Processing of moire fringes when scanning screened originals
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With the development of technology, people often use scanners to scan some beautiful images for storage or processing by computer. Among the scanned originals used, a considerable part of the originals belong to printed matter. This part of the manuscript, which we call the "screened" manuscript, mainly refers to the image made up of dots in traditional prints. These network points and the network lines formed by the dots interfere with each other in the scanning process, causing invisible visual interference, which is a moire fringe. Prints are not the only factor that causes moiré, and such streaks may occur during both the input process and the output process. In the output process, if the screening control is not good, moire fringes will be generated. What's even more harmful is that the image content also produces moiré, such as textiles with repeating patterns. For the moire fringes in the output process, the usual solution is to eliminate the angle of each color plate.
So, what method can I use to eliminate moiré when scanning prints?
First, eliminate the moiré by rotating
First, a method similar to adjusting the screen angle is used to eliminate the moire fringes. Scanning an original at an angle and then placing it correctly in Photoshop can sometimes avoid the creation of a stripe pattern. If you are using a platform scanner, place the image along the edge of the platform and then in the opposite direction so that you know exactly where the image is. There are a range of different angles to choose from, and if one doesn't, try another one. The way to rotate the image is by selecting the menu: Image/Rotate/Angle and then entering the angle in the dialog that appears. Of course, you can also choose to rotate freely.
Second, eliminate the moire fringe by raising the height
When scanning an image with a flatbed scanner, you can keep it out of focus by slightly raising the distance between the image and the glass platform. Usually the mid-range scanner has a limited depth of field, about 1/4 inch. The easiest way to raise the image away from the table is to use the frame, which not only accommodates different sizes of originals but also does not bend the original image. When this kind of off-focus scanned image is obtained, it can be sharpened with a soft mask.
Third, eliminate moiré by softening
The core technique for eliminating moiré is the amount of image softening. Most of the image softening, how to do it accurately is actually a problem that makes the image change and makes you feel more reasonable. The key is how much softening can remove the moiré without damaging the image quality. Although the softening method will cause some loss of image, it is still the most reliable method to eliminate moiré and has the greatest flexibility. The flexibility mentioned here refers to the possibility of user input. The basic steps are as follows: scan the image at 2 times the desired resolution, which is usually 4 times the number of screen lines; soften the image; halve the resolution; soften again when needed, use other filters to polish; sharp To get the final satisfactory image.
This will be explained in detail below.
The first thing to do first is to double the resolution you need. This has two purposes: first, adding more image information, reducing the damage of various filters; second, when half of the pixels are removed, the moire fringes based on these pixels will be more difficult to see.
Although scanning at such a high resolution is time consuming, it must be done. However, a quality factor as low as 1.5 is acceptable for many output devices, which means that if you output on a 150lpi device, the pixel resolution is only 225, which is acceptable, doubled to 450dpi when scanning images. Compared to 300dpi doubling to 600dpi, in a 4✕5吋 CMYK scan image, that is the difference between 27.5MB and 15.5MB.
We can use two filters to eliminate moiré: Despeckle and Gaussian Blur. There are two reasons for using a Gaussian softening filter: First, Gaussian softening has more freely chosen parameters. We must decide which parameters to use, which means we can know the results of these commands without doubting these filter pairs. What the pixel really does; the second is to use the Gaussian softening to get a better softening, the Gaussian distribution is to place the filter effect along the bell curve, the bell curve is the middle strong two weak curve, it will weaken a little filter The effect, but does not completely eliminate the effect of the filter, other softening filters can not do this, resulting in less than ideal results. Gaussian softening works in 1/10 pixels, and the selected softening intensity is determined by the intensity of the moiré fringes. The key is to choose a value that softens the image so that it can eliminate moiré without causing loss of detail. The softening value used should not exceed 1.8, and is usually close to 0.7.
Before softening the image, you should quickly check all the channels to find out where the moire fringes exist. Usually, the moire fringes only affect one or two channels. The channels in which the fringes are located are easily detected. If only the channels in which the fringes are present are softened, the quality of the image will not drop too much.
Perform a further step-by-step detection of the moiré fringes. Once the channel in which the fringes are found, select the stripe and the surrounding area, find out the range of the space occupied by the stripe, select the determined range of tones, and select the color range command. Select the color, select the stripe, and increase the softening value until the pixel value of the moiré stripe area selected in the channel is equal to the adjacent pixel value. Check Gaussian softening and check the work. Repeat these steps for the other channels in which the moiré is present. In order to get a high quality image, it is worthwhile to have more trouble.
The next step is to halve the resolution and the option to determine the size change is not selected. This feature uses the same number of pixel grids but limits the file size by adjusting the pixel size. Canceling this option will allow Photoshop to add pixels, but what is usually needed is to reduce the pixels. The resolution can be reduced by the required file size. When half of the pixels are removed, the image quality should be improved. If the moire fringes still exist, choose another smaller amount of softening to eliminate. It is also possible to find a ring of rosettes made up of dots. The Despeckle filter can be used to eliminate rose spots, the filter searches for edges in the image, and then slightly softens all pixels except the edges.
Finally, the image can be sharpened to get the final satisfactory image effect. If the moiré stripe that we spend a lot of effort to eliminate is reappeared at this time, then only the pixels in the area where the stripe is located are not sharpened. Change, only sharpen other parts.
The method of eliminating moire fringes not only removes the apparent spatial separation between the dots, but also produces an image that appears to have continuous edges.
I hope that these introductions will help you get the ideal image when scanning screened prints.