In today's prepress workflow, digital manuscripts have increasingly become the first choice for printers. However, in actual work, it is often necessary to use some printed products as originals. However, many image quality products that use printed matter as originals often have large problems, such as color cast, lack of layers, imaginary haze, strong graininess, presence of moire, etc., and thus cannot be well used. The reason for these problems is that, besides the poor quality of the printed manuscript itself, the scanning operation and the improper setting of its parameters have a lot to do with it. For professional printing companies and prepress output centers, image scanning and color separation generally use high-end roller scanners. This article uses the Heidelberg Primescan D7100 roller scanner to scan printed manuscripts as an example to introduce image enhancement in the scanning process.
1 Hardware Preparation
1) Preparation of the scanner
Before scanning begins, make sure that the scanner is placed on a stable, absolutely flat surface to prevent vibrations and that the air around the scanner is as clean as possible so that dust does not fall on the original; ensure the scanner before scanning. And the monitor is fully warmed up, typically about 30 minutes.
2) Original Analysis and Installation
Under the standard light source, according to the original tone level, color status, sharpness standards and defect levels, identify the status of the original, that is, standard, medium, and other originals and non-copyable manuscripts, and then distinguish between scanning to faithfully reproduce, Minor adjustments still need to be corrected.
The installation of the manuscript is directly related to the ability to obtain high-quality color separation films. If the manuscript is not installed properly, Newton rings will appear and the color separation quality will be reduced. The rollers and originals should be wiped clean with a clean cloth, and then the originals should be installed. Do not damage the originals during installation.
2 scanning process and scanning parameters of the specific settings
The scanning software used in the scanning process described herein is Heidulberg's NewColor700, and parameter setting and image processing are also completed under the software environment. Baseline corrections for the scanner include focus adjustment, white balance, and color adjustments. The scanner reference is adjusted to ensure that the image input, gray balance of the image, descreening, color misregistration, control of image size and resolution, and the like meet the set control requirements. The rough process is as follows: After the document is installed, the focus adjustment of the scanner is first performed, and then the white balance correction, pre-scan, setting of the formal scan parameters, and formal scan are respectively performed.
1) Focus adjustment
Before performing white balance correction on the scanner, focus adjustment is performed. The original is evenly attached to the surface of the scanner roller, and an original with an area of ​​1 cm×1 cm is used as a focusing target with a large density range, a rich central layer, and the like. Nowadays, high-end scanners generally control the focal length automatically, and for originals scanned from printed materials, the virtual focus should be properly adjusted to eliminate Moiré and smooth and rough particles.
2) White balance correction
The white balance correction of the scanner is achieved by adjusting the maximum output working voltage of the photomultiplier tubes of the three heads (R, G, B) of the scanning head in order to balance the signals of the three channels (R, G, B). Different types of originals have different white balance settings. The white balance point of the transmitted document is in the clean area of ​​the roller, and the white balance point of the reflected document is in the white area of ​​the original or on the white coated paper.
3) Pre-scan
The pre-scan is a low-resolution scan to determine the scope of the formal scan and the setting of scan parameters. General scan driver software has this function. To facilitate accurate delineation of the formal scanning area and observation of the analysis of the original image, the resolution of the pre-scanning is of course as high as possible, but excessively high resolution wastes time. The general pre-scan resolution is 72dpi.
4) Setting of formal scan parameters
The setting of the scan parameters is very important and it will directly affect the scan results. The setting of the formal scan parameters includes the verification of the original type, the determination of the scan color mode, the setting of the resolution and the scan image size, the calibration of black and white fields, the level and color correction, the setting of sharpening and smoothing processing parameters, and the like.
1 Check original type
In the "Scansetup" dialog of the scanning software, you can find the icon options for the original type, which are Reflected Photo, Natural Color, Negative, Printed Original, Black and White Line, User Defined, and so on.
2 Determine the scan color mode
The scan color mode in NewColor7000 is RGB/CMYK/LAB and Gray modes. If you need to further process in Photoshop or other software after the scan, you should use RGB mode; if the scanned image is directly applied to the printing, you can directly select the CMYK mode; and for the linear script and monochrome continuous tone The original should use Gray mode. Whether it is RGB/CMYK/LAB color mode or Gray color mode, NewColor7000 can provide multiple output color management
Profile files are optional for easy color management in the printing process.
3 Resolution and Scan Image Size Settings
The resolution to be set here is the final output resolution of the scanned image, but it cannot exceed the scanner's maximum optical resolution. The setting of the resolution has a great influence on the quality of the scanned image. If the resolution setting is too low, the scanned image is coarsely grained, the edges of the image are jagged, and the quality is poor; if the resolution setting is set too high, unnecessary details such as spots, wrinkles, and images on the screen may be present in the original. Other background highlights will also make the scanned image storage space too large, affecting the scanning speed. Therefore, the resolution of the scanned image must be set correctly to obtain a clear image. The calculation method is: scanning resolution = number of screen lines × magnification × quality factor. When the scanner is determined, choosing an appropriate quality factor is also critical to the quality of the output image. To ensure output image quality, the quality factor is often 2.0.
