Reading the "Printing Technology" period 2006.11, Mr. Ding Yi compiles a "high-precision printing" article, which makes the author benefit a lot. Mr. Ding is recognized as a highly respected expert in the printing industry. He can read Mr. Ding’s article, and his experience in printing new technologies is a blessing.
Under the heading “2. Printing effect†in the “Technical Discussion†section of the article, Mr. Ding’s discussion of the effects of high-precision printing is refined and accurate. The article states: "High-precision printing has become a technology brand in recent years, and it is gradually used by printing companies to print high-end product catalogs, posters, large calendars, landscapes, and cultural relics. There are mainly the following reasons: to promote the technical strength of printers; easy to see the quality gap, in order to seek business expansion; increase product added value, can increase the printing fee standards; with the printing effect gradually recognized by customers, there are Help printers increase business volume." “With the promotion of high-precision printing technology, there are more and more printing objects that are applied in comparison with previous years, such as fur, landscape, skin, and precious metals, Buddha statues, hand-printed materials, etc. In the past, cars and The fur is most suitable for high-precision printing. Nowadays it has reached a point where it is not limited to objects, and it can meet the needs of the market.†The colleagues in the printing industry should really appreciate it.
Discussion on the Influencing Factors of the Resolution of Print Tune
Under the heading “2. Printing effects†in the “Technical Discussion†section of the article, the following subheadings were used in the specific analysis of the image effects produced by high-definition printing: “(1) Reproduce the smooth reproduction of the cleanliness†and “(2) improve the resolution Force "," "(3) suppresses moire," and "(4) eliminates rose spots." The above four subheadings point out the typical features of high-precision printing. In particular, under the heading (1), it is pointed out that "in visual terms, prints are more closely aligned with photos." The author thinks this is the most important criterion for the quality of the printed matter. However, the author would like to discuss with Mr. Ding whether it is because “increasing the number of screens and reducing the area of ​​screens can eliminate the phenomenon of inconsistencies at the outlets, which can result in smooth reproduction and clear tone reproductionâ€.
The author sincerely believes that since the mention of "increasing the number of screens" is intended to refer to the use of AM screening methods. The often-uncleanness at the edge of AM sites is due to the exposure light source focus point. A part of the edge of the focus point is displayed on the edge of the dot, forming a large number of outward projections, the size of which depends on the size of the focus point. The higher the exposure accuracy, the smaller the size of the focused spot, the smaller the protrusion, and the smoother the edge of the dot. Assume that the high-precision screening uses the same exposure accuracy as the conventional 175 lpi screening and that the bumps have the same size. Under the same grayscale condition, compared with 175 lpi, the number of high-precision dots in the unit area increases, the single dot becomes smaller, and the ratio of the bulge to the theoretical dot perimeter is larger, so the bulging will make the dot edges relatively less smooth. In fact, under the well-controlled conditions, high-precision dot-printing is indeed closer to continuous photo-taking than conventional 175-litre printing, but this is because high-resolution dots can achieve higher tone resolution than dot edge The effect of finish.
In my previous work, the author quoted Professor Feng Ruiqian of the Beijing Institute of Graphic Arts as a translator of the Pocket Digital Printing Guide, which expressed the concept of print clarity. The translation elaborates: "The overall resolution of the image reproduction system is two independent characteristic functions. The ability of the system to distinguish the details of an image is called spatial resolution. The ability to reveal the tone or color during reproduction is called the tone resolution or Color resolution: The image quality is the product of the spatial resolution and the tone resolution.†The author thinks that Mr. Ding’s (1) part of the title describes the issue of the resolution of the print tone and points out the important indicators for evaluation of print quality. one.
Why can high-resolution amplitude modulation screens improve the tone resolution? From the definition of tone resolution, it can be understood that the tone resolution of the amplitude modulation dot is the variable dot number from the smallest to the hundredth. The expression is
The tone resolution = (exposure accuracy dpi / screen number lpi) 2-1
Assume that the exposure accuracy is 4000 dpi, which is compared with 175 lpi and 350 lpi.
The 175 lpi tone resolution is 521, 350 lpi is 130. In a unit area of ​​175 lpi, four 350 lpi dots can be arranged, so the total tone resolution of 350 lpi in the equivalent area is 130 x 4 = 520. Since the image electronic file is conventionally represented by 8 bits, the maximum number of gradations is 28=256. Under this condition, when the single-tone tone resolution is higher than 256, the accuracy becomes redundant and the performance cannot be achieved. If the image electronic file is 350dpi and 175lpi is screened, the average grayscale value of adjacent four pixels is used as the dot grayscale, and the maximum number of available grayscales is 256. 350dpi copies one pixel per dot, there is no precision redundancy. The problem is that 520 gray scales in the equivalent area can be used. The above brief description shows that high-resolution screening can achieve higher tone resolution, and the printed image therefore appears more delicate and softer. If the performance of the equipment is poor, such as exposure accuracy of only 2800 dpi, 350 dpi screening can not achieve higher tone resolution at the time, and only increase the spatial resolution. What Mr. Ding described in heading (3) is exactly the way and effect of changing spatial resolution.
In the article, Mr. Ding stated that “since the outlets are small, the images are more delicate, and the images can be clearly identified with a magnifying glassâ€, we must also discuss this with the gentleman. The author thinks that the more delicate effect of the image is due to the increase in resolution of the tone. "Small network" seems to only change the spatial resolution. Only the increase of the spatial resolution can improve the detail of the image and the image looks clearer, but it does not make the change of color and light more delicate and soft. The sharpness and delicate softness of prints are affected by the two independent color characteristics of spatial resolution and tone resolution, respectively. The reason why the difference between the two strategies is that the difference between the technology and the actual application effect is long neglected. The opportunity to discuss with Mr. Ding was once again put forward, hoping to arouse the attention of colleagues in the industry. When discussing the selection of more applicable new technologies, the printing quality can be evaluated more reasonably and accurately.
