Image tone adjustment processing in color report production
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Various factors to consider when choosing CTP
Many large printing companies and prepress companies have already launched CTP systems and have gained some experience. However, there are many companies that are considering purchasing CTP systems that can gain some experience and lessons from CTP pioneers.
The color tone of the image we often talk about contains two layers of meaning, namely color and tone (hierarchy). In the color image of RGB additive mode, the equal amount of R, G, B forms the gray part of the image, and the brightness level is mainly reflected. It depends on this part, it reflects the light and dark details of an image, constitutes the structural skeleton of the image, and the other parts determine the details of the color in the image. The shades and shades constructed with neutral gray components form a saturation. Different colors are produced, resulting in a rich image detail structure. To get a good image, one requires a wider black and white layer and a color level, and the other requires a more reasonable distribution of the image to maximize the important detail in the image. As we all know, RGB images with wider color gamut should be reflected on newsprint with CMYK images with narrow color gamut. It must be properly adjusted by prepress producers. The specific steps are as follows:
First, analyze the manuscript, grasp the original theme
The manuscripts that are handed to our prepress producers are varied in type and subject matter, and some news photos have many problems, so it is important to grasp the manuscript. The manuscript is the basis of our production. First of all, we must thoroughly understand the spiritual characteristics of the manuscript, grasp the key points, and have a good heart. Secondly, we should actively communicate with the customer to determine the customer's requirements for the color and level of the image. Sometimes, we can present it to the client. Some appropriate suggestions to better handle the image.
Second, the white field and black field inspection and setting
White and black fields are the brightest and darkest tonal values on an image. If you don't have work experience, first use the eyedropper tool to check if the diffuse highlights and shadows in the image are within the printable range of the press. If the white and black fields meet the printing requirements, then there is no need to reset their values. However, adjustments are needed in many cases because color photographs have a density range of approximately 2.5 and color positive films can reach 3.0 or more. The greater the density range of the original, the stronger the ability to express its level, the greater the range of color gamut, especially the dark tone, but the surface of the newsprint is rough, the surface flatness and brightness are low, and the dot reduction It is much worse than coated paper. According to the provisions of GB/T17934.3-2003 "Newspaper Cold-Finishing Ink Offset": the number of mesh lines should be 85-120; the tone reproduction range is 5%-85%; the sum of the tone values does not exceed 260% . It can be seen that less than 5% of the tonal value is difficult to guarantee during the printing process, and the details of the highlighted area of the image are lost. Similarly, more than 85% of the dark areas are difficult to ensure the reproduction of the tones, and the dark details mean loss. In order to compensate for the influence of such printability on the reproduced image hierarchy, it is necessary to hierarchically compress the image for printing.
The method of adjusting the white and black fields can use a high light dropper and a dark drop pipe. In the PHOTOSHOP curve and level tool, there are dropper tools, the corresponding dropper is used to set the white field and the black field. Their setting values are based on the specific printing conditions, generally using C, M, Y, K color values. Settings. Double-click the dropper to enter the CMYK highlight value in the color picker to be 5, 3, 3, 0. Similarly, double-click the dark-adjusted dropper to input the common black-field values C, M, Y, and K in the color picker to 65, 53, 5, and 95, respectively.
When the black and white fields are set, the overall level of the original has been compressed. We must also make appropriate adjustments to the original level, and appropriately pull the theme level of the original to be , and the non-thematic level is compressed to better. The theme of the theme is adjusted, and the adjustment of various levels is as follows: (A) The concave curve on the lower part of the figure indicates that the middle adjustment is to the high light level, and the dark level is compressed, so that the image level of the first half is highlighted. B) The concave curve on the graph indicates the dark tone of the emphasized image, (C) the positive sigmoid curve indicates the emphasized image midtone, and the (D) inverse s-shaped curve indicates the highlight and dark tone levels of the image.
As can be seen from (A), if the adjustment point is moved up and becomes (E), you can compress the dark tone level and move the adjustment point down (Figure F). According to the (B) diagram, move the adjustment point up (as shown in Figure G), you can pull the dark tone level apart, and move down (as shown in Figure H) to open the middle light level.
