Image mode selection and conversion points (below)

6) Convert to CMYK mode for printing (printing) output

The CMYK image is an image pattern consistent with the four-color printing process. It consists of four color channels, each of which is represented by 8 bits. The generation of the black version depends on the setting of the separation parameters. From the CMYK four-color channel, dots are generated according to the color channel data, four color separation films are output, and four color printing plates are produced for printing.

1 Convert from grayscale mode to CMYK mode

After a grayscale image is converted to a CMYK image, four color information is added to the image. These color values ​​are related to the current color separation setting parameters. For example, 36% of gray can be converted to C40%M33%Y33%K1%, C37%M30%Y30%K7%, C26%M20%Y20%K22%, etc. These actual data illustrate the gray balance of the CMYK image.

2 RGB mode and CMYK mode conversion

The mutual conversion process between the RGB mode and the CMYK mode is bidirectional, and the Lab mode is the transit mode. The process of converting from the RGB mode to the CMYK mode belongs to the color separation process, and the reverse process belongs to the soft proofing process.

Before converting from ltGB mode to CMYK mode, there are two ways to understand the final result: one is to use the CMYK Preview command to preview the converted result; the other is to use the Gamut Warning command to understand the color area that is currently out of print range. , and in its active state, correct with the sponge tool so that the color of the overflow area enters the CMYK color space.

The key from the RGB mode to the CMYK mode is how to define the CMYK color space. There are three methods: Define the CMYK color using the built-in color separation setting function, using an ICC sample file definition, or using a previously defined color separation table.

Because RGB color gamut and CMYK color gamut are not completely identical. Therefore, during the conversion of the RGB image and the CMYK image, some color loss will result. For the RGB image, the color information falling in the CMYK color space is not substantially lost; beyond the color space, the color space mapping algorithm is not completely reversible and will cause color loss. Therefore, you should use as few as possible high-color, high-brightness colors that can only be displayed on the screen, and switch back and forth between RGB and CMYK modes as much as possible to avoid irreparable distortion of the colors.

Note: When converting an RGB image to a CMYK image, different conversion results will be obtained if different black generation methods are used; the CMYK mode file is larger than the RGB mode file, and some important filter functions cannot be used. Therefore, it is better to convert the image editing work to CMYK mode; when converting from RGB mode to CMYK mode, it is better to planarize the image before conversion, which will help to ensure that the expected mixed layer mode appears. Effect; Color ink jet printers, ink jet printers generally use dyes for dye, its color is more bright, high saturation, color range is also wider than the printing ink, so better to use RGB color mode expression. If you first convert to CMYK mode, the print color is basically the range of the print ink color gamut, and some colors that are supposed to be very vivid and saturated will not be reflected.

7) Convert to multi-channel mode for special printouts

The Multichannel mode uses 256 gray levels in each channel. Particularly suitable for certain professional printing areas. For example, two-color screen printing in the Scitex CT format.

You can convert images to multi-channel mode according to the following guidelines:

Convert any image synthesized by more than one channel into a multi-channel mode image, the original channel will be converted to a spot color channel.

When converting a color image to multiple channels, the new grayscale information is based on the color values ​​of the pixels in each channel.

Converting CMYK images to multiple channels creates cyan, magenta, yellow, and black spot channels.

Converting RGB images to multiple channels creates cyan, magenta, and yellow spot channels.

Deleting a channel from an RGB, CMYK, or Lab image will automatically switch to multi-channel mode.

3 Conclusion

The image mode conversion will damage the image information. Different mode conversions will produce different conversion effects, and will become better and worse. Therefore, whether it is before, during, or after conversion, consider carefully to ensure the best results. It must be remembered that not changing the appearance of the image does not mean that the image information has not changed. Try to select the appropriate color mode carefully so as not to enlarge the conversion chain. No matter what kind of conversion, the less the better.

In short, the conversion of the image mode requires more than just selecting a color mode, but also reconsider the appearance of all parts of the image and the image in order to get the best output of the conversion.

Source: "Printing World"
Author: Department of School of Design, Packaging Engineering, Shaanxi University of Science and Technology Liu Xiaoxia

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