Color Management and Prepress Workshop
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Chairman Zhang Zhongyi delivered a speech:
For the relevant academic seminars, the Institute has developed new practices in the future. In addition to continuing the foundations of the former chairman, a related academic seminar will be held every two months. Today's academic seminars are mainly printed by consortium corporations. It is sponsored by the Xingcai Culture and Education Foundation and is jointly handled by the Institute and the China Printing Society. In addition, the Institute will also handle teacher-study camp activities, hoping to promote the application of printing technology.
Here, I would like to introduce you to the two vice presidents of the Institute, one is Huang Yisheng, the general manager of the industry Yongfeng Paper Company; the other is the academic circle, Professor Chen Changlang of the Cultural University; the other is the standing supervisor of the Institute, Li Xingwei Doctor; We jointly develop the direction of development and introduce this high-tech to the industry. Color applications are used in a wide range of applications, so how to expand the color management perspective from printing to other industries is a very important topic. People with lofty ideals are also welcome to join the China Printing Technology Society. In particular, student members, the recent revision of the charter by the China Printing Technology Society not only reduced the annual membership fees of student members, but also canceled the membership fees for student members. Thank you also for coming to this seminar today.
Moderator: Li Xingwei, Executive Supervisor
Thank you for participating in today's seminar. I believe that you are participating in the expectation and learning mood of color reproduction management. Today, I will invite you to invite well-known experts and scholars from China to give you an incisive speech. First introduce the two speakers today: the first is Dr. Chen Hongxing, Ph.D. in Imaging Science from Chiba University, Japan, and the topic is the theory and practice of color reproduction. The second is Dr. Sun Peili, Ph.D., Color University of Darby, UK. The theory and practice of color management.
Introduction: Professor Luo Meijun
In the past, the machinery and equipment in the working environment were closed, and the equipment was set by the engineers. The color reproduction practitioners only need to make the manuscript according to the relevant settings. Today, the equipment is open, and the equipment may come from different manufacturers. Therefore, the color correction of the screen, the printer and even the printing machine is very big, although the color management system is very popular in use. But we must understand the theoretical application of the color management system. I also hope that today's topic is only the beginning. In the future, companies should pay more attention to employee training on related topics, and promote this experience to the printing industry in order to improve the quality of printing technology.
The theory and practice of color reproduction: Professor Chen Hongxing
First of all, when talking about the source of color, there is a color before the foundation of scientific theory. From a physics point of view, light is an electron wave, short wavelength belongs to high energy, long wavelength belongs to low energy, color occurs in a small range of visible wave, and its range is about 380-780μm; The sunlight is transmitted through the prism, and when the white light is broken down into color light, and the color light that has been destroyed and decomposed is re-polymerized through the prism and the lens, white light can be formed again. This theory is applied to photography and printing, that is, after the color light is destroyed, it is reproduced by polymerization. Secondly, color is actually a feeling. After the light enters the human eye, it generates stimulation and transmits it to the brain. The eye is actually a feeling entrance. How does the human eye feel the color? First: there are sensible cells in the human eye: three kinds of cells with short wavelength, medium wavelength and long wavelength; the other is the cytological theory of sensible color in the human eye: black and white, opposite color <contrast color>. Later, through scholars' research, it was found that the synthesis of the three primary colors and the comparison said that the human eye accepted the first stage of the color light to produce the three primary colors first, and the messages interacted to produce opposite colors. That is, the human eye has black-and-white rod-shaped cells and sensible color-shaped cone-shaped cells; when it comes into contact with color, the light first stimulates the RGB of the cone-shaped cells, and produces a contrasting color when transmitted to the brain.
▼Additional color mixing:
From a physical point of view, the color light is the decomposition of the white light, partially reflected, partially absorbed, thus producing color. Adding color method: RGB two-color addition will produce CMY, and the middle will produce white; the name of the addition will be brighter and brighter, for example, RGB color will add up and finally produce white; subtractive color mixing principle: when photographing or printing When we use the color filter, when the C filter is used, only the light of C can pass through, and the other light is blocked. The primary color is CMY, and the secondary color is two-phase. , produces RGB, the last three colors produce black, that is, the energy is completely absent. The subtractive color mixing principle is far more complicated than the additive color mixing principle. The most predictable in the subtractive color mixing principle is the use of color positive films, and color paper and color printing are far more complicated than using color positive films.
