The main task of applying color separation is to analyze the configuration of technical points and technical control parameters in image composition and color separation processing of common manuscripts in electronic color separation platemaking, and to analyze the internal laws. To improve the image copy quality and job efficiency of the application color separation job.
First, the color contrast operation of the original contrast is too small
Too small contrast of the original refers to the size of the image's density. There are two important reasons for the production of this kind of image. One is that the original creators used a series of techniques to produce special artistic effects (such as high-profile images), and the other is that the original creators were not skilled in the technique. Or control mistakes (such as underexposure). The following four types are common in electronic color separations. (1) High-profile originals, (2) low-key originals, (3) overexposed originals, and (4) under-exposed originals. Among them, the high-profile and low-key manuscripts of (1) and (2) are consciously shot and produced by the manuscript author. The separation of color should be based on the characteristics of faithful manuscripts. Excessively overexposed and underexposed manuscripts of (3) and (4) are caused by improper handling during photography or photographic processing, and appropriate compensation is required for color separation.
The high-profile manuscript is shown in a of Figure 6.1, which is only in the high light to midtone range. If the black field is set at the deepest part of the original during the color separation scan, the entire tone of the scanned image will be hardened, and the copied image will lose the high-profile characteristics of the original document and become like a normal photograph. Therefore, high-precision manuscript separation should adopt the white/black calibration of the curve (1) in Figure 6.2, otherwise the color separation result will be the same as the curve (2). Therefore, during the color separation operation, the artistic features of high-profile manuscripts should be fully understood when setting up the black court. Select a high density level on the gray scale to use as the black field of the image or select the darkest part of the original to reduce the black scale calibration values ​​C, M, Y, and BK, and increase the intermediate tone level appropriately. , can fully reproduce the characteristics of high-profile manuscripts. Therefore, the black field calibration is the key to its color separation technology. Selecting a dark field as much as possible when selecting a black field on a ladder scale can make the copying effect better.
A low-key manuscript is the opposite of a high-profile manuscript. As shown in b of Figure 6.3, the image has only intermediate and dark tone levels, but is different from the under-exposed manuscript. Therefore, when performing color separation, a low-density area should be selected as a white field on the ladder scale, and the black field selection should also be higher than the density value in the darkest part of the original document. No changes to the hierarchy are required for correction.
The image of an overexposed original is similar to a high-profile original but it is due to overexposure in photography. Therefore, when the color separation, the white field should be selected in the original density of the normal position, the black field is selected in the original density of the most people. When the gradation is corrected, the midtones are properly increased, as shown in curve (2) in Figure 6.2. In contrast, under-exposed manuscripts are similar to low-profile manuscripts and are caused by underexposure in photography. When the color separation is calibrated, white-field is set at the position where the manuscript is high-profiled. The black field is set at the position where the density is the darkest and the middle tone is appropriate. reduce.
Second, color separation manuscript color separation operation
The color original color cast is caused by the poor balance of the three primary colors of the photosensitive material, the unbalanced gamma ratio, the color temperature error of the photo exposure light source, and the improper photographing processing, as shown in Fig. 6.4 and Fig. 6.5, thereby causing the image to be partially or partially biased. color.
The color correction of color-originating manuscripts is difficult to perform with only the color correction function in the electronic color separation machine. The functions of white balance, white/black calibration, CAST, gray balance curve adjustment, gradation correction, background color removal, background color gain and restriction need to be comprehensively applied to realize color correction of color-originated originals.
1. Correction of high-profile color-shifted originals
A typical high-resolution color-shifted manuscript is shown in Figure 6.6. That is, the measured R, G, and B spectral densities are C87%, Y83%, and M98% in the image white field. That is, there is no M between C and Y. Get the ideal gray balance. There are two ways to solve this color cast, as shown in Figure 6.7. The first is to use the MAN/AN mode to adjust the C and Y values ​​to set C96%, Y98%, and M98% when setting the white field. That is, move the color separation curve of C, M, and Y respectively. Second, after setting the white field calibration, use the CAST function to adjust the relationship between C, Y, and M according to the degree of color shift and the purpose of replication. Color balance.
2. Mid-tone color correction manuscript correction
The midtone color cast refers to the condition that the R, G, B spectroscopic density of the neutral gray density portion or the halftone dot of C, M, Y is unbalanced, or the "need to add a little red" or "yellow" appears when the skin color is reproduced and reproduced. Too much "state. As shown in Figure 6.8, C48%, M70%, and Y54%.
The method of midtone correction is: (1) Use CAST function to take any one of Y, M and C as the standard and adjust the other two colors to balance value, as shown in Figure 6.9, when the value of C is 48% When, Y, M adjusted to an equilibrium value of 59%. (2) When correcting, first use the gradation correction to determine the tone pattern, then use the cAST function to adjust the three-color curve shape, and finally use the color correction function to calibrate the color, as shown in Figure 6.10. (3) Correct the middle tonal offset with white/black calibration. The midtone shift color is mostly caused by the inaccurate color temperature of the shooting light source, and often occurs simultaneously with the high-profile color shift or the dark tone shift color shift. Therefore, the correction function should be comprehensively applied and cooperate with each other.
