With the improvement of color management technology, prepress staff can choose to perform color separation in image input, image processing, and even image output. One of the most important means is to use the output device ICC profile for color separation. Since the ICC profile accurately describes the color gamut of the device, and it is also defined by a device-independent color mode, it is more convenient for color exchange among different users. Therefore, more and more users will adopt ICC in the future. Feature file separation!
1 How to get information from ICC profile?
The use of ICC profiles for color separation, on the one hand, allows the fixed CMYK settings to be saved as ICC profiles for later recall; on the other hand, spectrophotometers, IT8 standard manuscripts, and ICC profiles can be used to create software (eg Heidelberg's PrintOpen software) gets the ICC profile of other output devices (such as color inkjet printers, printing equipment) to make the color separation more consistent with the output requirements, and to better preview the effects of the final product.
In production, different separation settings are always used according to the original or output mode. It is not possible to use a profile or a setting to complete all separations. Sometimes it is necessary to modify the data in the separation to make the image more consistent with the output requirements. For example: Change the color separation method (UCR or GCR) or the number of black versions. It is therefore particularly important to obtain some of the information in the ICC profile!
The ICC profile cannot be modified in ordinary software. If you get an unfamiliar ICC profile, you can sometimes get some information from the profile file name, but this is not a lot. In Photoshop, you can also find information on the color separation method, network coverage, and maximum black limit in the output device ICC profile. The steps are as follows:
1) Open Photoshop and select the desired ICC profile in the CMYK settings.
2) Create a new grayscale image or RGB image in Photoshop, pull a gradient from white to black from left to right in the image;
3) Move the mouse left and right on the image and read the CMYK data through the information panel.
Reading the CMYK data by moving the mouse can get the gray balance curve of the ICC profile. The biggest difference between UCR (Background Removal) and GCR (Gray Component Substitution) is that the black version has a different range of effect. The black version of UCR only works in the dark part of the image and is an auxiliary version; GCR The black version can be said to be full-tone and is the main color version. Slowly move the mouse to observe when the black version appears and the number of sizes, can determine the separation method. As can be seen from the data in the above information panel, the black version of this ICC profile only works in the dark tone, and it can be determined that the color separation mode is UCR. If we move the mouse to the darkest part of the image, we can also learn the black limit maximum and the maximum dot coverage.
2 What kind of conversion intents?
The color management module (CMM-Color Matching Methods) converts the source color space into the target color space according to the conversion method, and the conversion method determines how the color is converted. The conversion method is called "intention" in Photoshop and there are four kinds of "inductive", "saturation", "relative colorimetry" and "absolute colorimetry". The source color space (RGB or Lab) in color separation is larger than the target color space CMYK. The color in the source color space may fall outside the color space range that can be represented by the output device. The conversion mode is to decide how to handle these super color spaces. color.
One of the important principles of image processing is to retain the hierarchical distribution of the original image. Only “perceptual” in the conversion method can satisfy this requirement. “Perceptual” means that if one or more colors in the original image are not in the target color space, the source color space will be globally compressed to the target color space. Although the conversion may cause the colors that are already contained in the target color space to change, the color conversion of the image is maintained visually relatively stable by compressing the entire color gamut. The conversion methods “absolute colorimetry” and “relative colorimetry” are the opposite. If a color exceeds the target color space (CMYK), the color is compressed into the target color space. This conversion method may cause color in the source color space. The color coordinates with inconsistent degree coordinates have the same chromaticity coordinates after the conversion to the target color space. In fact, this is the slight level of loss!
Therefore, as long as the image is color-coded through the ICC profile, the “perceptual” option should generally be selected, and only it can ensure that the color-separated image remains in the original hierarchical distribution.
3 How to make the screen display closer to the print?
The "absolute colorimetric" option in the conversion method has taken into account the paper characteristics (eg whiteness) present in the ICC profile. Depending on whether the image is color-separated or soft-proofed only on the screen, you decide whether you want to select the "Emotional" or "Absolute colorimetric" option in the "Color Settings" conversion method. However, switching between conversion methods is not convenient, and you may sometimes forget to change the settings back.
The author suggests using the following method to solve this problem: Use the "Color Settings" menu in Photoshop 6.X only for on-screen display, and the Image Separation uses the "Convert to Profile" menu. In the "Color Settings", select "Absolute Colorimetric" for "Transformation Method", and "Perceptual" for "Conversion Method" in the "Convert to Profile" menu. The purpose of doing this:
1) Image color separation can maximize the level of original image color;
2) Taking into account the characteristics of the paper, look through the display as much as possible to the actual color.
4 Select black compensation?
Whether to choose "black market compensation" is most likely to cause controversy. Some media have said that they should not choose “black market compensation” because it has nothing to do with color management; other media ensure that only by selecting this option can the loss of secret adjustments be avoided.
The function of "black field compensation" is to replace the darkest gray in the original image with the darkest gray in the target color space, and the colors of other ultra color areas are also adjusted accordingly to ensure that the details are not lost. If you only look at this statement, you should really choose "black market compensation."
However, I chose different images to test and got two conclusions:
1) Whether to choose "black field compensation", the image does not have obvious visible details lost. If "black field compensation" is selected, the image on the display is larger than the actual print contrast;
2) Sometimes when converting RGB images to smaller color gamut CMYK images (such as newsprint printing), selecting "black field compensation" produces even brighter colors.
Therefore, it is recommended to cancel the “black market compensation”. Although the image looks relatively small, it is closer to the actual printing!
5 Table
The "Color Separation Model/Table" option in Photoshop 5.0 "CMYK Settings" is no longer available in Photoshop 6.X. If you want to apply the separation settings (such as Photoshop 4.0) of the previous version of Photoshop to the current Photoshop, just Open the separation settings in Photoshop 5.0 and save again in the “Table” to generate an ICC profile that can be called flexibly in Photoshop 6.x or other software.
6 Conclusion
The "CMYK Settings" menu in Photoshop serves two purposes: it determines how the image is converted to CMYK and determines how the CMYK data is displayed on the screen;
Gray Component Replacement (GCR) Gray for the multicolor portion of the image is partially or completely replaced with black ink, and the black range is full tone; while the Under Color Removal (UCR) is the opposite, black is only used for the dark tone of the image. Some appear;
Graphics, logos and other graphics should use the GCR color separation method, the largest black version to avoid black overprint;
If you use an unfamiliar brand of ink, you can use a spectrophotometer to test the ink, and then use Lab color coordinates to define the color of the ink in Photoshop.
Dot gains, inks, dot coverage, and color separations that have been identified in the ICC profile cannot be changed in Photoshop;
By making RGB test strips and Photoshop preview functions in Photoshop, you can get important data in the ICC profile;
In the separation of color conversion method should use "perceptual" option;
Conversion method "absolute colorimetry" takes into account the whiteness of the paper;
It is best not to use the "use black field compensation" menu, as this will give a screen display that better matches the final product;
The "Separation Model: Table" in "CMYK Settings" in Photoshop 5.X converts the previous separation table or built-in settings into an ICC profile.
Author/Xu Zhaohui
Source: Print World
