Comprehensive pre press technology collection 1

Prepress comprehensive technology collection data (1)

what mode is the best to deal with color pictures in newspaper typesetting

in the process of newspaper or magazine typesetting, we often encounter the processing of color pictures. When opening a color picture, it may be RGB mode or CMYK mode. When using Photoshop, do you use 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 one thing. For an open picture, whether it is a picture in CMYK mode or an RGB mode, do not switch between the two modes, let alone turn the two modes around. Because in the dot matrix image software, every time the image color space is converted, a part of the details of the original image will be lost. If you turn an image into RGB mode and CMYK mode for a while, the information loss of the image will be great. It should be said here that the picture mode used for plate making and printing in the publishing process of color newspapers must be CMYK mode. The cost of picture components is 40% less than that of competitive multi-material 3D printers, otherwise printing will not be possible. However, it does not mean that the printing effect of processing pictures in CMYK mode must be very good when processing pictures, but it should be determined according to the situation. In fact, the effect of choosing RGB mode for Photoshop processing pictures is stronger than that of using CMYK mode. As long as the pictures are processed in RGB mode, and then converted into CMYK mode pictures, and then output the film, you can make plates and print

during image processing, if the image opened for processing is an RGB image itself, or the original image is allowed to be scanned in RGB mode during the scanner input process, this situation is no better for the layout of color newspaper. When using Photoshop to scan the original picture, as long as the RGB setting option in the color setting option is selected in the file menu bar, the color picture input through the scanner is the picture in RGB mode. In a word, if you can get the picture in RGB mode without first transforming the picture mode, you can use this mode to process the picture, especially the picture downloaded from the Internet. In order to ensure the printing effect of the picture, you must use RGB mode for processing. The following aspects can illustrate this point

(1) RGB mode is the color mode adopted by all devices based on optical principles. For example, the display works in RGB mode. The color range of RGB mode is larger than that of CMYK mode, so RGB mode can show many colors, especially bright and bright colors (of course, the color of the display must be corrected to avoid the distortion of picture color). This color is difficult to print. This is also the main reason why the picture will darken when the picture color mode is converted from RGB to CMYK. When displaying pictures in RGB mode in Photoshop, you must first select the CMYK preview command in the View menu (if Photoshop is in Chinese version, select the view option in the bright picture menu bar and select the CMYK option therein), that is, process the pictures in RGB mode and display the pictures in CMYK mode, so that the picture color seen by the operator on the display screen 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, there is one more color channel than RGB. In addition, it also uses RGB display mode to simulate the display effect of CMYK. Moreover, the operation mode of CMYK is completely different from the TGB principle based on optics. Therefore, the efficiency of processing pictures in CMYK mode is lower, and the quality of processing pictures is also worse

(2) when using Photoshop to process pictures, some filters in Photoshop do not support CMYK mode. In addition, the processing of pictures often goes through many subtle processes. For example, it may be necessary to combine the contents of several pictures. Because the primary color tones of each component cannot be the same, they need to be adjusted. It may also be necessary to make each component synthesize and filter in some way. No matter what effect the image processing wants to achieve, the operator hopes to produce and retain all kinds of subtle effects as much as possible, and make the image have real and rich details as much as possible. Because the color range of RGB mode is much larger than that of CMYK mode, when processing images in RGB mode, a wider color space and national and micro variable effects will be obtained in the whole processing process. If these effects are used well, Most of them can be kept. Although we still have to switch to CMYK mode in the end and there will definitely be color loss, it is much better than losing the color of the picture at the beginning

(3) when converting RGB mode pictures to CMYK mode pictures, color separation parameters will play a decisive role in the effect of 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 the picture processed in RGB mode needs to be converted into CMYK mode for output, the color loss during mode conversion can be reduced by adjusting the color separation parameters during the conversion process

shoddy doors and windows make it more rampant (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. After color correction and correction, the RGB mode image data information can become an effective document for long-term storage, so that the RGB mode images retrieved from the archive in the future can be known on different output devices. For RGB mode picture data information needs to be emphasized in many publishing workflow in the future, so when it is newly used, it is very convenient whether the color separation method is system level color management method or image conversion method in Photoshop

