A new method to go to the net

If we come across a mottled scene caused by Moire fringes on the screen, we must find a very wonderful way to avoid or eliminate this phenomenon. When we do a copy of the print, we often encounter this kind of headache, although we are not willing to copy an image that has been screened as an original. People have been recommending various methods to solve this problem. This article also adds new members to many solutions. Figure 1 Portrait of Albright ab There are two images below, which is the same photo of the U.S. Secretary of State Albright. One is the presence of a moiré (Fig. 1a); the other is an effect treated using the method recommended in this article (Fig. 1b), and no moire is present. So how do you do it? In fact, the same effect is used for both effects and the same trimming is done. Why is there such a dramatic change in print quality for the same image? This shows that the original manuscript on the screen has its own problems. Many of these problems can be solved when we remember the following concept: 30 degrees. This is also the core of our solution. First of all, let's review the major elements of screening. Let's look at black and white manuscripts first. To successfully complete the copying process, we must pay attention to the reproduction of the entire tone: white, black, dark gray, light gray, and gray tone. However, on the press, there is no grey ink involved in the printing process other than black ink and white paper. We will consider using some strategies to accomplish this gray task. How to do it? By subtly using a small, large number of black ink dots and a small number of white paper dots in common use, the purpose of obscuring the viewer's vision can be achieved, and the eyes feel a gray effect. There are several ways to do this. Using random ideas will get the desired result. If the idea can be easily observed, we call this effect Mezzotint; if the idea is very small, it is called FM. However, the printing industry uses some sort of regular ink overprint pattern to achieve darker or brighter effects to reproduce different shades of gray. At present, this model has not changed. The idea it uses is the dots that will change in size, but the mutual spacing of the dots will not change. This mode is called amplitude modulation screen. Each inch length, the number of all outlets is called the screen frequency or screen ruling. The higher the number of screens, the better the printing quality: for newsprint, the number of lines for 65 lines/inch is appropriate, and the number of screens printed for magazines is generally 133 lines/inch. If the number of dot lines is exactly in the horizontal or vertical direction of the page, the dot pattern will generally be more pronounced. Therefore, the practice has to specify the angle of the screening mode. In general, there is no error in the exact web angle unless the image is printed using multiple inks or we are trying to copy the screened image. 30-degree angle solution Most of us have a relatively unclear understanding of the use of four different screening angles to print on the four colors of blue, yellow, yellow and black. In general, it does not matter whether we know what angle to use or why. However, if you are ready to process the screened manuscript, an understanding of the screening angle setting theory is critical. Whenever two regular screening patterns are overprinted together, some kind of interference effect or moire can occur. It can be from very significant to almost undetectable. Of course, we want to see the second case. However, if we use the same screen angle for four-color printing, the result we are sure to see is the first case. The CMYK printing method using halftones has undergone a long period of practical trials. Very early, people knew that when using multi-color printing to use screens with many angles, the result obtained by a difference of 30 degrees between them was the most reliable. If you think carefully about four-color printing, you will find that there are only three ink angles that differ by 30 degrees. If the angle of an ink is 0 degrees (the dots are completely horizontally arranged), it can be said to be 90 degrees, because the dots are also completely vertical, we can put the second color ink at 30 degrees, and the third color ink is placed on 60 degrees, but the fourth color angle will not be so regular. Fortunately, yellow is lighter than the other colors, so it doesn't matter what the screen angle is. As long as the angles of blue, goods, and black differ from each other by 30 degrees, you can rest assured. In general, the color is 75 degrees (measured clockwise from the horizontal direction), black is 45 degrees, and cyan is 15 degrees. The relationship between the angles of 30 degrees between these three angles is very important, and the actual angle value It does not matter how much it is. Now, we look back at Albright's photo. The original image appeared in the American "New York Times" magazine. It was printed on the same angle as the previous one. That is, the angle of the black version is 45 degrees. When we scanned it, it got its dot pattern. Although for this scanned image, the original may not be in an accurate vertical position, it can also be judged that the angle of this mode may be between 43 degrees and 47 degrees. When we printed again, the secondary screening angle used was also 45 degrees, so that moiré was obtained in Figure 1a. The effect of Figure 2 is that there is no moire, because Figure 1b itself is not at the horizontal or vertical position, so it is different from using two 45 degree screen angles. This situation is equivalent to the two screens used for this manuscript, one of which is 45 degrees and the other is 75 degrees. So although the angle of the screening we use has not changed, the image effect has undergone great changes. This argument allows us to achieve significantly improved quality results by rotating the image 30 degrees on the printed page when processing the black-and-white original that has been screened. Unfortunately, this method is somewhat unrealistic. We all know the fact that moths fight fire, and from the point of view of ape, it would think: "Why are