image when using the Founder Feiteng or Weiss typesetting software.
It can be seen that when editing and processing pictures, use Lab mode or RGB mode as much as possible, and then convert to CMYK mode when it is unavoidable. Once you turn into a CMYK mode image, don't switch back easily. If you really need it, switch to Lab mode to process the image. If the original picture used for scanning input is a color picture, but the picture is used in the grayscale layout, when inputting the picture with the scanner, do not input the original picture directly into the grayscale mode. You should input the picture in RGB mode. After the RGB mode processes the picture, it is first converted into a picture of the Lab mode, and then the channel separation command is used to select the picture of the L channel as the grayscale picture for printing.
Most operators are relatively straightforward in the process of actually processing the picture. What kind of picture is needed is directly scanned into the desired picture mode by the scanner, and then used for layout after a little processing, and looks like when printing. Still the same thing. In fact, in order to really get a good picture printing effect, don't be afraid of trouble, follow the standard operating procedures.