How to Avoid the Shortcomings of Traditional Lithographic Offset to Improve Color Copy Quality (I)

The development of traditional lithographic offset printing technology has become a mature technology, but it still has some limitations. As an applicator, we should try our best to avoid the weaknesses of technology and use its strengths to serve our work to achieve satisfactory results.

1 Compression of color gamut - RGB color gamut is larger than CMYK color gamut

As we all know, the scope of CMYK rendering color is much smaller than that of RGB color space. Why is this? This is mainly caused by the printing of raw materials, the first is the ink, we compared the yellow, the product, and the blue three-color ink ideal absorption light map and the actual absorption of light, you can find the difference, the ink absorption of light in the absorption of the spectrum is not complete Absorbed, the reflected spectral segments are not completely reflected. In this way, the actual ink produces “grayscale”, and this “grayscale” greatly compresses the range of the print's expression color; followed by paper, paper is the basis for color rendering, and the whiteness of the paper determines the color brightness of the picture at high light. And saturation, which affects the contrast of the color; as well as the absorptivity and smoothness of the paper, play a crucial role in color rendering. Therefore, print cannot truly reflect the rich colors in the natural world. What we are doing is just simulation. Here, on the one hand, we hope that ink factories and paper mills can produce better inks and papers to better represent colors; on the other hand, as print designers, in order to allow prints to reflect bright colors, the cost allows Use inks and papers with good printability as much as possible.

2 Tone compression and adjustment - the original contrast is much larger than the print contrast

The ideal reproduction condition of the tune is a linear copy, but the reproduction density that the print can achieve is only 1.8 to 2.0 at the highest, and the normal density of the natural color original has a maximum density of about 3.0, and most of the contrast is above 2.2, so the original must be In order to compress the gradations, it is necessary to lose some of the subtleties. In practical operation, a reasonable level curve should be adjusted according to the contrast of the manuscript and the specific content of the tone, that is, the level of the minor part of the manuscript should be compressed. Make the main site level emphasized. Now we can use PHOTOSHOP's curve palette to adjust RGB channels for different themes of the image. As shown in Figure 1, the vertical axis represents the output density, and the horizontal axis represents the input density. (A) The concave curve represents the highlight of the emphasized image, and the (B) concave curve represents the shadow of the emphasized image. (C) Positive sigmoidal curves represent the midtones of the emphasized image, and (D) the inverse sigmoidal curves represent 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.

From the above it can be seen that the level adjustment in PHOTOSHOP is very convenient, but there is one point to note that adjusting a part of the hierarchy will affect the other part of the hierarchy. We should adjust a reasonable level curve according to the characteristics of different manuscripts and correct it. The insufficiency of the manuscript maintains the main part, giving up the minor part and making the picture better.

3 Gray balance--CMY three-color dot ratio curve does not coincide

The same amount of CMY blending results in a gray color, while the actual ink yellow, product, and cyan blending results in not a gray color but a partial brown color. To make the composite colors of the three primary colors at all levels be neutral gray, the gray balance data of the three color versions of yellow, magenta, and cyan can not be controlled on a single curve. Through practice, it is proved that three kinds of printing primary ink dot sizes of gray printing Usually different, cyan dots are usually larger than yellow and magenta dots. As shown in Table 1, a set of neutral gray reference data.

The gray balance in color reproduction has become the scientific basis for our prepress production. As long as the proportions of the three color version dots on the gray part of the print are correct, the three color dot ratios of other colors can be surely correct. Based on the gray balance data, we can use the dropper tool on the PHOTOSHOP's curve palette and refer to the values ​​displayed on the information palette to find the point where the gray balance appears in the image, determine whether the image is color cast or neutralize the pole. .
(to be continued)