Third, the network expansion compensation
Let's take a look at the 60L/cm square dot gain rate table (for reference only):
From Table 1, it is found that from 10% to 50%, the percentage of the original Internet access point is 40%, and correspondingly, from 19.1% to 68.4%, the percentage of printed Internet access points is 49.3%, that is, the level of print reproduction is higher than The original level range is wide; from 50% to 90%, the percentage of the original Internet access point is also 40%, but from 68.4% to 96.1%, the percentage of dots on printed materials is only 27.7%, that is, the level of reproduction of prints is higher than The original version is narrower. The expansion of the Internet point is not measured in newsprint printing, but it is consistent with the trend of change in newsprint printing, and the dot enlargement rate in newsprint printing is larger, and the expansion rate of 50% of outlets is over 30%, so it is printed on newsprint. In this case, the widening or narrowing of the level of printed dots will be more serious. For this reason, we must make pre-print compensation for the expansion of the outlets. The most common method is to directly enter the 50% dot gain rate in the dot gain setting box in PHOTOSHOP, or use the curve tool to pull 50% of the dot to the corresponding output value or use the transfer function (choose including pass Function box) Appropriate reduction of the corresponding dot and save the output in EPS file format.
Fourth, the adjustment of color cast
First, to clarify the gray balance, the ash in the RGB space is a combination of equal amounts of R, G, and B. The gray balance in CMYK space is that the amount of Y and M is basically the same, and the proportion of green is higher. Remember that these values will increase the speed of your school color, as shown in Table 2:
Secondly, how to judge the color shift, use the screen density meter to measure the original should be gray, such as the scattering highlight area, the shadow of the building, the character's hair, etc. If the RGB values are not equal, the original has a color shift.
Then, use the dropper in the curve tool to define the neutral gray. The specific operation is to move the defined neutral gray dropper to the gray area with color cast in the image, and replace the part with the defined neutral gray value. The value, so the color shift around this tonal range can be eliminated. In general, the setting of the black and white fields is the same as the color correction.
Five, memory color correction
Memory color is a color that people are very familiar with. People appreciate the prints first and notice these colors. If they have problems, they are easy to find. Therefore, the performance of memory colors in prints is very important. It must be made in prepress production. Careful correction of these colors, according to some of the experience of previous people, we should remember to remember the color values of some memory colors, so that the correction can be targeted.
1. Skin tone correction
Skin tone correction is a very detailed job, and Table 3 lists some of the main reference values for the three types of human skin. There is a certain difference in the color of each kind of human race. As far as the Asian race is concerned, it belongs to the yellow race. In the CMYK space, the basic color components are usually yellow and magenta, of which yellow is more than 10% more than magenta. It is the third color, which plays a role in balancing skin tone and tissue contour. Its value is generally 1/2 to 1/3 of magenta. This is a rough standard. Because there are many differences in skin color, there are old people and young people by age. The distinction between children, women and girls, in which the living environment is different, can also make the skin color of their peers very different. Therefore, you must pay attention to the calibration. When correcting all colors, you should adjust it in small steps, then adjust the effect and then adjust it. The correction for skin color is no exception. In some local newspapers, there are more advertisements. It is best to put some beautiful head pictures with the same color tone on the same layout for easy adjustment and printing. There are many important figures in the newspapers such as People’s Daily and Liberation Daily. These images must be carefully processed and the processed curves are saved so that the curve is loaded the next time you encounter a similar image.
2. Blue sky correction
We live with blue sky on our heads, the blue sky gives people a very comfortable feeling, the memory of the blue is profound, and the pictures with larger areas are generally landscape photos, of which the green and yellow content of green plants is heavier and green. Do not arrange magenta in the pan-colored area, but arrange yellow in the sky, not only the blue looks impure, but the picture will lack space. To deepen the blue sky, apply a neutral black. Moderate color exaggeration, the visual experience of the human eye is comfortable. The basic color of the blue sky is blue, followed by magenta, as shown in Table 4:
3. Some typical color components
Not only skin color, blue sky is memory color, there are many common colors in daily life, they are deeply imprinted in people's brains, these colors are listed in Table 5:
Sixth, the problem should be clear in the color correction process
The scanners, monitors, etc. we use should be calibrated. Don't neutralize everything: keep some picture color special effects, such as some performing light pictures; for pictures with severe color, leave some color cast to avoid too much distortion of the original. The color of the normal manuscript must also be processed: because the newspaper itself has poor whiteness and has a gray bottom, from the perspective of color processing, the color should be used less, the primary color and the intermediate color should be used, and the amount of background color removal should be large. In particular, the advertising version, try to increase the saturation of the color, so that the advertising color is bright and bright. If you use a scanner with a higher grade, the scanning software has adjustment tools such as exposure, density range setting, black and white pole, and tone curve. Then the image level correction and color correction should be performed during scanning. Correcting colors in RGB space allows for fine adjustments in CMYK space.
Production is the basis of printing. The production must reflect the original theme, but also meet the specific printing conditions, avoiding some quality problems, such as uneven color, unclear level, paste, scum, etc., so in the production process, for some non- At the main level, the “and” is “and”, so that the main level has a certain degree of expansion, and the general color should not be placed in the opposite color to make the color more pure and vivid.