Feel the color management around you (2)

2. Color space and color profiles

Image mode conversion is one of the main functions of the color management system. In fact, the conversion of the image mode is the conversion of the color space of different image modes. Each image mode has its own unique color space. The same image mode usually has a common color space.
The same model, but the colors of different images can find a fixed position in this space, so you can convert each other in this common color space. Although different modes of images have different color spaces, as long as they have a fixed color space, they can also establish a contrast relationship between different modes of colors so as to realize the interchange of color spaces.
However, images that are also CMYK modes often have no common color space because the color space of CMYK depends on the color characteristics of the device that produces the color. The CMYK mode expresses a certain color by the percentage of dots of printing inks of cyan, magenta, yellow, and black colors. Its color space is not only limited by the color of the ink, but also affected by factors such as the printing material, the characteristics of the printing equipment, the operation of the printing reproduction process, and the environment. Therefore, the color space of different printing equipment is often different; the same equipment is changing the raw and auxiliary materials such as ink, paper, and even the color space will change with the change of product, time, and environment. To realize the conversion of the CMYK mode color space, only a common standard reference that can be accepted by everybody can be established, so that each color in the respective color space can find a corresponding position on the standard reference object, so as to be mutually compatible. For conversion purposes. This common color space is the so-called device-independent color space.

The common color management system now uses Lab image mode as a common standard reference. Photoshop is also an internal format with Lab as its display image. When you open or enter an arbitrary-mode image in Photoshop, Photoshop first converts it to a Lab-mode image for display and processing. Therefore, when processing an image, although it is possible to arbitrarily convert the image mode, in fact, the image is always kept in Lab mode inside Photoshop until it is decided to register or output to be converted to the selected format. Therefore, when the image mode is converted, the color of the image often does not change from the display alone. Converting a certain image mode to Lab mode converts each color on the mode image into a corresponding Lab color space position.

The color profile of the device, or the device's color profile, is the basis for the mutual conversion of the image modes. Using Lab mode as a standard reference for mode conversion requires only the establishment of a color correspondence relationship between this mode and the Lab mode. The color of a position on a pattern image is set to A. The color of point A is consistent with the point C of the Lab color space, and point C on Lab corresponds to point B of the color space of the other mode. So you can confirm that A is exactly the same as B. This is the basic principle of image mode conversion, and it is also the way of color management.

The color space of CMYK depends on the color characteristics of the device, and the color characteristics of this device are established in correspondence with the Lab color space. Then when the image mode changes, the color of this image is first converted to a corresponding point in the Lab color space, and then converted to the color corresponding to this point in the Lab color space of the other image mode. In this conversion process, it is no longer necessary to correspond to the first color space. This situation is the same whether it is converted from R6B to CMYK or from one CMYK to another in image mode.

Obviously, a very important precondition for the correct use of the color management system is the strict determination of the device's color profile.