The reflective object is the reflectivity, and the transmitted object is the transmittance. For example, a white object is 100% of the reflected energy, and a black object is a complete absorption of energy, which produces different ash to varying degrees. The cross-platform principle of color is to focus on the human eye's position, regardless of whether it is CMY language, RGB language, or other languages. By relying on this platform, cross-media problems can be solved.
▼L a b equal color space:
The Lab color space has a total of three axes. The vertical axis is L for light and dark. The brighter the upper, the darker the lower, the +a and -a are red and green, and the +b and -b are yellow and blue. The opposite of the color perception of the human eye can also be used to illustrate the color space of the device; it can also be used to explain the hue, lightness and chroma: hue: for example: red, yellow... all are hue, and the red hue angle is 45 degrees. The yellow hue angle is 90 degrees; the brightness: the brightness of the color; the chroma: the vividness of the color, the distance from the gray axis, the more vivid the outside, the darker the inside.
The color image composition will vary depending on the device or the principle of production. For example, the screen is composed of RGB colors, and the printing is composed of CMY. YCC, Y stands for the degree of light and darkness - the gray level signal, and C refers to the CRCV. The red-green color is poor, and the other is yellow-blue difference; therefore, RGB or YCC is usually used in image processing. Here is a phenomenon: the apples that are actually seen by the human eye and the apples copied from the TV, if you use the instrument to measure, you will find that the color is not the same, that is, the actual apple and TV copy Apple has different reflectances, which have a lot to do with light source conditions. Finally, whether it is television or print reproduction, it is often added to the worker's favorite color, to remove the skin color or skin defects, so even if the copied value is quite accurate, it will eventually be trimmed because of people's favorite color. This is called color correction. The principle of color correction was first used in the scanner, that is, the color system of the scanner is to simulate the real color, and the color separation is to divide the color into CMYK dots; the reason why the earliest scanner failed is that the problem of accurate color density dots must be related to paper. Factors are taken into account, and as a result, the current scanner is constantly improving.
There are three kinds of inks on the printing. In fact, there are eight kinds of compositions. If C=M=Y, it will produce gray. However, due to the difference in the proportion of ink components, it is not very ideal gray. Therefore, CMK will be used instead of printing. CMY printing can save costs and produce better results, but the use of UCR must also be appropriate, excessive use, although there is a good contrast, but lost the level of detail.
In terms of color, the color gamut of the screen is larger than that of the printer, so the color on the screen may not be faithfully presented by the printer. Therefore, in color copying, it is necessary to first consider which colors are not printed by the printer, but must first make The compression process, after being compressed, is guaranteed to be within the color gamut of the printer, and must be converted by the color conversion program, and then by CMY, from the screen to the printer, from the additive method to the subtractive method.
Theory and Practice of Color Management: Professor Sun Peili
First, the system that introduces you to color management first includes closed systems and open systems. Open color management: Manage color exchange between different systems, such as printing presses, scanners or screens... The color correspondence between each device must be correct, but there are many input values and output values. Therefore, it is necessary to have a very complete color management system, the so-called open color management, which corresponds to all the colors of a device-independent color space, and the relationship is like the communication between people from all over the world, or first conversion. Become a common language - English, and then correspond to the language that is familiar to each other, you can communicate. Isn't the closed system not good? In fact, it must be based on the nature of the equipment. If it is the printing work of the newspaper today, it can use closed color management because of the simplicity of its equipment.