3. Correction of dark-shaded originals
Dark color cast refers to a condition where the spectral densities of R, G, and B in the dark tone that should be neutral gray are inconsistent. Normally, even the color originals that have been properly photographed and correctly photographed have a certain degree of color cast in their dark areas. In fact, it can be assumed that all color originals have dark spots or color imbalances, as shown in Figure 6.11. , shown in 6.12.
There are four methods for color correction of dark originals. (1) Use the AUTO/AN EB function, that is, use the AUTO/AN EB function to set the black field calibration value of the selected black field to a standard neutral gray value during black field calibration, but after calibration Care should be taken to check its effect on midtones. (2) Using the MAN EB function, the shaded color to be selected is treated as a neutral gray at the time of calibration, and the percentage values ​​of the three color dots of C, M, and Y are directly adjusted to neutral gray. Shadows affect the color of the image area and must be handled carefully. (3) With the CAST function, after the calibration is applied, there is still a color shift at the dark tone, and the CAST function can be used to shift the color separation curves of the three primary colors of C, M, and Y at the dark tone. Until the color shift of C, M, Y is eliminated. The CAST function is performed in different regions. In the dark tone portion, only the dark tone plays a significant role and has little influence on other regions. (4)Using the UCR or UCA function, it can be seen from the foregoing that the UCR and UCA can eliminate dark tonality, as shown in Figure 6.13. When UCR/UCA is used, the Y version should be used as the reference, and the starting point of UCR/UCA should be selected at the same time. The slope, removal amount/gain amount should be combined with the process and controlled within a suitable range.
In summary, the correction of color-originating manuscripts should be considered as the analysis of the color cast state of the manuscript and the main body of copying, and the white-field/dark-field calibration, gradation correction, CAST, UCR/UCA, and color correction functions should be flexibly applied to adjust , And check the corrected image status at any time to ensure the correct color reproduction.
Third, the manuscript printing manuscript operation
The use of prints as originals in electronic color separations is extremely high. When prints are used as manuscripts for color separation, only the following three issues are mainly addressed: (1) moire and sharpness, (2) gradation reproduction methods, and (3) backside photocopying to ensure image separation quality.
1. Moire and sharpness: Since the printed matter is the dot to reproduce the image, when the print job is performed as a manuscript, the dot shape of the original, the scanning line, the number of the screen lines, and the screen angle will cause the problem. Moire occurs in the color-processed image, resulting in deterioration of image quality. The method to eliminate the moiré in the actual color separation includes (1) making the network point halo, that is, adjusting the focal length of the sampling head lens until the print halo. (2) Change the number of scan lines, dot shapes, and net angles, that is, when scanning, use different scan lines, dot shapes, and Y, M, and CBK four-color screen angle distributions when the original is copied. (3) Change the subtle level of control parameters emphasized in the electronic color separation machine. The first is to change the relationship between the SHARP and SMOOTH binary values ​​in the subtle level, that is, to decrease the SHARP to improve the SMOOTH; second is to change the subtle levels to emphasize the multiple parameters in the composition. (4) Increase the scanning aperture. Specific operations should be combined with the above methods according to the status of the originals and the purpose of copying. At the same time, we should let customers know that the separation and copying of printed manuscripts will inevitably reduce the clarity of finished products, that is, reasonable choices should be made when both the degree of moire and the degree of clarity decrease are taken into account.
2. Hierarchical reproduction method: the original manuscript contrast is relatively small. When the white/black calibration is performed, the white field should be raised two steps forward. The black field should be advanced one step. The correction can be the same as the color photo. Methods, the intermediate tone levels need to be properly improved.
3. Back photocopies: Back photocopies are caused by too thin or double-sided printing on the printed manuscript. Therefore, in the actual color separation, when the original towel is attached to the roller, the original should be lined with black paper. If the original is a single-side printed matter printed on thin paper and the image contrast is to be increased, the white is covered with white. Paper is appropriate.
When the printed product is scanned for color separation, it should also be noted that the white balance point needs to be selected on the white portion of the original, and the white balance is selected after the automatic balance calibration is completed.
IV. Separation of manuscripts from Chinese paintings and ancient paintings
The manuscripts of traditional Chinese paintings and ancient paintings are composed of five artistic elements consisting of black rhyme, lines, hue, background color, characters, and seals on rice paper. From the color point of view, it has ink, strokes, light color, heavy color four kinds of common characteristics is "based on black, color supplemented"; from the tone of the tone is divided into coke, thick, heavy, light, clear. Therefore, the reproduction of Chinese paintings should be based on a full understanding of the connotation of the original in order to make the reproductions come alive.
Chinese painting
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