Photoshop color mode and application (2)

2 Comparison and Selection of Color Modes

Through the above description of each color mode, we understand the difference between the various color modes. How do we choose between using Photoshop to process images? Let's talk about some of the major color modes used when working with images using Photoshop.

The color modes commonly used to process images using Photoshop are RGB, CMYK, and Lab.
The CMYK color mode is a color mode that must be used when an image is printed, that is, CMYK is a printed image color mode. However, the use of RGB color mode in the processing of images is superior to the CMYK color mode in processing effects because:

1) RGB is a color mode adopted by all devices based on the principle of optics, and compared with the CMYK mode, the color range of the RGB mode is larger, so RGB can express more colors, especially bright and bright colors, but This bright and bright 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.

2) For the same image, only three channels of R, G, and B colors can be processed in the RGB mode, and four channels need to be processed for the CMYK. In addition, the operation mode of CMYK and the operation method based on the optical RGB principle It is completely different, so the efficiency of processing images with the CMYK mode is lower, and the quality of the processed images is also worse.

3) Some filters in Photoshop do not support the CMYK mode when working with photos using Photoshop. In addition, the editing of pictures often goes through many subtle processes, such as combining the contents of several pictures together. Since the tones of the components are not 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 finer editing effects will be obtained during the entire editing process. 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.

4) At present, for newspaper publishing, the pictures used need to be stored for a long time. It is ideal to retain the picture data in RGB mode. After correction and correction of the RGB mode image data can be stored for a long time, so as to facilitate future retrieval and retrieval from the archive.

5) When using various devices for picture output, proofing, and printing, the main method for observing (and measuring) the color difference of pictures reproduced by the above printing output device is to measure the cyan, magenta, and yellow required for producing neutral ash. The amount. 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 should be changed, otherwise it will produce bad printing results. For example, an image originally suitable for a high-quality sheet-fed press would be smeared if printed on a web press (especially newspaper printing!), although the processing method can be achieved by correcting the CMYK image. However, after the picture is separated into CMYK, all the pixels in it are within the CMYK tone range. With RGB mode pictures, larger RGB color gamuts can be used to reproduce better results.

As can be seen from the above discussion, using Photoshop to process color pictures should be done in RGB mode as much as possible. However, there are several points to note in the application process:

1) When using RGB mode to process pictures, be sure to ensure the authenticity of the picture colors when outputting in CMYK mode. When dealing with RGB mode images, we can first select the CMYK Preview command in the View menu, that is, edit and process pictures in RGB mode, and display pictures in CMYK mode so that the operator can see the picture colors on the screen. In fact, it is the color needed for printing, which is a good image processing method when applied to printing.

2) Make sure that the picture is fully processed and converted to CMYK mode for output. It is best to develop such a habit to back up an RGB mode picture before conversion. Because in the conversion process, Photoshop actually first converts the image from the original RGB color mode to the Lab color mode, and then produces a final CMYK color mode. This inevitably results in the loss of some pixels. Therefore, it is best to convert the image before conversion. Original backup. However, it is not possible to switch back and forth between RGB and CMYK color modes. The random conversion between them will cause a lot of loss of image information.

When the RGB mode is converted to the CMYK mode output, adjustment of the color separation parameters can reduce color loss during the picture mode conversion.

Another very important color mode in Photoshop is CIELab. This color mode also has good characteristics in processing images. The RGB mode is based on the optical principle, while the CMYK mode is the color mode in which the pigment reflects light. RGB has too many transition colors between blue and green, and there is too little transition between green and red. The CMYK mode is in the editing process. There are more colors lost during the picture, and the Lab model compensates for these aspects. In fact, the Lab mode is similar to the RGB mode, and the Lab mode can be regarded as a mode in which two channels of RGB mode plus one luminance channel are added. Lab mode is device-independent and can produce a color space that matches any device. We can use this mode to edit and process any picture (including gray image), and it is as fast as RGB mode, which is several times faster than CMYK mode. Lab mode has a large color space, which provides transitional space for the conversion of other color modes. This ensures that there is no loss of color within the CMYK range when performing color mode conversion. In the production of non-colored newspapers, the use of Lab mode to convert pictures to grayscale is often used. For some of the RGB images downloaded on the Internet, if you do not use Lab mode transitions to convert to grayscale images, then using Founder Feiteng or Versin's layout software may sometimes not work.

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. And once it becomes CMYK mode, it will not be easy to turn back? If you really need it, turn it into Lab mode to process the picture.

Source: "Printing World"
Author: Zhai Ru Guangdong Press and Publication Department Senior High Technical School