The Subject Notebook With Dividers of color management is the method of color transfer to reduce color loss

The Subject Notebook With Dividers of color management is the method of color transfer to reduce color loss

At present, in color management technology, the so-called color feature connection space adopts the chromaticity space of CIE1976Lab. Colors on any device can be converted to this space, forming a "universal" description way, and then color matching conversion. In the computer operating system, the task of implementing color matching conversion is completed by the "color matching module", which is of great significance to the reliability of color conversion and color matching.

So, how to achieve color transfer in the "universal" color space, to achieve color loss or as little as possible?

This requires each set of devices to generate a profile, the color signature file of the device. We know that different devices, materials and processes exhibit different characteristics when it comes to rendering and transmitting color. In color management, the color presented on one device is presented on another device in high fidelity, which requires us to understand the characteristics of color presented on various devices. Since a device-independent color space, CIE1976Lab chromaticity space, has been selected, the color characteristics of the device are represented by the relationship between the description value of the device and the chromaticity value of the "universal" color space, which is the color description file of the device.

Device color feature description file

In color management techniques, the most common device color feature description files fall into three categories. The first type is scanner feature file, it provides Kodak, Agfa, Fuji Company standard manuscript and the standard data of these manuscripts, the use of scanner input these manuscripts, scanning data and standard manuscript data of the difference reflects the characteristics of the scanner; The second type is the display feature file, it provides some software, can measure the color temperature of the display, and then generate a color block on the screen, the color block information reflects the characteristics of the display; The third type is the feature file of the printing equipment, which also provides a set of software, the software generates a graphic containing hundreds of color blocks in the computer, and then the graphic output on the output device output, if it is a printer directly proofing, printing press will first out the film, proofing and printing, the output of these images are measured to reflect the feature file information of the printing device.

The generated profile, that is, the color feature file, its format is composed of file header, tag table and tag element data.

File header, which contains the basic information of the color feature file, such as file size, type of color management method, version of file format, device type, color space of the device, chrominance space of the feature file, operating system, device manufacturer, color restoration target, medium of the manuscript, light source chrominance data, etc., the file header accounts for 128 bytes. Tag table, which contains the tag number name, storage location, data size information, but does not contain the specific content of the tag, tag number name occupies 4 bytes, and each item of the tag table accounts for 12 bytes. Tag element data, it is in accordance with the tag table instructions, in the specified location to store the color management required by a variety of information, according to the complexity of the tag information, the size of the tag data varies.

For the printing enterprise equipment color feature file, graphic information processing operators have two ways to obtain.

The first approach is the profile that comes with the device at the time of purchase. It can meet the general color management requirements of the device. The profile is loaded into the system when the application software of the device is installed.

The second way is to use a special profile software, according to the actual situation of the existing device, to generate the applicable color feature description file, which is usually more accurate, and more in line with the actual situation of the user. The state of equipment, materials and process processes can change or shift over time. Therefore, the profile needs to be remade every once in a while to match the color response of the time.

Color is transferred through the device

Now, let's take a look at how colors are transferred across devices.

Firstly, for a manuscript with normal colors, scan input is carried out by scanner first. Since the scanner profile provides the corresponding relationship between the colors on the scanner (namely the three stimulus values of red, green and blue) and the chroma space of CIE1976Lab, the operating system can obtain the chroma value Lab of the manuscript color according to this conversion relationship.

The scanned image is displayed on the monitor screen. Since the system has mastered the corresponding relationship between Lab chromaticity values and the red, green and blue driving signals on the display, the red, green and blue chromaticity values of the scanner are not used directly during the display, but the conversion relationship given by the profile of the display from the Lab chromaticity values of the previous manuscript. Get the red, green and blue display driver signal that can correctly display the color of the manuscript on the screen, and drive the display to display the color. This ensures that the colors displayed on the monitor match the colors of the original manuscript.

After observing the accurate image color display, the operator can adjust the image according to the screen color processing according to the customer's requirements, and because of the profile of the printing equipment, after the image color separation, the correct color can be observed on the display after printing. When the operator is satisfied with the color of the image, the image is separated and stored. During color separation, the correct dot percentage is obtained according to the color conversion relationship carried by the profile of the printing equipment. After RIP (raster image processor), recording printing, printing, proofing and printing, a printed copy of the original manuscript is obtained to complete the entire process.