Color Tone Adjustment in Image Making

When selecting a CTP, various factors to consider

Many large printing companies and prepress companies have launched CTP systems and have already had some experience. However, there are many companies that are considering purchasing CTP systems. These companies can gain some experience and lessons from CTP pioneers.

The image hue we often talk about contains two meanings: the color and the tone (level). In the color image of the RGB additive mode, the same amount of R, G, and B forms the gray part of the image. Rely on this part, it reflects the details of the brightness and darkness of an image, constitutes the structural skeleton of the image, and other parts determine the details of the color in the image. The neutral gray component adds color to the light and dark levels of the construction to form saturation. Different colors, resulting in a rich image detail structure. To achieve a good level for an image, one is to require the image to have a wider black and white levels and color levels, and the other is to ask the image level to have a more reasonable distribution in order to maximize the important details of the image. As we all know, RGB images with a wider color gamut need to be embodied on newsprint with narrower color gamuts of CMYK images. They must be properly adjusted by prepress producers. The specific steps for adjustment are as follows:

First, analyze the originals and grasp the originals to reflect the theme

The originals passed to our prepress makers are diverse in terms of type or subject matter, and there are also many issues in news photos, so it is very important to grasp the original manuscript. The manuscript is the basis for our production. First of all, we must thoroughly understand the spiritual features of the manuscript, seize the main points, and have a heart. Second, we should actively communicate with customers to determine the customer's requirements for colors and levels in the image, and sometimes, it can be presented to the customer. Some appropriate suggestions for better handling of images.

Second, the inspection and setting of the white field and the black field

White and black fields are the brightest and darkest tone values ​​on an image. If you don't have work experience, first use the dropper tool to check if the scattered highlights and shadows in the image are within the printable range of the press. If both white and black fields meet the printing requirements, then there is no need to reset their values. But in many cases it needs to be adjusted because the color photo density can reach about 2.5, and the color positive film can reach 3.0 or even above. The greater the density range of the manuscript, the stronger its ability to express the level of performance, the greater the range of color gamut performance, especially the better performance of the dark tone, but the newsprint surface is rough, the surface roughness and brightness is low, dot restore It is much worse than coated paper. According to the requirements in GB/T17934.3-2003 "Newsprint Coldset Offset", the number of mesh lines should be 85-120; the tone reproduction range should be 5%-85%; the sum of tone values ​​should not exceed 260%. . It can be seen that less than 5% of the tone value is difficult to guarantee in the printing process, and the details of the highlight area of ​​the image will be lost. Similarly, more than 85% of the shadow areas are difficult to guarantee the reproduction of the tone, and dark tone details means loss. In order to compensate for the influence of this printability on the level of the reproduced image, it is necessary to perform gradation compression on the image for printing.

The method of adjusting the white field and the black field can use a high-light dropper and a dark dropper. There are dropper tools in PHOTOSHOP's curve and level tools. The corresponding dropper is used to set the white field and the black field. Their setting values ​​should be based on the specific printing conditions, generally with C, M, Y, K color values. Settings. Double-click the eyedropper to enter the CMYK highlight point value in the color picker as 5, 3, 3, 0. Also, double-click the dark tone pipette to enter the common black field values ​​C, M, Y, and K in the color picker respectively 65, 53, 5l, and 95.

When the black and white fields are set up, the overall level of the manuscript has been compressed. We must also make appropriate adjustments to the original manuscript level to properly pull the theme level to be reflected in the manuscript. The non-theme level should be compressed so that it can be better. The details of the theme are set off, and the adjustments of various levels are as follows: (A) When the curve of the concave level represents the middle tone to the high light level, the dark tone level is compressed, so that the first half of the image level can be highlighted. B) Concave curve in the graph indicates the dark tone of the emphasized image, (C) Positive sigmoid curve indicates the midtone of the emphasized image, and (D) The inverse sigmoidal curve indicates the highlight and shadow levels of the image at the same time.

From the (A) diagram, we can see that if the adjustment point is shifted upwards to become the (E) diagram, the level of the dark tone can be compressed, and the adjustment point can be shifted down (as shown in Figure F). Gradually, some layers are opened; according to (B), shifting the adjustment point upwards (as shown in Figure G) can open up some of the darker levels, and moving down (as in Figure H) can pull some of the mid-level highlights off.

