The hottest screen six color overprint Part II

2022-08-19
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Silk six color overprint (Part 2, Part 1)

(a six color cure for screen printed halftones, Part 2) it is the dream of every silk printer to apply the theory to practice and accurately copy the colors with a wide range of color gamut. However, it can be seen from the content of the first part that the limitations of the traditional four-color printing process in printing methods limit the realization of this dream. In order to solve the problems contained in four-color printing, it is necessary to find a non-traditional scheme without moire, a small amount of dot expansion and tone compression, so there is a six color overprint reproduction scheme. Expand the ink unit to six colors, that is, add two tones of magenta and cyan in addition to black and yellow. This method does increase the cost, but at the same time, we should also see the benefits of the scheme to the image. The reliability of rubber can only be known when it is made into tires and on the road, as can the six color overprint scheme. Only practical practice can prove its usefulness. Here we will evaluate the color separation technology and see how some important problems are, including gray balance, color separation channel, number and angle of halftone lines, proofing, and using dcs2 0 for output, printing color sequence, ink consumption, registration, etc. When to use six color overprint? Before dealing with the problem of color separation, it is best to determine whether the images processed by the two industrial clusters of polyurethane materials and high-performance high molecular materials are necessary or suitable for the use of six color reproduction scheme. There are two kinds of images that are most suitable for six color reproduction schemes (also known as two tone reproduction). The first is an image with darker details and full and rich colors, which is difficult to be obtained by traditional printing processes, such as black feathers, dark wine, black bottle glass, walnut, reddish brown and other dark wood products. In traditional printing, in order to obtain these colors, black must be added to the color separation, but black suppresses the fine tone of the image, and the effect pursued will be erased. Another type of image is those with cubic color components in high-profile details or high-profile areas. Such images often appear in pop panels and similar extreme images. Examples of such images are water colored soap bubbles, shadows of white eggs, lace and details of white clothes, white clouds or barbecue food, etc. Natural color images such as flowing water, rocks, stones, beaches, feathers, bark, shells, leather, carved gemstones and ice are also the choice for two-tone reproduction. Note that in an image, the possibility of extremely high light and extremely dark tone appearing at the same time is very small. Generally speaking, either extremely high light or extremely dark tone occurs. In addition, we should also remember that the two tone process is not omnipotent. Like other printing methods, it also has certain limitations. The more details in high-profile or dark areas of the image, the more careful and serious it needs to be. Since the chromatic method is also used in six color reproduction, many colors that can be perceived by human eyes are still outside the color gamut range or replicable range, especially very bright green, purple and violet/magenta. For simplicity, the focus is on images with a large number of dark details and a large number of highlight colors and tertiary colors. These images are often problematic images in silk printing. Generally, there will be dark tone merging and high-profile dot expansion and too many moire. The color separation object used for testing here is the familiar Kodak Q60 test image. Photoshop is usually used for color separation. Before using six colors to work, you need to make some basic settings in the color separation settings dialog box. In windows and Mac OS, open the file> Color Settings> CMYK setup dialog box (Figure 1)******* Figure 1: CMYK settings *********** there are several color separation settings in image color separation that need to be adjusted for six color separation. First, adjust the ink color and black version generation value in the CMYK setup dialog box************************************* If the colorimeter or spectrophotometer is used, measure and record the l*a*b* value of each printing color, and input it into the ink color box in Photoshop (open the ink option (ink option operators need to add sufficient high concentration absolute ethanol ons to the cooling tank before operation) window). For the purpose of this article, the measurement is not performed, but the default setting is swop uncoded. Only highlight high-profile colors without darkening normal color separation inks. Here, the focus is on improving the fine soft tone of high-profile areas while maintaining the density of dark tone images. A key adjustment here is to set the black version generation in color separation. In order to make this setting, you need to open the color separation option window, select black generation to enter the black generation curve. The next step is to click 0% of the point (lower left corner) with the mouse and drag it to the right to 50% of the point. This means that there is no black version below 50%. At the same time, click the upper right corner of the middle and drag it down to 75%, so that the black version will not have a value greater than 75% in the darkest area of the image (Figure 2). The purpose of this is to make the image generated by chromaticity achieve gray balance at all hues. This is a typical background color removal (UCR) process, especially suitable for the brighter part of the image. It