Getting started with color correction

A good printed picture gives a person a feeling of comfort and naturalness, as if he had witnessed the scene with his own eyes. Good copying depends on good manuscripts, but most of the light or photo manuscripts used in daily printing often have color casts due to environmental or light limitations. These coloured images are very different from our daily visual experience, such as the purple skin tone, the greenish sky and so on. If these images are printed directly without any treatment, the effect is certainly not good. Visible, exactly the same as the original, is often not the best way to deal with. A good designer corrects color casts so that the final print results are consistent with the human eye's visual habits.
Making the image look natural is the basic principle of color correction. However, this is easier said than done. If you do not understand the principle of printing, this requirement is often difficult to achieve. It is common to see that some inexperienced designers are very careful to use photoshop to work in front of the screen and spend a lot of time thinking that they will get the ideal picture. However, the result of printing is disappointing. It is a pity. The reason for this is that designers rely on screen effects to judge the color of a print. Almost all screens have their own color cast, but only to a certain extent. Looking at the same photo with different screens often has different effects. And the screen is a kind of light source. If you look at it for a long time, the human eye will have a natural adjustment effect and adapt to the color cast of the screen. The color judgment will lose its objectivity, resulting in inconsistent results of color correction. Therefore, the screen is not an ideal tool for judging the color of printing. To master the professional color correction skills, the first is to understand the principle of printing, know the color dot combination, and at the same time know how to read the image of the dot data. In this way, it will not be confused by the screen. Even if another screen is changed for work, a consistent and ideal color correction effect will also be obtained. Before correcting color pictures, the first thing to do is to make correct color judgments. The basis for judgment is the gray balance of printing. The part that appears to be neutral grey on the screen, if it is not gray, is a picture that looks like a biased image. How to predict the result before the picture is printed and predict what color it will be is what designers need to master. There is no mystery about this kind of knowledge. If you understand the principle and practice, you will be able to master it. In theory, the same amount of CMY (cyan, magenta, yellow) inks can be mixed to produce a neutral gray. (In other words, 50C+50M+50Y can produce 50% gray). However, the ink in the real world is limited by the industrial production conditions. The color rendering capability of CMY is not the same. Among them, the color rendering power of magenta is the strongest, and the color rendering power of blue is the weakest. Therefore, the same amount of CMY ink will not be produced. Neutral gray (will be reddish gray), in order to get neutral gray, must increase the proportion of blue (C) ink weight, which is the basic principle of Grey Balance. For example, to produce 50% neutral gray, the proportion of green is 13% more than that of magenta, and the proportion of magenta and yellow ink may be about the same. If 30% of neutral gray is to be produced, it is required to have 9% more than magenta. . Different levels of neutral gray, magenta, and blue may also have different mixing ratios. In the light gray part, be especially careful because the human eye is most sensitive to light-colored color casts. The proportions of cyan and magenta in different situations may refer to the attached table. Note that the numbers here are only a reference. Actually different brand inks can make the ratio of outlets 2% to 5%. It's best to compare with your printing company. They provide different proportions of green and magenta mixed neutral gray for your reference. This will make the judgment more accurate. Neutral gray density Green magenta/yellow 0.204%2%0.3010%6%0.4320%13%0.6030%21%0.7840%29%0.9850%37%1.2560%46%1.5870%57%2.1090%71%2.6890%82%2.8095% 87% With neutral gray dot data, it is equivalent to having a reference color scale. It is very convenient to judge image color cast. As long as the neutral gray part of the image is found, the ratio of the dot is displayed in the photoshop using the color picking tool. If the reading obtained is in agreement with the standard, it can be concluded that the figure has no color cast, otherwise it will be based on exceeding or Lower than the standard part, to make corrections to the image. Naturally-looking pictures are often the results of meticulous care by designers. Some color masters can even correct color pictures with black and white screens. In the process of learning to correct colors, it is very important to understand the correct concepts. Mastering the relationship of printing gray balance is the basic task of repairing color.

In the previous article, we must first sharpen our tools to make good use of our products. Because of the color pictures we produce, we finally present them in a printed form. We understand the principles, characteristics, and limitations of the color printing process. It is very good for making high-quality pictures. important. Without this knowledge, it would be like sailing without a compass. All relying on speculation, it may reach its destination, and it may never reach it. Off-line offset printing technology is now used to express colors in the shape and arrangement of dots. Common dot shapes include dots, squares, and ellipses, and the dot size is expressed in hundredths. In general, the minimum point printed on a four-color offset printer is 2%, and the maximum is 85%. These numbers represent the proportion of ink coverage in a single dot area. Why is the maximum area of ​​dots not 100%? This is due to the size of the printing dots and is also affected by two other factors. The first factor is the total amount of ink in the printing grid (ie, the amount of four-color ink that can be tolerated per unit area). The second factor is "dotgain". Due to the excessive supply of ink, the paper will be excessively wetted, causing problems in production, such as affecting registration due to paper deformation, the total amount of four-color printing is not more than 350%, so the maximum value of individual printing dots It will not be more than 85%. The dot gain value is the result of the interaction of the ink characteristics, the printing pressure, and the ink receiving ability of the paper. The factors of printing pressure and paper are more decisive. In general, high-quality light flour paper has a smaller dotgain (about 15% to 20%), and the dotgain of newsprint can be large (approximately 25% to 35%), so that the same set of film is used to make the zinc plate and print on different papers. , color effects can be very different. Relatively speaking, printing pressure is a variable that can be grasped by the printing factory. Some printing companies even set the average outlet value of each printing press under normal conditions to be used as production quality control. Since the dotgain will affect the final color effect of the picture, it is a variable that we need to take into account when we do the image correction. Before the color correction of an image, there is usually a review step, which is to see if the level of the scanned image is normal. Most of the pictures can be divided into three parts, namely the bright part (light color part), the middle tone part and the dark part (dark part). This part of the picture constitutes the light and dark levels of the entire picture. The troubles of color correction and pitfalls in beginners are excessive attention to color and ignorance of levels. In fact, whether or not pictures are expressive, and the level is often the key factor. To grasp the changes in levels, we need to start with the light and dark positions. The light position is the smallest part of the total CMYK points in the picture. The dark bit is the largest part of the total CMYK points. These two places are the start and end of the picture hierarchy. To make it easier to understand, imagine the picture as a room. The light position is the light source in the room. If the light source is bright, the whole house will be brightened. If the light source is dark, the whole
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