The ideal and reality of color management

Color reproduction is an important task for plate printing. For a long time, the color inconsistency between originals, display, proofing, and printing has caused great confusion to users. The color that the user sees on the screen is completely different after printing, or the color of the printed product and the proof of the sample are inconsistent, causing unnecessary disputes to the merchant and the customer. This situation has plagued the industry for many years until the proposal of color management made people see the hope of changing this situation.

There are many kinds of color devices, such as scanners, digital cameras, monitors, printers, printers, etc., different types of devices, and even devices of the same type and different types, and the ability to express colors is also different. For example, different scanners have different results when scanning the same originals. Different displays have different effects on the same image. Not to mention the difference in the color of printed products caused by the difference in the ink sheets, and the fundamental reason for this difference is. The reason is that each device represents color in its own color space. In other words, the definition of color is device-dependent. For example, it defines a RGB value of a color, which is suitable for a user's certain display, when the same When the value is passed to another monitor, 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 mode of color management is: from the color of a device's color space, the characteristic file of the device is converted to the mediating color space, and then combined with the characteristic file of the target device, and then converted into the target device color space. In this process, due to the use of a device-independent color space as an intermediary, the color consistency is guaranteed.

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) Each device must have a standard specification describing its color characteristics and parameters.
(3) There must be a precise conversion algorithm to achieve the color conversion between different spaces.

In order to standardize the standards of color management, the International Cocor Consortium has established the ICC specification, which is supported by a large number of vendors today. The ICC specification is the result of negotiations among many vendors, and is independent of the device and platform. It describes the method of converting between the device's color space and the mediating color space.

The formulation of the ICC specification is a clear element of color management.

(1) The color space as an intermediary is CIE XYZ or CIE Lab.
(2) The file format that describes the color characteristics of the device is the format defined by the ICC. For CMYK type devices, ICC files usually store multidimensional lookup tables. For RGB type devices, ICC files usually store algorithms that are converted between RGB and CIE Lab.
(3) The color matching algorithm is completed by the manufacturer according to the ICC specification.

To give an example of digital proofing, let's take a 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 colorimetric value after printing (CIE Lab). Now according to the ICC profile of the color printer, we can calculate this. (CIE Lab) Another set of color (CMYK) colors needed, this set (CMYK) value and the original (CMYK) value are definitely not the same, but after this color printer output, will have the same visual effect as printing That is, it has the same (CIE Lab) colorimetric value.

The implementation of color management requires a stable color environment. If the result of digital proofing is required to be consistent with the printing result, then the premise is that the printing environment must be stable, and there are some units that do not have strict quality control to do well. If it is required that the display, proofing, and printing be the same as the original, the prerequisite 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 manuscripts, the plate-making company is not responsible and the customer will not pursue it. This is very difficult in China because of the uneven quality of the manuscript. The customer's request to the plate maker is not faithful to the copying of the manuscript, but requires the quality of the manuscript to be further improved by one grade. Under such realistic conditions, the implementation of so-called full color management will not achieve the desired results. Some domestic manufacturers have realized this problem. They are accumulating and analyzing the characteristics of domestic manuscripts. They hope to classify domestic manuscripts and sum up the scanning color separation parameters for each type of manuscript. Under such premise, the entire range of colors will be implemented. Management makes sense.

Source: pack.cn