Small Gold Gift Paper Bag The Impact Of Prepress Processing Of Images On The Quality Of Printed Matter (2)

Small Gold Gift Paper Bag The impact of prepress processing of images on the quality of printed matter (2)

 

Two, digital image prepress processing

The pre-press processing of digital images should generally be carried out from the aspects of image hierarchy, color and clarity. If an image is good in these aspects, it is an image that meets the requirements of reproduction from the printed manuscript. In terms of printing requirements, it is required to faithfully restore the original manuscript, but the limitations of printing tell us that it is difficult to faithfully restore, so the consistency of the visual appearance effect is a good supplement to the faithful restoration, but because everyone's visual habits and aesthetic habits are different, everyone's scale is not the same when adjusting, so it can only be described qualitatively.

 

1. Adjustment of image level

The adjustment of the level is to deal with the high profile, middle and dark tones of the image, and reproduce each level as much as possible. But as has been said before, under normal circumstances, the density of the original manuscript can reach 3.0 or higher, while the density of printed matter can only reach 1.8 or 2.0, the density range of printed matter is far less than the density range of the original manuscript, the level must be compressed, then how to compress printed matter to make the original manuscript restore better, we are currently using the method of process compression curve, The purpose is to make the appearance of the print effect is basically close to the appearance of the original manuscript, the visual appearance effect here refers to a judgment of our eyes, rather than using density instruments to measure, in fact, that is to say, "look close to the effect" even if the purpose is achieved. The adjustment of image level can be analyzed from the following two aspects.

 

① Calibration of high light and dark tone

Highlight and dark refers to the brightest and darkest tone values on an image, also known as white field, black field, if you want to adjust the image for printing, you need to consider how to set the value of highlight and dark, because of the high light of the printed image, generally brighter than 3% ~ 5% of the area is not printed, that is, 3% ~ 5% of the area has become 0%, that is, the white of the paper. In this way, the layers of the high-brightness areas of the image are lost. On the contrary, more than 95% of the dark areas will be printed as 100% black, and this part of the dark level will be lost. This is the shortcomings of printing, in order to compensate for the impact of this deficiency on the level of the reproduced image, it is necessary to carry out hierarchical compression of the image for printing, for example, compress 0% white to 5% gray, compress 100% black to 95% dark gray. It's not necessarily 0% or 100%, it's usually 2% or 98%.

 

The best way to set the highlights and dimmers for printing during image processing is to use Photoshop's highlight and dimmer droppers. The dropper tools are available in Photoshop's Curves and Levels tools. One of their features is specifically for setting the poles. The hue between poles on the image is redistributed according to the range set by the poles. The setting of the pole color value depends on the specific printing conditions, there is no unified standard, in most cases, the commonly used CMYK high light pole value can be set 5, 3, 3, 0, and the dark pole value can be set 65, 53, 51, 95 or 95, 85, 85, 80. To set up, just double-click the highlight and dimmer droppers in the Curves and Levels tools and enter the Settings, as shown in Figure 3. If it is set in RGB color mode, then the equivalent highlight point of RGB is 244, 244, 244, and the dim pole value is 10, 10, 10, and the same can be set. After setting up the highlight and dimmer dropper, all you need to do is click the highlight and dimmer dropper at the selected highlight and dimmer points in the image. So how do you determine highlights and darks in an image?

 

Usually we understand the printing highlights can be divided into two kinds, one is the point without information, that is, 0%, called mirror highlights. The other is a highlight point with information with details, called a scattered highlight. To determine the printed highlight is actually to find the appropriate scattering highlight in the image, which can be used in Photoshop to check the color value of the important scattering highlight, determine whether it is located in the printing range (5% ~ 95%), if in this range is not adjusted, if the scattering highlight is not in the printing range, For example, 2%, and this point is important and needs to be printed, then use the defined highlight dropper to click on the scattering highlight point on the image. Also select the darkening pole and define it with the darkening dropper. In this way, after the two highlight and dark poles we choose are defined, the tone between the highlight and dark poles on the image will be redistributed according to the range of highlight and dark Settings, that is, the image level is corrected according to the characteristics of the printing. After setting the highlight and dark poles, in order to achieve the consistency of visual effects with the original manuscript, it is also necessary to adjust the process compression curve of the image. Below we will analyze several typical process curves.