Secondly, when it comes to the reproduction of cross-media colors, different color media, such as screens and printers, differ in the number of colors subordinate to the media and the relationship between the human eye and color. Different media have their RGB correspondence to CMYK, so the color description of different media must rely on the media color characterization job to describe the device dependent Independent. The relationship between color and the RGB or CMYK of the human eye. In addition to the above factors, the feeling of color must overcome the impact of the surrounding environment; in addition, the problem of the color gamut between different media must be solved through the gamut corresponding program; To accurately perform color reproduction, the three differences in color media must be modeled; these three differences include: color characterization, color appearance mode, symmetric algorithm; use color characterization to describe the RGB color on the screen Convert the tristimulus value of the adult eye to color: XYZ, in the reference white color of the reference color environment, in the mode of entering the color appearance to calculate the hue, brightness, chroma feeling, hue, lightness, chroma of color-independent I feel that we use LCH to represent the correction of the image, and then to overcome the gamut difference between the original media screen and the printer, and use the inverse operation process to convert the RGB values on the screen into the CMYK of the printer. The difference in color between the original and the copy is reduced. When you want to convert color into a conversion job related to the human eye, you must first determine that the relevant calibration job is ready, so that color conversion will make sense. If the correction of the device is changed, the description or conversion process of the characterization must be recalculated in the category describing the characteristics of the device. Model There are two major types of mathematical modes: one is device-specific, also known as media-specific, that is, the mode associated with the color rendering characteristics of a particular device must be derived from the expertise of an imaging device. The mathematical mode, using this mode to convert color, convert RGB into XYZ; the advantage of this mode is that it can perform very accurate conversion operations without too much data, and its embedded parameters have meaning and predictable The color correction is achieved without the need to re-measure. The general type of category Model This mode is that we do not need to know whether the media uses additive or subtractive methods when performing color conversion. It does not need to have too much professional knowledge. Just use the mathematical model to describe the relationship between the two color modes. The advantage of this mode is that any imaging medium can be used, but the disadvantage is that the number of measurements required is large.
In order to meet the color conversion needs of different media, manufacturers hope to propose a standard color management architecture to meet the needs of different markets. This demand comes from the print-centric plane communication market and the home and office centered on display and network. Market; In 1993, ICC Color International Alliance proposed a set of cross-platform color management system standards, including ICC and sRGB architecture; ICC architecture for the use of non-device-dependent XYZ or Lab color space as a bridge for color reproduction communication, color of ICC architecture The accuracy is higher, but the computing cost is higher; while the sRGB architecture is initially developed to match the same color seen on different screens of different networks, and the color is mapped to the same space by a simple method, which has a certain accuracy. The flexibility is small. The architecture of sRGB is initiated by Microsoft and HP. The concept is to use an RGB colorimetric system to produce screens. The flexibility is small, and different media cannot make correct color conversion. Some important terms in the ICC architecture: PCS: non-device dependent color space for connecting different signals, ICC specifies the use of XYZ or Lab; Profile: description file describing color characteristics, providing color management system to capture colors such as RGB or CMYK All necessary information between the data and the non-media-dependent XYZ and Lab during color conversion are stored in the profile file. This job must be converted by the DMM color management module.
Another basic architecture for the Profile Connection Space is a bridge between Lab or XYZ, which has an Input Device and an Output Device, for example, scan the file into RGB values. We make this RGB value and its corresponding XYZ value as a profile in advance And model this data, and how does this XYZ value be correctly rendered on the printer? You can convert it to a Lab value first, then compare it to the desired CMYK value and print it out if the gamut is very When approaching, the colors on these two sides will be very close. Devices can be divided into three categories: first, Input Device devices, including: digital cameras and scanners. Output Device device, including: Printing System. Display is a particularly independent class, it can be used as Input Device and Output Device is separated by the screen view and the printer output. And other special color spaces can also be converted using profile, which is called Color Space. There is also a name called Profileline, the content is that we have determined that only two D e v i c e are used. You can connect them into a profile, which will form RGB in, CMYK Out, he has one less operation in the process of calculation, and the speed will of course be faster. As for the transmission reversibility of the profile, it is divided into two categories, one is One way, the other is Two Way. Oneway means that its transmission is only in one direction and there is no reverse. Two way means that it converts RGB to XYZ, which also provides XYZ conversion to RGB, which is a classification. There is another classification: we must have a source of data in the color conversion process, and it must have a purpose in the end. Scanners or digital cameras are all part of One Way, is to convert the RGB value into XYZ value, it does not need to convert the XYZ value into RGB value, because we can't see it on the screen, so it is always one-way. As for other output devices, they belong to Two Way, because we can copy the copied copy to the color gamut on the screen for proofing work.
The ICC stipulates the performance of the color gamut, making it the color space of our ideal reflective print. It is not the XYZ, Lab that we see on the screen, it has a certain observation environment. It has four different chromaticity correspondence spaces: including perceptual, relative, saturated, and absolute chromaticity correspondence. There are two ways to store colors: one is matrix, it can only be used in three-dimensional space, so we can't use it on printing; one is the corresponding one, use internal difference to convert its value.
Finally, let's understand the architecture of ICC. First, we will convert all the colors into one place and translate them all together. Every time there is a conversion, there will be errors. Each company's translation method is different, so each time, each result, each Homes are not the same, and the scope of the ideal is not specified, so future color management will focus on this aspect.