(5) when using various printing machines, digital proofing equipment or computer monitors to print, proofing and output pictures, the main method to observe (and measure) the color difference of the pictures copied by the above printing output equipment is to measure the amount of cyan, magenta and yellow required to produce neutral gray, which is called the gray balance of the replication system in printing. If the picture is converted to CMYK mode, when reusing different output devices, the picture requires adjusting the highlight, intermediate and dark points of CMYK picture, and changing the total gray balance and color saturation. In order not to affect the printing quality of the picture, the amount of black in the picture should be changed, but if the black gjb150.5 temperature impact experimental color data is not corrected and the picture is printed, bad printing results will be produced. For example, CMYK mode pictures that were originally color separated by a high-quality sheet fed printer will have a large amount of ink in the pictures if they are printed on a web press, and the processing method can only be to modify the CMYK mode pictures. RGB mode pictures can use a larger RGB hue range to reproduce brighter and more saturated colors. However, after the picture is divided into CMYK, all pixels in the picture are within the CMYK hue range

it can be seen from the above discussion that RGB mode should be used as much as possible to process color pictures with Photoshop. However, in the process of operation, it should be noted that pictures processed in RGB mode must ensure the authenticity of picture colors when output in CMYK mode; When using RGB mode to process pictures, make sure that the pictures have been completely processed before converting to CMYK mode pictures. It is best to leave an RGB mode picture for standby

in addition to processing pictures in RGB mode, Photoshop's Lab color mode also has good characteristics. RGB mode is based on optical principle, while CMYK mode is the color mode in which pigments reflect light. The advantage of lab mode is that it makes up for the shortcomings of the previous two color modes. RGB has too many transition colors between blue and green, and too few transition colors between green and red. CMYK mode loses more colors in the process of processing pictures, and lab mode compensates in these aspects. The lab mode consists of three channels. The L channel represents brightness, which controls the brightness and contrast of the picture. The color of channel a ranges from dark green (low brightness value) to gray (medium brightness value) to bright pink (high brightness value), and the color of channel B ranges from bright blue (low brightness value) to gray to burnt yellow (high brightness value). Lab mode is similar to RGB mode, and the mixing of colors will produce brighter colors. Only the value of the brightness channel affects the brightness change of the color. Lab mode can be regarded as RGB mode of two channels plus one brightness channel. Lab mode is device independent. It can be used to process any picture (including gray-scale pictures) and is as fast as RGB mode, which is several times faster than CMYK mode. Lab mode can ensure that there is no loss of color within the CMYK range during color mode conversion. If you convert RGB mode pictures to CMYK mode, you should add an intermediate step to the operation steps, that is, convert them to lab mode first. In the process of achromatic typesetting, it is often used to apply lab mode to convert pictures into grayscale images. For a picture in RGB mode downloaded from the Internet, if it is not converted to gray-scale image after transition without lab mode, sometimes the picture cannot be typeset when it is typeset with Founder Feiteng or Weisi typesetting software

it can be seen that when processing pictures, try to use lab mode or RGB mode first, and then turn to CMYK mode when you have to. Once the image is converted to CMYK mode, don't convert it and then turn it back. If necessary, convert it to lab mode for processing. If the original picture used for scanning input is a color picture, but the picture is used in the gray-scale layout, when using the scanner to input the picture, do not directly input the original picture into the gray-scale mode, but use the RGB mode to input the picture. After processing the picture in the RGB mode, first convert it to the picture in the lab mode, and then select the picture in the L channel as the gray-scale picture for printing through the channel separation command

most operators are relatively straightforward in the actual process of processing pictures. They can directly scan what kind of pictures they need into the required picture mode with the scanner, and then use it for typesetting after a little processing. It still looks like that when printing. In fact, if you want to really get a good picture printing effect, don't be afraid of trouble and follow the standard operation steps. (to be continued)