Several color modes of digital images (below)

Third, scientifically apply various color modes in professional teaching

When a color picture is opened, it may be in 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) Do not convert between these two modes while using Photoshop to process the picture. Because, in the dot matrix 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 loss of information in the picture will be great. It should be noted here that the picture mode used for platemaking in the publication process must be a CMYK mode picture, otherwise it cannot be printed. 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.

(b) In the image processing, RGB mode is the best choice. If the picture being opened for processing is an RGB mode picture or the original picture is allowed to select the RGB mode to scan during the use of the scanner input, this situation is better for the printing and typesetting. In the case that it is not necessary to first convert the picture mode, pictures in the RGB mode can be obtained, and the pictures are processed in this mode. In particular, pictures downloaded from the Internet have a good printing effect. The following aspects of the statement illustrate this point.

1. The RGB mode is the color mode used by all devices based on optical principles. The color range of the RGB mode is larger than the CMYK mode, so the RGB mode can express many colors, especially bright and bright colors. 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 images in RGB mode in Photoshop, you must first select the CMYK Preview command in the View menu (if you are using Photoshop for Chinese, select the preview option in the View menu bar and select the CMYK option). That is to say, the picture is edited in RGB mode, and the picture is displayed in CMYK mode, so that the color of the picture on the display screen seen by the operator is actually the color needed for printing. This is a kind of color when applied to printing. Very good image processing method. When Photoshop is working in CMYK mode, there is one more color channel than RGB. In addition, it also uses RGB display mode to simulate the effect of CMYK display, and the operation mode of CMYK is completely different from the RGB principle based on optics. Therefore, the efficiency of processing pictures with CMYK mode is lower, and the quality of processing pictures is also low. To be worse.

2. When using Photoshop to process images, some filters in Photoshop do not support CMYK mode. In addition, the editing process of a picture often goes through many subtle processes, such as the possibility of combining the contents of several pictures together. Because the original tones of the components cannot be the same, they need to be adjusted, or the components may be combined in some manner, and filter processing and the like may be performed. Regardless of the effect of the picture processing, the operator wants to create 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, when the picture is processed in the RGB mode, a wider color space and more nuanced editing effects will be obtained during the entire editing process. If you use it well, most of you can keep it. 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 greatly affect the conversion of pictures. 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, when the picture output, pay attention to adjust the gray balance. When using various printers, digital proofing devices, or computer monitors for printing, proofing, and outputting of pictures, when viewing (and measuring) the color difference of pictures reproduced by the above printing output device, the main method is to measure the generation of neutral grays. The amount of cyan, magenta, and yellow needed is called gray balance in printing. 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 model image originally color-separated by a high-quality sheet-fed printing press may cause smearing if it is printed on a web printing press. The amount of black in the picture is large, and the processing method can only be modified CMYK. Pattern 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.

As can be seen from the above discussion, using Photoshop to process color pictures should be done using RGB mode as much as possible. However, during operation, it should be noted that the pictures processed using the RGB mode must ensure the authenticity of the picture colors when outputting in the CMYK mode, and the pictures processed using the RGB mode must be sure that the pictures are completely processed and then converted into 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 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. When editing and processing pictures, try to use Lab mode or RGB mode as much as possible, and change to CMYK mode when you have to. 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.

More and more design and printing separation departments recognize the main advantage of RGB - flexibility. By color balancing and archiving RGB image data, users can freely create multiple CMYK images with their own gray balance characteristics, special blackplates, and a specified tonal range (including appropriate highlights and dark tones and dot gain compensation). Archived saved RGB images can also be used for new media, including monitor-based content delivery.


Source: Journal Author: Zhao Hao

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