Troubleshooting Color Management ICC Profiles (1)

What is the general structure of the ICC profile?

The ICC profile typically consists of a file header, a tag table, and tag element data.

The file header occupies a total of 128 bytes, mainly describes the file attributes, such as the characteristics of the file type, capacity, version number, color management module type, device color space, feature file connection space (PCS), color matching method, PCS space The tristimulus value XYZ of the light source.

The label table mainly includes the total number of labels in the property file and the description of each label. The total number of tags occupies 4 bytes; each tag description occupies 12 bytes, the first 4 bytes are tag identifiers, and the middle 4 bytes is the offset of the tag relative to the start byte of the property file. The 4 bytes are the length of the tag element data.

The tag element data is mainly used to store the conversion data between the device's color space and the PCS. The capacity of each tag element is different.

Is a display (eg display) profile basically a table with two-dimensional numbers?

The display profile is actually very complex, they can be a series of tables, or a table followed by a matrix, followed by a table, the result seems to be a large table. The various types of profiles are the same.

What is the difference between the output (eg printer) profile and the display profile?

Most display profiles are table-matrix-table types. Printer profiles are a series of table types. However, these differences are almost irrelevant to users. The main difference is that all display profiles are from RGB to PCS (and vice versa), and many printer profiles are from PCS to CMYK (ie 4 channels compared to 3 channels). However, many printers require RGB data to be converted into CMYK in their driver, so that the printer profile has only 3 channels.

The printer profile differs from the display profile in that it pays attention to all variables (paper, ink settings, gamma, etc.) and the measurement scale of the device during printing, and generates a profile through these measurements. Is this right?

this is correct. It is also worth noting that not only the printer profile, but also the display settings (contrast, brightness, color balance, gamma, etc.) must be corrected. The display must also be measured to generate a display profile. In order to better maintain the consistency of the input and output colors.

What is a "connection profile"? How is it different from the device profile?

A device connection profile converts data decoded in one device's color space to data in another device's color space, and it applies only to those specific devices.

Different devices have different ability to represent colors, and the color range they can represent is not the same. Therefore, the device profile defines the device's color profile information, which can be used to obtain the range of colors that the device can capture, display, or reproduce.

The device profile can be converted to PCS, or converted from PCS, and federated during processing, mixing the input and output profiles according to the needs of the workflow. The connection profiles allow people to add their own "plots" to a specific pair of devices, or keep the black in CMYK to perform CMYK conversion under two different printing conditions.

How many figures are in each profile? For example, can the display profile list all the colors of the RGB and CIELAB that the device has (24-bit RGB display 16.77 million colors)?

The size of the profile is changed with the profile creation software. It can be a combination of 8 bits for each RGB, ie, 224 = 16.77 million colors, but RGB can be increased to a combination of 16 bits or more. The combination of 16 bits is 248 = 281.4749 billion colors, which are not explicitly listed in the table. Any color in can be obtained by CMM interpolation.

As can be seen in the ICC profile specification, it is correct to include only one LUT without the TRC/matrix profile. Check the scanner profile for some plants. All feature files contain a TRC/matrix element and LUT. What is the role of TRC/matrix elements in the scanner profile?

The ICC allows the creation of a simple TRC/matrix for the scanner profile as much as possible. When it is displayed, it works in this way. However, the spectral sensitivity of most scanners is not sufficiently close to the color matching function (effective human eye sensitivity) to allow the TRC/matrix to be used well alone. For this reason, ICC also specifies a LUT (look-up table) method that allows non-linear transfer, which is used by most profile creation software. However, relying solely on the functionality of the LUT itself is not enough to make the data more accurate, it is necessary to increase the TRC (tone reproduction curve) to improve it. For this reason, all profiles use TRC in addition to one LUT. Adding a separate matrix is ​​not required for accuracy reasons; it can be combined with LUTs, but there are also some feature file authors who have found that separating the matrix from the LUT may be more advantageous.