Third, 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 sites is 40%. Correspondingly, from 19.1% to 68.4%, the percentage of printed Internet sites is 49.3%, that is, the level of print reproduction The scope of the original level range is wide; from 50% to 90%, the original percentage of Internet sites is also 40%, but from 68.4% to 96.1%, the percentage of printing sites is only 27.7%, that is, the level of reproduction of the print range than The original level is narrower. The above network expansion is not measured in newsprint printing, but it is in line with the trend of changes in newsprint printing. Moreover, the expansion rate of outlets in newsprint printing is even larger, and the expansion rate of 50% of outlets is more than 30%, so it is printed on newsprint. In the middle, the widening or narrowing of the range of printing outlets will be more serious. For this reason, we must make pre-print compensation for the expansion of outlets. The most common method is to directly enter the 50% dot gain rate in the dot increase setting box in PHOTOSHOP. You can also use the curve tool to pull the 50% point to the corresponding output value or use the transfer function (selection includes passing Function box) Make appropriate reductions to the corresponding outlets and save the output in EPS file format.

Fourth, the adjustment of color cast

First of all, to clarify the situation of gray balance, the ash in the RGB space is the synthesis of equal amounts of R, G, and B components. The balance of gray balance in CMYK space is that Y and M are basically the same, while the proportion of blue is higher. Remember that these values ​​will increase your speed of color correction, as shown in Table 2:

Second, how to determine the color cast, use the screen densitometer to measure where the originals should be gray, such as diffuse highlights, shadows of buildings, human hair, etc. If the RGB values ​​are not equal, then there is a color shift in the original.

Then, neutral ash is defined by the dropper in the curve tool. The specific operation is to move the defined neutral ash pipette to the gray area with color shift in the image. Click to replace the site with the defined neutral ash value. The value, then the color cast around this hue range can be eliminated. Under normal circumstances, the black and white fields are set at the same time as the color correction.

Fifth, memory color correction

Memory color is the color that people are very familiar with. People appreciate that the prints first notice these colors. If they have problems, they are easy to find. Therefore, the memory color in prints is very important, and it must be done before printing. Carefully correct these colors. Based on some of the lessons learned by previous researchers, it is best to remember the color values ​​of some memory colors so that they can be targeted for correction.

1. Skin color correction

Color correction is a very meticulous task. Table 3 lists some of the major reference values ​​for the three categories of skin color. Each kind of human skin color has certain differences. For Asian races, it belongs to yellow species. In CMYK space, the basic color components are usually yellow and magenta, and yellow is 10% more than magenta. It is the third color, which plays a role in balancing skin tone and tissue contours. Its value is generally 1/2 to 1/3 of the magenta color. This is a rough standard because there are many differences in skin color. There are old people and young people according to age. The distinction between children, women, and young girls, including differences in their living environment, can also make the skin color of their peers different. Therefore, care must be taken when correcting. When correcting all colors, you should make small adjustments, look at the effect and adjust it later, and correcting for skin color is no exception. In some local newspapers, there are many advertisements. It is better to place some of the beautiful head pictures that are basically the same color in the same layout for easy adjustment and printing. People’s Daily, the Liberation Daily and other newspapers have pictures of many important people. These pictures must be carefully handled and the processed curves must be saved so that the curves are loaded the next time they encounter similar pictures.

2. Correction of blue sky

We live on a blue sky. The blue sky gives us a very comfortable feeling. The blue-blue memories are profound. The pictures of a large area of ​​the sky are generally landscape photos, in which the green plants have a heavy yellow and blue content. Do not arrange magenta in the pan-colored place, and arrange yellow in the sky. Not only does the blue look unclean, but the picture lacks space. To deepen the blue sky, apply neutral black. Moderate color exaggeration, giving the human eye visual experience 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 the memory color, there are many common colors in daily life, and they are deeply imprinted in people's brains. These colors are listed in Table 5:

Six, color correction process should be clear

The scanners, monitors, etc. we use should be calibrated. Don't neutralize everything: Maintain some special color effects for pictures, such as some performing arts lighting pictures; for color-difficult pictures, leave some color deviations to avoid distortion of originals. The color of normal manuscripts must also be handled: due to the poor whiteness of the newspaper itself, with a gray bottom, so from the perspective of color processing, should use less complex colors, multi-primary, inter-color, the amount of background removal should be large, Especially the advertising version, try to increase the saturation of the color to make the advertisement bright and bright. If you use a higher-grade scanner, the scanning software has adjustment tools such as the exposure amount, density range setting, black and white poles, and tone adjustment curves. The picture gradation correction and color correction should be performed during scanning. Correcting colors in RGB space can be finely tuned in CMYK space.

Production is the basis of printing. Production must reflect the original theme, but also meet the specific printing conditions, to avoid some of the quality problems, such as uneven color, unclear level, paste, dirt, etc., so in the production process, for some non- At the main level, the “harmonization” is “harmonized” so that the main level has a certain degree of expansion. The pan-colored office should not be placed in the opposite direction, so that the colors can be more pure and bright. ◎

Reprinted from: "Guangdong printing" 2005 4 Author: Wu Ming root