Precautions during image processing

In order to become a qualified image processing personnel, it is necessary to master the following aspects of knowledge: basic printing knowledge, basic color science and color management knowledge, the basic printing process of the unit or downstream, PHOTOSHOP about "color Basic understanding of settings, understanding of the characteristics of commonly used tools, data structure characteristics of type pictures, reasonable processing steps combined with specific processing content and corresponding correct data control.

It can be said that all the content of the quality control of offset printing is based on the outlets and the transmission of the outlets. If there is no basic knowledge of the outlets, at least it is impossible to understand the significance of the data changes on the information panel in PHOTOSHOP . Without the basic knowledge of color science and color management, it is impossible to accurately control the color separation process; at the same time, the most accurate screen display also needs to be achieved through color management. "Color setting" is closely combined with the printing process. Without knowing the back-end printing process, the correctness of the color setting cannot be guaranteed, and the correct color separation is even more impossible to talk about. The so-called common tools are not just "brightness / contrast", "color scale" and "curve". If only these three are used, I am afraid that the visual effects and work efficiency are also unsatisfactory. At the same time, the data control of key content It will also be out of control (I personally do not advocate the use of the "brightness / contrast" tool, which cannot effectively control the accuracy of data control). As for the characteristics of each tool, I think it should be based on in-depth understanding of strengths and weaknesses, complement each other.

The above said I think it should be the basis for good image processing. In fact, the process of image processing is not only a process of visual beautification, but also a process of data control. Only visual beautification based on a reasonable data structure can be realized. In layman's terms, it looks good when printed on the screen. It must be fine. After all, all the efforts in the early stage are for the back-end printing reproduction. If the printing reproduction is uncontrollable, all your efforts will be less effective.

Relevant knowledge, screen correction, color separation settings and other content will also have a special article to introduce to you, this article is limited to the precautions in the image processing process.

The same is image processing. The characteristics of business and newspaper are very different. Let's talk about the business field first.

In commercial applications, high-quality manuscripts and poor manuscripts also have different concerns in the processing process. Generally speaking, the high-quality manuscripts obtained by the processing personnel are already lascivious by electrical separation and other processes. Some people will think that all manuscripts must be good, because such manuscripts are often good in visual effects, but it should be emphasized that this is a misunderstanding. We must check whether the data structure of the picture is through our own inspection. Reasonable, because the screen display effect does not fully reflect the real situation of the picture data, especially on the gray axis. The quality of a picture does not entirely depend on the color and gradation. Errors in gray balance data and black plate structure will often give you a fatal blow during the printing process. Really at that time, the loss of materials will not be mentioned for the time being. I am afraid that time is irretrievable, so it is very important to check in advance. Generally speaking, the overall contrast of the picture depends on the total ink volume. The length and thickness of the black version are determined by the content characteristics of the picture. The gray balance data is determined by the back-end printing. If the data deviation is large, do n’t expect to be able to Make corrections in PHOTOSHOP, if necessary, re-distribution. In fact, a good manuscript does not need to be adjusted too much, it is nothing more than the optimization of details, but it is a manuscript with average or even poor quality. It will be difficult to process. At this time, calibration is very important, without calibration. Of manuscripts often fail to guarantee overall contrast and color reproduction. Some people are used to calibrating in CMYK mode, I think it is not appropriate, should be calibrated in RBG mode, and after the appropriate adjustment of the brightness and contrast of the picture before color separation, because adjusting the brightness and contrast in CMYK mode will It has a very bad effect on the gray balance data.

How a good manuscript is faithfully reproduced for business, but for the newspaper industry, any manuscript needs to be re-optimized, it is nothing more than the difference in the number of steps and the difficulty.

To be realistic, the production of the newspaper industry is characterized by its strong timeliness, which requires higher production efficiency per unit time, and in terms of the scope of adjustment and the range of change, the newspaper pictures must be much larger than the commercial pictures. In this case, the idea of ​​adjusting the image becomes more important. First of all, we need to analyze the pictures. What is the problem and what is the nature of the problem? What should be adjusted first, what should be adjusted later, what tools should be used to adjust, etc., must be thought in advance. The manuscripts of newspaper pictures, whether they are the manuscripts of the newspaper ’s photographers, Xinhua News Agency or online pictures, are basically RGB, and the newsprint printing network has expanded far more than coated paper. 4 colors use a large amount of background color removal, which determines that the picture adjustment process basically needs to increase the brightness, but also control the color separation. How to maintain the coordination of brightness and contrast during the adjustment process, how to ensure that the local adjustment will not affect the overall data structure of the picture, how to ensure the coordination and unity of color saturation and neutral gray, this is before real hands-on It is necessary to consider clearly, because fundamentally, the process of adjusting the picture is actually a process of constant loss. For example, the brightness of the mid-tone tone is increased, the surface of the mid-tone tone is more distinct, and the contrast is greater. At the cost of sacrificing the level of dark tone, but you think this sacrifice is acceptable, and after you have adjusted a few steps, if you find that a previous adjustment is inappropriate, often unwilling to retreat to the original because of time. That step was readjusted, but a patch was made on the existing basis. In fact, this is upside down. Not only the original inappropriate places may not be repaired, but also may have undesirable effects on the places that were not used, so it is very important to analyze the pictures before starting.

Regarding calibration, I would like to say a few words. For high-quality manuscripts, it is generally necessary to borrow black-and-white fields during the time-sharing, which is necessary from the perspective of restoration, but for general-quality manuscripts, especially those used for newspaper printing Most of the industrial pictures are social and humanistic manuscripts). Compared with high-quality art manuscripts, such manuscripts do not matter how to restore, but they pay more attention to how to enhance the visual impact. Quite different. First of all, it is reflected in the calibration. Because the quality of such originals is uneven, the black and white fields are directly selected in the picture content. The calibration can first solve the problem of overall color cast and insufficient contrast to a large extent. The white field of individual manuscripts will deviate during automatic calibration and need to be corrected manually, but generally speaking, the calibration will lay a good foundation for the following further adjustments. Everyone is used to adjusting the picture level with a curve. If the scale is not calibrated first, this adjustment will often result in insufficient dark contrast. Commonly used tools are not limited to curves and color levels. Skilled use of tools such as color balance, hue / saturation, matching colors, replacing colors, and optional colors will greatly improve the quality and efficiency of image processing. For tools related to color adjustment, users must at least know the relationship between hue, saturation and brightness, the relationship between different color spaces, how to control the super color gamut, etc. At the same time, the meaning and adjustment of each parameter in the tool You must also have a certain understanding of the impact of the image data in the process, otherwise blind use will do more with less.

Finally, I want to share with you the experience of using the USM sharpening tool. I think that USM sharpening is actually a damage to the subtle level, but only from the visual effect to the sharper edges, so the sharpening step should be placed Finally, after all the content of the picture has been processed, it should be sharpened. At the same time, the error of the screen display should also be considered. The screens we use are all displayed in the form of pixels, and TIFF and JEPG as bitmap format are also reflected in pixels, so when the screen resolution and the picture resolution do not correspond to one-to-one or integer multiples, then There will be a display error. At this time, the sharpening, the preview effect we have seen is also inaccurate. In addition, if the display size of the picture on the screen is close to the actual size, the reference of the sharpening effect is also helpful. Of course, it is also essential to have a clear understanding of the concepts of "quantity", "radius" and "threshold" in the tool and the correlation between the sharpening process.

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