Discussion on whether high-precision printing can enlarge the color gamut
Mr. Ding’s point of view under the headline “(5) Enhancing the Colourfulness and Enlarging the Scope of Print Reproduction†is the content the author wants to discuss.
First, the meaning of "print reproduction range" is clarified. In the title, Mr. Ding proposed "print reproduction scope". In the discussion under the title, he mentioned "color expression scope." The author thinks that it is the same content, that is, the print color gamut that is usually used. Based on this, the author tries to analyze what the influencing factors of the printing color gamut range are, and whether high-precision printing can expand the printing color gamut.
The range of printing gamuts, strictly speaking, refers to all colors on and within the boundaries of the printing gamut. When the colors are expressed in different color modes, the color parameters appear in different dimensions. The print gamut range refers in most cases to the gamut boundary. Printed color gamut boundary, the combined color of all the colors used in the printing of the base color ink, the maximum chromaticity's x, y coordinates (CIEYxy) or the maximum chromaticity's a*, b* coordinates (CIELAB). The connecting line can be regarded as the vertical projection of the three-dimensional space along the luminance axis of the printed color, and the closed surface of the surface of the three-dimensional space formed on the two-dimensional plane of the chromaticity. Obviously, the coordinates of the color gamut boundary do not change and represent the same range of color gamut. In practical applications, the closed polygon formed by the solid color printing of the primary ink and the chromaticity coordinates of the secondary overprint color of different primary colors is simply used to represent the boundary of the printed color gamut. Because these colors not only have the maximum chromaticity value, but the sample color preparation is relatively simple.
For any screen of screens for any number of screens, and any size of FM network, 100% of the screen area is on the ground and there is no difference between each other. The field printing effect is only affected by the ink itself, the substrate, the fountain solution, and the printing pressure, and has nothing to do with the screening method. In other words, if the above effects are not changed, any existing screening methods cannot change the chromaticity or color index of the ink in the field. In this sense, high-resolution printing cannot expand the color gamut. Why does it seem that high-precision printing can “raise the color brightness and expand the scope of print reproduction?†I suspect that the main reason for this is that it mistakenly changes the gradation of color within the color gamut as a gamut boundary, ie, the color gamut. Changes in the scope.
In the “Technical Issues†section of the article, Mr. Ding’s choice of “the relationship between the area of ​​outlets and the increase in outlets†clearly shows the number of screens with different numbers of screens added. The amount of dot gain during printing is obviously different. Therefore, if a high-resolution printing method is used, if the dot area on the film or plate (CTP method) is not properly adjusted beforehand, the image printed on the high-precision dot is printed at a relatively low number, and the tone value is increased. More, the image looks more saturated. The increase in saturation has two basic characteristics. First, the tone of the tone curve is darker, and it becomes the turning point of the tone curve. Below the inflection point, the gradient of the saturation tone curve increases, the saturation contrast increases, and the printing image is closer to the effect of continuously adjusting the picture. However, the improvement of the effect of this interval is at the expense of the contrast reduction of saturation in the interval above the inflection point. For shadow-dominated images, this print copying effect is unacceptable. Second, the increase in saturation of printed images is due to a decrease in brightness. The image pixels are printed and reproduced with a relatively larger ink coverage area, and while obtaining a more saturated color, they also have to accept the fact that the brightness is lower. In recent years, many psychophysical experiments in the imaging field have shown that image reproduction with the aim of visually perceiving authenticity and maintaining brightness contrast are generally more important than saturation contrast. For print copying, the printed color space always appears to be narrow compared to the visual perception of the color space in the natural scene, or the relatively continuous original color space. In the printing of color space, it is necessary to optimize the trade-off between color brightness and saturation in order to make the printed copy image get the best visual perception effect. Mr. Ding mentioned in his article "Improve the purity of color" and the color "get a bright reproduction effect." If the color is outside the printing color gamut, high-precision printing cannot achieve the effect described above because the printing color gamut cannot be expanded. If the color is within the printing color gamut, it should be the effect that the brightness and saturation of the color increase at the same time, at least the effect that the saturation does not increase the brightness. If the original electronic file is not properly adjusted, high-definition printing cannot increase the brightness and saturation of the color at the same time, so the above effect cannot be achieved. These effects can only be achieved with high-fidelity color printing technology. In fact, the saturation level obtained by high-precision printing is linear, which can be easily achieved by adjusting the tone curve in advance during the conventional 175 lpi printing. For a long time, people did not use this adjustment method, because such a linear adjustment is not a quality product should have.
In summary, high-precision printing cannot expand the printing color gamut, and cannot simultaneously increase the brightness and saturation of printing colors. Therefore, it cannot improve the purity of color, and cannot be used in the four-color plate printing for green and orange in color expression. , Violet and other complementary colors, get a bright reproduction effect." If the color saturation within the printing color gamut is excessively increased compared to the conventional printing, it should be caused by improper adjustment of the high-precision printing tone, which is a problem rather than an advantage.
Regarding the linear control of high-precision printing tone adjustment, under the heading of “1. Mainline Screening Lines†in the “Technical Discussion†section of the article, Mr. Ding has already stated that “there is a need for high-precision printing with tone adjustment and dark tone adjustment.†I agree very much. However, I was puzzled by the "printing must use 8-color printing machine" requirements that were followed by it. Four-color printing using high-precision screening technology, why must use 8-color printing press? Only high-fidelity color printing