these guys so silly? Or why are they so? Do they not feel it?" In fact, the 45-degree screening angle is normal, but there is no compulsory measure, and it must be allowed to change like a moth. If the original image and the newly added screen have the same angle, and we cannot rotate the original, rotate the screen. If you store images in EPS format in Photoshop, you can choose a screen angle and screen frequency, these options will override the default settings of the imagesetter. If the photograph of Albright in Figure 2 is turned back to the normal position of Figure 1, but does not want to appear nasty moire, then what should be done? As long as the angle of the black screen is set to 15 degrees or 75 degrees, both of these angles are 30 degrees to the original screen angle. The screen frequency and dot angle can be changed to get excellent results. If we use this image elsewhere, we can use this method to get the best results for completely different features. Photoshop did not warn that EPS files have built-in screen settings. If we want to prevent problems in the future, we can create a built-in prompt file named Albright.screens.in or similar. PageMaker software menu item printing > Color selection also allows us to select the frequency of the screening angle, and their inclusion in the image file does not affect the subsequent operations. QuarkXPress software does not have this feature. Built-in screens For black-and-white images, we can easily consider the number of built-in screen lines to minimize moire. However, for color images, there is a great danger, so this problem should be a strategic issue for experts. But in black and white images, if we change the angle to 15 degrees or 75 degrees, this has an optimized effect on originals that have been screened with 45 degrees, and there will be no problems in other places. However, in doing so, the question of the number of screens should be taken into account at the same time, because this is also a way of dealing with the once-paged manuscripts that is different from the usual methods. We use a portrait of U.S. President Bill Clinton (Figure 2) to explain why we can't just think of using a dot-breaking approach. Although this already-screened image was not corrected during scanning, we did not adjust the screen angle, but we could barely see any moire on the screen. In fact this image and Albright's portrait come from a news newspaper. The only change in this image processing is that Albright's portrait is replayed using 65 lines per inch, while Clinton is re-screened with the standard 133 lines/inch. The thicker the number of screen lines, the more pronounced the moire pattern. The finer the output screen, we can get better results. Figure II Clinton portrait. Results obtained with different processing methods for secondary screened originals (1) Uncorrected scanning method. (2) Use the same image file with built-in 200 lines/inch screening. (3) The same scanned image was corrected using the method recommended in this paper. (4) The same image file, using a 15 degree black angle, 150 lines/inch screen. (5) Scan the effect of the image with the automatic density range correction and descreening function of the Lenno-Haier scanner. The downside is that the smaller the outlets, the easier it is to control the printing process. If the printer can not guarantee the integrity of the network, the image will show a blurring effect. Often, we think it's a terrible thing, but it's a good thing for second-screen images. In general, we believe what printers say about the maximum number of screens, but generally do not refer to such situations. For example, the recommended number of magazines is 133 lines/inch. If a black-and-white original with a second screen is used, it is recommended that a 150-thread/inch screen be built in, and the black angle should be placed at 15 degrees instead of 45 degrees. We did a very interesting test. One of the Clinton images has 200 lines/inch. If the printer knows this in advance, it may pay more attention to it. Web printing does not have any means for this number of lines. Even the most sophisticated sheetfed offset press that uses the best paper, it is difficult to achieve a 200 line/inch screen number, and the same is true for laser imagesetters. We have never done this before for magazine printing, so we are very curious about the results we have produced. The role of overly detailed screens in generating certain problems is overestimated. We do not want to see very bad image results, and we certainly admit that when printing in magazines, images with normal 200 lines/inch screens will have worse results than those with 133 lines/inch. There is no way to distinguish between these cases from the proofs. We can see that using 200 lines/inch screens combined with our recommended method would be better. In addition to the 30 degree angle, in addition to the original angle and the angle of the laser imagesetter, there is a third angle that must be considered. This angle is the scanning angle. Scanner sampling occurs in a very flat direction. Or in order to be consistent with the previous statement, we say that it is carried out along the 0-degree angle. This process did not remove the relationship with the 30-degree angle. If, like scanning the Albrecht image, the image of the screen that was once screened is thrown onto the scanner, the result of the scan will be poor. If you want to get a good copy effect, the scanning process also has problems with the angle arrangement. If the original is placed in the vertical direction on the original holder, there is an unsatisfactory 45 degree angle arrangement in the scanning direction and the screening mode. So we can rotate the manuscript 15 degrees. For black and white images, it doesn't matter if the direction of rotation is clockwise or counterclockwise. In either case, the angle will be 30 degrees apart, which is exactly what we want. Arranging the scanning angle has long been mentioned in many articles, but most of these articles are for the staff to try, which angle is the most suitable. If the manuscript is not good enough, we also admit that it will not have any harm to the scanned image. But why guess at this angle? I