also reduces the amount of black plates attached to the darkest areas of the image********* Figure 2: black version production ********* one advantage of color reproduction with double cyan and double magenta inks is that it reduces the amount of black ink required to replicate high gray and dark gray and rich colors. By adjusting the curve, the black version is applied in the 3/4 tone area, and the other five colors are used to obtain clear gray and dark details********************************* Grey balance grey balance is the core and skeleton of color separation. When using two tone technology, we must pay attention to keeping the gray balance of high-profile colors. This can be a skill, but it's not too difficult. By limiting the amount of black ink in color separation, neutral gray can be obtained by color overprint. However, when these color inks are overprinted into gray, the ink volume of yellow, magenta and cyan must be adjusted. Gray is a pure gray without obvious color deviation. Again, these adjustments need to be done in Photoshop. In order to achieve the balance of CMY, it is best to make a neutral gray scale and add it to the test image, which is very helpful. Usually start with RGB images, because Photoshop only applies color separation settings when converting RGB images into CMYK images. In order to make a neutral gray adjustable ladder rule, use the test strength of instant paper and core paper to calculate. The rectangle selection tool makes a rectangular strip, and set the background color to (0, 0, 0) and the foreground color to (255255255). This is the RGB value entered by double clicking the color picker on the toolbar. Next, apply the "blend" tool to apply the foreground to background color fill mode to mix and fill in the narrow rectangle. Photoshop will produce a transition effect from white to black in the narrow rectangular bar. Next, go to the image>adujst>posterize dialog box and set the number of steps to 21, which makes a gray ladder with 21 steps. Note that neutral gray is defined by RGB value, so when checking neutral gray, you must also use the "straw" tool to absorb different points in the image, and look at its value in the information column. The definition of neutral gray is r=g=b. For example, (128128128) presents a neutral gray halftone color. Next, attach the RGB gray scale to the paper area of the color separation image. There is a similar gray scale at the bottom of Kodak Q60 image. Interestingly, not the whole scale is neutral gray, which makes it a reference for additional scales. When converting the color mode from so many plastics being used RGB to CMYK, Photoshop uses the ink value entered in the color setting to calculate and establish a color separation table. This calculation accurately determines the amount of cyan, magenta, yellow and black that make up the ash balance. Photoshop also determines at which point the dark tone begins to add black ink. All of these are very important, because we must manually adjust the high-profile and gray balance in the area below 25%. In this particular exercise, the amount of swop cyan and magenta was reduced and replaced by light cyan (LC) and light magenta (LM). To make this change, first copy the swop cyan and magenta channels, name them "light cyan" and "light magenta" respectively, and specify each channel as a spot color channel in the channel options window. You also need to set the color value of the new channel and input "0" to the field. To establish LC and LM color values, first establish a printing sample of LC and LM inks. It is recommended to use LC and LM inks with a density of 50% of standard cyan and magenta for printing. Using a densimeter, obtain the l*a*b* value of each color, and enter this value as the channel color (double-click the channel name). If there is no densimeter, this value may be adjusted by the human eye until the color obtained in the color picker visually matches the printing effect. Once these new channels are named, the curves of the original cyan channel and magenta channel need to be readjusted. The goal is to adjust the standard cyan and magenta to fall in the dark area of the primary tone area, while LC and LM mostly fall in the high-profile and below 25%. For this sample image, first open the curve dialog box (image>curves), and move the starting point of cyan and magenta to 25%, so that the standard cyan and magenta below 25% are completely removed. Instead of making a separate adjustment to the middle tone, let the curve adjust automatically. For LC and LM curves, open "Preview" and point a node at 50%. Then slowly drag the curve upward at the 25% point until the gray balance of the image looks better. This requires some experience, but it is not difficult. After adjusting the LC and LM curves, restore to the composite image (including all channels), and use image>adjust>selective color to make necessary adjustments to the high-profile and 25% area. This process is described in detail in the Photoshop user manual. After adjusting the high tone and 25% hue, the influence of these two channels on the intermediate and dark tones should be reduced by adjusting the LC and LM curves and applying the selected color adjustment. Once all these adjustments are completed, leave the six color separation setting, and the effect of these changes is shown in Figure 3************ Figure 3: dark cyan/dark magenta color separation * * * * * * * the dark cyan and dark magenta versions used in six color printing are lighter in color than the traditional four-color cyan and magenta versions. Note that when these colors are separated from yellow and black, the flesh tone shows the opposite appearance, and the high tone of the gray ladder moves towards yellow**********************************************

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