 

② Several typical process curves

For a given print manuscript, after the high light and low tone calibration, it is also possible to artificially change its hierarchical reproduction, the level of the digital manuscript is compressed, extended or left unchanged. Since the density range of the print is far less than the density range of the original manuscript, the level must be compressed, and in the compression process curve, the level of one area of the image is compressed, and the corresponding level of another area will be extended, the purpose of this compression and expansion is to make the print as close to the original as possible from the visual effect. In practical application, according to the characteristics of the original manuscript, we should grasp the key levels of the image to be copied and emphasized, and make corresponding abandonment of the secondary levels. For manuscripts with normal contrast, the low density area can reach 0.2-0.3D, the high density area can reach 2.1-2.9D, the maximum density contrast can reach 2.7D, the color is bright, the level is rich, and the highlight to the intermediate level of such images should be emphasized as the main body of the image. And some manuscripts, such as night scenes, backlight photography, etc., the dark part of the area is large, is the main body of the picture, at this time need to emphasize the dark level. In short, it is necessary to discriminate according to the characteristics of the original manuscript and master it flexibly. Figure 4 shows several typical stepwise process curves.

 

The curve (1) is the distribution of the high light level, the dark level is compressed, suitable for the dark area is small, need to emphasize the light level of the manuscript, such as the overexposed manuscript during photography; The curve (2) represents thinning and opening the darkening level, the highlight level is compressed, suitable for the darkening area is large, the picture is dull, the darkening level needs to be pulled apart, so that the entire picture brightens the manuscript, such as the manuscript caused by insufficient exposure during photography and the night scene image; The curve (3) is that the highlight and dark tones are pulled apart, and the middle tone level is flattened, which is suitable for the original manuscript with dull picture and flat highlight level, such as the snow scene picture; Curve ④ is the case that the high light and dark levels are compressed, and the middle tone level is pulled, which is often used as a curve, suitable for the original manuscript is thin and the density contrast is small.

 

From these several typical process curves, we can see that in order to make the print closer to the original manuscript, we must consider all aspects of the factors, choose the best tone process curve.

 

[NextPage]

2. Image color correction

Color reproduction refers to the decomposition of color, transmission, synthesis of a complex process, color reduction is also a major aspect of printing reproduction, in the process of color reproduction, subject to factors such as scanning light source, lens, color filter, photoelectric conversion system, sensitive materials, paper, ink and other factors, color error is inevitable. In particular, the problem of printing level compression and ink, which has a crucial impact on the original color, in order to obtain the ideal color reproduction, we must try to correct these color errors and achieve the ideal color reproduction.

 

① Preparation before color correction

First of all, we must carry out equipment calibration and system calibration, these devices include scanning equipment, display equipment, output equipment and proofing equipment, these devices have to undergo strict professional calibration, the other is to have a relatively perfect color management program between these devices. These are the basis for our color correction. Here to talk about the display equipment, in image processing, the appearance of the image color is reproduced by the display before printing, the display is based on RGB mode, and the final product we want is to use ink to restore the printed matter on the paper, is CMYK mode, using RGB display equipment to reproduce the CMYK image, is bound to affect the appearance of the color. Therefore, this conversion (color management system) in the display must be accurate, but also to maintain the consistency and stability of the lighting environment light source, in order to make the screen display and proofing as consistent as possible.

 

Secondly, the level of correction should be carried out before the color correction. Because according to the color mechanism, the color is presented on the basis of neutral gray level, so the level should be corrected first and then color correction, otherwise, after the color correction is completed, the color will change when the level is corrected.

 

Then there is the question of what color mode is reasonable to correct colors in. In Photoshop, no matter whether the image is RGB mode or CMYK mode, it can be corrected in order and color, which color mode is different. The advantage of using RGB color space for correction is that the color gamut space is large, which is consistent with the color space of the display. However, because it must be converted to CMYK space when it is used for printing output after correction, there will be some colors that cannot be displayed in the CMYK gamut, that is, the color of the image exceeds the printed gamut, which is called overflow color. The advantage of color correction in CMYK color space is that the corrected image is directly used for printing without color overflow. In addition, because the CMYK color space is a color space that conforms to people's visual habits, it is easier to grasp the color changes when representing a certain color and its changes. In view of these aspects, it is generally possible to correct the image in the RGB color space, and then fine-tune the image in the CMYK color space.

 

② Discrimination of color bias

Gray balance is a very important concept for color correction. We need to use the concept of gray balance when we judge the color bias of the digital image after scanning. If you know the primary color components needed to produce neutral gray of various brightness (see Table 1), you can use the neutral gray area in the original manuscript for color correction. Use the Screen Density tool (Info) in Photoshop to measure the color value of a digital image. If the value of the area that should be neutral gray is not the value of the gray balance, it indicates that the image has a color bias. According to the proportion of the gray balance, it is easy to determine which color is more and which color is less.

 

The scattered highlight area in the image is the best area to check the neutral gray, the highlight area is not all neutral gray, but relative to other brightness colors, its gray component is more, so from here to check, not only can judge the color bias, the previous highlight pole calibration is also started from here.

 

In addition, there are many other experiences in the identification of color bias, such as memory colors, such as blue sky and white clouds, green grass and so on, these colors have a very deep memory in people's minds, for professional image processing personnel, it is more important to remember the CMYK ratio of these colors. The more you remember, the better you can judge if the color is accurate.