4 black and white calibration
Scaling refers to determining the gradation range of the manuscript reproduction, that is, the highlight point and dark tone point of the manuscript respectively correspond to the maximum and minimum values ​​of the output signal of the scanned image, which is the basis and key of the entire image reproduction. In terms of tone, the white field is darkened and the high profile darkens. If the black field is set, the entire picture is not black enough.
The correct white-field and black-field selection and setting should make full use of the paper according to the characteristics of various types of manuscripts and printability conditions.
The maximum density of white and four-color inks is superimposed to achieve the best visual contrast. The setting of black and white fields should expand the tone of the main body of the manuscript as much as possible, increase the intensity of contrast, and at the same time understand the essence of the manuscript, and know how to abandon the level of bright tone and dark tone in order to highlight the main purpose of the manuscript.
5 levels and color correction
The level and color correction functions of the common roller scanner's scan drive software are very strong, and special professional, it is very suitable for color separation correction. In NewColor7000 it is possible to change the level of a mixed channel or monochrome channel. When adjusting the level, it can be more strictly controlled within a certain range without affecting other levels.
For color cast problems in certain colors in the image, NewColor7000 uses the ColorSector partition for selective correction. If red is selected as the correction target during correction, the red C, M, Y, and K content can be changed to achieve the purpose of adjusting the color.
6 Sharpening and Smoothing
Both the scanning process and the printing process will reduce the sharpness of the image, so in order to restore those sharp details, it is necessary to use sharpening techniques. Sharpening is the emphasis of image sharpness, and its core is to improve the edge contrast.
Sharpening adjusts the degree of sharpening for different image content. If the image contains relatively distant scenery or city views, distant people or animals, and a lot of details, then the right amount of sharpening is allowed; if the image contains Fewer details, simple shapes, or close-ups of people or animals, the degree of sharpening should be as small as possible. Excessive sharpening introduces too much detail and makes the image look ugly.
In the image input process, the image may be disturbed by various noises and mixed into various high-frequency noises, such as noise in the photoelectric conversion process and errors in information transmission, and thus cannot correctly reflect the digital image information. To get the right information, you must eliminate these noises. In addition, in the process of color reproduction, in order to ensure the need for reproduction and artistic reproduction such as skin color and silk texture, it is also necessary to make the image smooth, soft, and reduce sharpness.
In the sharpening option interface of NewColor7000 scanning software, the sharpness and smoothness can be adjusted by the width of the outline, the width of the white border, the width of the black border, and the smoothed value respectively. The higher the value, the sharper the first three are. The better, the smoother the value, the smoother the image.
5) Final scan
After all the basics and preparations have been done according to the previous steps, you can start scanning the original. When processing a scanned image, the following 4 points should be taken care of: 1 to ensure that the highlight part of the picture does not stop the network; 2 to ensure that the dark tone of the picture does not level; 3 to ensure that the middle tone is rich in levels, moderate brightness; 4 When doing background color removal, ensure that the dark color of the picture does not exceed 260% to 280% after the four-color overlay.
3 Netting processing
As we all know, when scanning halftone originals, if you do not go to the net processing, it will certainly appear visible moire, these moiré not only affect the subjective quality of the image, but also affect the subsequent processing of the image, such as This scanned image is used as a manuscript for plate making and printing, which ultimately affects the quality of the printed matter. Therefore, the scanner's de-networking capability is also an important measure of its performance.
The scanning software NewColor7000 also has a descreening filter function, which can directly process the scanned image. As long as a certain number of network cables is set in the network access field on the scan setting panel, the purpose of network removal can be achieved during the scanning process. The specific number of network cable lines can be set as long as the number of screen lines for printing files is consistent with the number of lines. Remove most of the texture.
In addition, the phenomenon of webprinting of some originals of printed matter is very serious. The web-going function in the scan still cannot make the image go well to the web. At this time, it is possible to continue to go to the web in Photoshop; sometimes the image may become Too imaginary, you can use USMUnsharpmask fine-grained adjustment in Photoshop filters to improve clarity.
Quantity: The degree of imaginary or sharpness emphasis. The final result of USM sharpening is to increase the difference in brightness between adjacent pixels in a black and white trim. For example, when the number is 100%, the contrast on both sides of the border is doubled. The number value is between 0 and 500%. The larger the value, the greater the emphasis enhancement effect. However, it should not be selected too large for specific applications, otherwise it will produce ugly borders or color changes.
Radius: refers to the radius of the emphasis of the sharpness, that is, the calculation centered on a certain pixel, and in how much adjustment the USM sharpening achieves sharpness adjustment. The specific scope of action is a range of pixel radius centered on a pixel and twice the radius. The greater the radius value, the greater the clarity enhancement effect. However, the radius value does not have to be too large, otherwise it will make the image appear very hard border.
Threshold: USM sharpening works when its significance is the starting point of USM sharpening, that is, when the difference between the gray values ​​of adjacent pixels is greater than the value of Threshold. The larger the threshold, the less pronounced the USM sharpening effect. The smaller the threshold, the larger the range of USM sharpening pixels, and the greater the emphasis on the sharpness of the image.