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(A Six-Color Cure For Screen-Printed Halftones, Part 2) theory applied to the practice of accurately reproducing a wide range of color gamut is the dream of every screen-printer. However, as can be seen from the content of the first part, due to the limitations of the traditional four-color printing process in the printing method, this dream has been limited. In order to solve the problems contained in the four-color printing, it is necessary to find a non-traditional no-profiling scheme, a small amount of halftone dot enlargement, and a tone compression scheme, and thus there is a six-color overprint replication scheme. Expand the ink unit to six colors, that is, add two shades of magenta and cyan in addition to black and yellow. This method does increase the cost, but at the same time we must see the benefits of the program for the image. Rubber can only be made to know its reliability if it is made of tires. The six-color overprint scheme is also the same. Only practical practice can prove its usefulness. Here we must evaluate the color separation technology and look at some important issues. The issues include gray balance, color separation channels, halftone screen lines and screen angles, proofing, output using DCS 2.0, printing color sequence, and inks. Consumption and registration, etc. When to use a six-color overprint Before dealing with color separation problems, it is best to determine if the processed image is not necessary or suitable for use with a six-color copy scheme. There are two types of images that are best suited for a six-color copy scheme (also known as two-tone copying). The first is images with darker details and full, rich colors that are difficult to obtain with traditional printing techniques such as black feathers, dark wines, black bottle glasses, walnut, reddish-brown and other dark woodwork. In the traditional printing, in order to obtain these colors, black must be added in the color separation, but the black color suppresses the fine color tone of the image, and the effect sought will be obliterated. The other image types are those that have tertiary color components in high-detailed details or high-profile regions. This kind of image appears in POP panels and similar extreme images. Examples of such images are water-colored soap bubbles, shades of white eggs, details of lace and white clothes, white clouds or barbecue foods. Natural color images such as flowing water, rocks, stones, sand, feathers, bark, shells, leather, sculpted gemstones, and ice are also images that use two-tone reproduction. Note that the very high light and dark shades in a single image are unlikely to occur at the same time. In general, either very high light conditions or very dark shadows occur. Also keep in mind that the two-tone process is not a panacea. It has the same limitations as other printing methods. The more detailed or dark areas of an image, the more careful and serious it is. Since the trapping method is also used in the six-color reproduction, many human-perceived colors are still outside the color gamut range or the reproducible range, especially the very bright green, purple, and violet/purple colors. For simplification, focus is placed on images with a large amount of shadow detail and a large number of highlight and tertiary colors. These images are often the problem of screen printing, and there will usually be dark tone and high-profile dot gains and excessive moire. The color separation object used for this test is the familiar Kodak Q60 test image. Color separation is usually done using Photoshop for color separation. Before working with six colors, you first need to make some basic settings in the Separation Settings dialog box. In Windows and Mac OS, open the File>Color Settings>CMYK Setup dialog (Figure 1). *******Figure 1: CMYK settings ********* There are several color separation settings in image separations that need adjustment for six color separations. First, adjust the CMYK Setup dialog box. Ink color and black version generated value. ************************************* If you have a colorimeter or spectrophotometer, measure each Print the L*a*b* value of the color and record it, enter it in the ink color box in Photoshop (Ink Options window). For the purposes of this article, no measurement is made, but the default setting is SWOP Uncoded. Highlight only high-profile colors without darkening the normal separation ink. The focus here is to improve the fine soft tones in the high profile area while maintaining the dark image density. One of the key adjustments here is to set up the black version in the separation. In order to make this setting, you need to open the Separation Options window and select the black version to enter the black generation curve. The next step is to click the 0% point (lower left corner) with the mouse and drag it to the right to the 50% point. This means that there is no black print in less than 50% tone. At the same time, in the top right corner, drag this down to 75% so that the black version will not have a value greater than 75% in the darkest part of the image (Figure 2). The purpose of this is to make the image generated by the trapping achieve gray balance at all shades. This is a typical Under-Color Removal (UCR) process that is particularly suitable for the brighter part of the image. It also reduces the amount