The ideal and reality of color management
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Color reproduction is an important task of plate-making. For a long time, the color inconsistency between the original, display, proofing, printing and other aspects has caused great confusion to users. The color that the user sees on the screen is completely different after printing, or the printed product is inconsistent with the color of the sample, which creates unnecessary disputes for the merchant and the customer. This situation has plagued the industry for many years, until the introduction of color management, so that people see the hope of changing this situation.
There are many kinds of color equipment, such as scanners, digital cameras, monitors, printers, printing machines, etc., different types of equipment, and even different types of equipment of the same type, the performance of color is also different. For example, different scanners have different results for scanning the same original. Different displays have different effects on the same image, not to mention the difference in ink color, which causes the difference in the color of the printed product. The reason is that each device represents a color in its own color space. In other words, the definition of the color is device-dependent. For example, it defines the RGB value of a color, which is suitable for the user's display, when the same When the value is passed to another display, the resulting color effect may change. The reason is that when a color is passed from one device to another, the conversion between color spaces does not perform a good color match.
The color management mode is: the color from a device color space, combined with the device's property file is converted to the mediation color space, and then combined with the target device's property file, and then converted to the target device color space. In this process, color consistency is ensured by using a device-independent color space as an intermediary.
From this we can summarize the three elements of color management:
(1) There must be a device-independent color space as an intermediary for color conversion between different devices.
(2) There must be a standard specification for each device to describe its color characteristics and parameters.
(3) There must be an accurate conversion algorithm to achieve color conversion between different spaces.
In order to standardize the standards for color management, the International Cocor Consortium has developed the ICC specification, which is supported today by a wide range of vendors. The ICC specification is the result of negotiation between many vendors and is device- and platform-independent. It describes the method of converting between the device color space and the mediation color space.
The development of the ICC specification is an element of color management that has been clarified.
(1) The color space as an intermediary is CIE XYZ or CIE Lab.
(2) The file format describing the color characteristics of the device is the format specified by ICC. For CMYK-type devices, ICC files usually store multi-dimensional lookup tables; for RGB-type devices, ICC texts usually store algorithms that convert between RGB and CIE Lab.
(3) The color matching algorithm is completed by the manufacturer according to the ICC specification.
For an example of numerical proofing, let's look at how color management works. There is a color value suitable for printing (CMYK). According to the ICC profile of the printing environment, we can calculate the visual color value after printing (CIE Lab). Now we can calculate this according to the ICC profile of the color printer. (CIE Lab) Another set of color (CMYK) colors required. This set of (CMYK) values is definitely different from the original (CMYK) value, but after output from this color printer, you will get the same visual effect as printing. That is, it has the same (CIE Lab) chromaticity value.
The implementation of color management requires a stable color environment. If the result of the digital proof is required to be consistent with the printed result, then the premise is that the printing environment must be stable, and some units with poor quality control are not doing well enough. If the display, proofing, and printing are required to be consistent with the original, the premise is that the quality of the original must be guaranteed. Foreign countries strictly abide by the principle of color reproduction. If the quality of the finished product is not high due to poor manuscript, the plate maker has no responsibility and the customer will not pursue it. This is very difficult in China because the quality of the original is uneven. The customer's request to the plate maker is not faithful to the reproduction of the original, but requires that the quality be improved to a higher level on the basis of the original. Under such realistic conditions, the implementation of so-called full-color management will not achieve the desired results. Some domestic manufacturers have already realized this problem. They are accumulating and analyzing the characteristics of domestic manuscripts. They hope to classify domestic manuscripts and summarize the scanning color separation parameters of each type of manuscript. Under this premise, the whole process color is implemented. Management makes sense.