Which mode is best for color pictures?
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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 images, you should first pay attention to the fact that for an image that is opened, whether it is a CMYK mode image or an RGB mode image, 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 where the picture mode is not required to be converted first, the picture in the RGB mode can be acquired, and the picture is processed in 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. The 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 darker when converting the picture color mode from RGB to CMYK. When editing RGB mode images in Photoshop, you must first select the CMYK Preview command in the View menu (if the Photo shop used is Chinese version, select the preview option in the view menu bar and select the CMYK option). That is to say, it is very important to edit the picture in RGB mode and display the picture in CMYK mode so that the color of the picture on the display screen that the operator sees is actually the color required for printing. This is a good way to deal with pictures. 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 Partially synthesized in some way, with filter processing 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 conversion of the image, and the 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 during the mode conversion by the adjustment of 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 printing, proofing, and outputting pictures using various printers, digital proofing equipment, or computer monitors, the main method of observing (and measuring) the difference in color of pictures reproduced by the above print 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, a CMYK mode image that was originally color-separated for a high-quality sheetfed press would 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, it should be noted during the operation: pictures processed in RGB mode must ensure the authenticity of the picture color when outputting in CMYK mode; when processing pictures in RGB mode, be sure that the picture is completely processed before converting to 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 gray image) 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 the RGB mode picture is converted to CMYK mode, an intermediate step should be added to the operation step, that is, first converted to Lab mode. In the process of typesetting a non-color newspaper, it is often used to convert a picture into a grayscale image using the Lab mode. For some images of the RGB mode downloaded on the Internet, if you do not use the Lab mode transition and then convert to grayscale, 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. The image should be input 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 more 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 typesetting after a little processing, and looks like when printing. Still so. In fact, in order to really get a good picture printing effect, don't be afraid of trouble, follow the standard operating procedures.