of blackplates attached to the darkest areas of the image. *********Figure 2: The black version produces ********** One advantage of using Duoqing and Magenta inks for color reproduction is that it reduces the duplication of high-profile gray and dark gray and The amount of black ink required to enrich the color. By adjusting the curve, the black version is applied in the 3/4 tone region, and the other five colors are used to obtain clear gray and shadow details. ********************************* Gray balance Gray balance is the core and skeleton of separation. When using a two-tone process, care must be taken to maintain a high-level color balance. This can be a trick, but it's not too hard. By limiting the amount of black ink in the color separation, neutral ash can be obtained by color overprinting. However, when these color inks are overprinted, the amount of yellow, magenta, and cyan inks must be adjusted. Gray is a pure ash with no obvious color cast. Again, these adjustments need to be done in Photoshop. In order to achieve CMY balance, it is best to make a neutral gray scale and add it to the test image is very helpful. It usually starts with an RGB image, because Photoshop only applies separation settings when converting RGB images to CMYK images. To make a neutral gray ruler, use the Rectangle Select tool to make a narrow rectangle. Set the background color to (0,0,0) and the foreground color to (255,255,255). This is the RGB value entered by double-clicking the Color Picker on the toolbar. Next, use the Blend tool to apply the foreground color to the background color fill pattern to blend fills in narrow rectangles. Photoshop produces a transition from white to black in a narrow rectangular strip. Now go to the Image>Adujst>Posterize dialog and set the number of steps to 21 so that you have a level 21 gray scale. Note that the neutral gray is defined by RGB values, so to test the neutral gray, you must also use the “pipette†tool to extract different points in the image, and see its value in the information column. The definition of neutral gray is R=G= B. For example, (128,128,128) presents a neutral grey midtone color. Next, the RGB gray scale is attached to the paper area of ​​the color separation image. The bottom of the Kodak Q60 image has a similar grey scale. Interestingly, it is not that the entire scale is neutral gray. This allows the additional scale to be used as a reference. When converting the color mode from RGB to CMYK, Photoshop calculates and creates a color separation table using the ink values ​​entered in the color settings. This calculation accurately determines the amount of cyan, magenta, yellow, and black that make up the ash balance. Photoshop also decided at which point to darken and start adding black ink. All of these are important because of the need to manually adjust the gray balance in high profile and below 25%. In this particular exercise, the amount of SWOP cyan and magenta was reduced and light green (Lc) and light magenta (Lm) were used instead. To make this change, first copy the SWOP cyan and magenta channels and name them "light blue" and "light magenta," and specify each channel as a spot channel in the channel options window. You also need to set the color value for the new channel and enter “0†for the field. To establish the Lc and Lm color values, first create a printed sample of Lc and Lm inks. It is recommended to print using Lc and Lm inks with a density of 50% of the standard cyan and magenta. Using the densitometer, you obtain the L*a*b* value for each color, and enter the value as the channel color (double-click the channel name). Without a densitometer, the 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 following curves need to be readjusted for the original Cyan channel and the magenta channel. The goal is to adjust the shaded areas where the standard cyan and magenta fall in the original hue area, while Lc and Lm mostly fall in the high profile and under 25% area. For this sample image, first open the Curve dialog (Image>Curves), move the starting points of cyan and magenta to 25%, so that the standard cyan and magenta below 25% are completely removed. There is no separate adjustment to the intermediate tone, but the curve is automatically adjusted. For the Lc and Lm curves, turn on Preview and point a node at the 50% dot. Then drag the curve up slowly at 25% mesh 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 the composite image (including all channels) and use Image>Adjust>Selective Color to make the necessary edits to the high-profile and 25% regions. This process is explained in detail in the Photoshop user manual. After adjusting for overtones and 25% tint, the effect of these two channels on midtones and shadows is reduced by adjusting the Lc and Lm curves and applying a selection of color adjustments. Once all these adjustments have been completed, leaving the six-color separation settings, the effect of these changes is shown in Figure 3. ************Figure 3: Dark Cyan/Dark Fuchsia Separation******* The dark cyan, dark magenta and traditional four-color cyan used in six-color printing Compared with the magenta version, the color is lighter. Note that when these yellows and blacks are color-separated, the meat shades appear to have the opposite appearance, and the high-graded gray scale shifts toward yellow. **********************************************