Prepress Production: Editing Images in CMYK

This is a cliché problem. When you open an image, it may be in RGB or CMYK format. If it is a CMYK image, the reason for converting to an RGB image is not sufficient. Because, in the dot image editing software, every time the image color space is converted, the original image details will be lost. If you turn an image into RGB for a while and turn it into CMYK, the loss of detail in the image will be great. Therefore, professional designers do not easily convert the color space of an image back and forth. The question is: If your image input process allows you to choose RGB or CMYK format, what choice would you make? And when you first select the RGB format for image adjustments, when is it most appropriate to convert the image to CMYK format? RGB is the color mode used by all optical-based devices. For example, monitors work in RGB. The color range of RGB is larger than CMYK, so many colors that RGB can express, especially bright and bright colors, cannot be printed at all. This is why the picture becomes dark when converting the color mode from RGB to CMYK. When editing RGB images in Photoshop, you can select the CMYKPreview command in the View menu, that is, edit in RGB mode and display in CMYK mode. This is a good method, because the efficiency of editing images using RGB is far higher than using CMYK. When Photoshop is working in CMYK, one color channel is more than RGB, and it also uses RGB. The display mode simulates the effects of the CMYK display. Moreover, the CMYK operation mode is completely different from the optical-based RGB principle. Therefore, the efficiency of editing images using the CMYK mode is naturally much lower.

Another reason to edit images in RGB mode is that some filters do not support CMYK mode. You can convert images to CMYK mode in Photoshop and then look at the filter menu. Also, image editing often goes through many subtle processes. For example, it may be necessary to combine the contents of several images together, and since the original tones of the various components cannot be the same, they need to be adjusted and may also be Combine parts in some way, perform filter processing, and so on. No matter what the process is, you will certainly want to produce and retain as many subtle effects as possible. Make the picture as rich as possible. Who likes the flat and lifeless picture? As I just said, the color range of RGB is much larger than CMYK. Therefore, when editing images in RGB mode, you will get a wider color space and more subtle editing effects throughout the editing process. If these effects are used well, most of them can be retained. Although in the end it still has to be converted into CMYK and there is no doubt that there will be color loss, why should it be "lost" at the outset?

By the way, regardless of whether the RGB image is converted to CMYK or the CMYK Preview is used to simulate the CMYK display effect, it is the separation parameter that plays a decisive role behind the program. The adjustment of color separation parameters will greatly affect the image conversion. At the very least, software like Photoshop has the ability to control color separation parameters. We specifically explored in other articles.

At present, few prepress departments are aware of the importance of RGB image data. Some departments have realized that scanning and digital photography should save files in RGB mode during the entire color correction and revision process, and after all adjustments are completed, the conversion to CMYK. Because of these corrected and corrected RGB data, the professional prepress department can store valid documents for a long period of time. This allows images retrieved from archives to be used on different output devices (or other replication systems). This emphasis on RGB image data has had a good impact in many publishing workflows, whether the color separation method uses a system-level color management method or an image conversion method in Photoshop.

The effects of copying the same image on various printing presses, digital proofing devices, or computer monitors should be strictly identical.

This is possible when separate separations are made for each device. Because each replication system can produce slightly different blends between cyan, magenta, yellow, and black to produce a similar appearance, individual color separations make the images look the same on different devices.
The primary method of observing (and measuring) the color differences replicated by these devices is to measure the amount of cyan, magenta, and yellow needed to produce neutral ash, which we call the gray balance of the replication system. If the image is converted to CMYK, then reusing different output devices requires the image to adjust the highlights, midtones, and shadows of the CMYK image and change the overall gray balance and color saturation. It is difficult to change the amount of black in an image without impairing the image quality. However, if an image is printed without correcting the black data, a poor result may occur.

For example, a CMYK image that was previously color-separated on a high-quality, on-line, dry sheet-fed press can cause smearing if printed on a cold-cured web press. The compromise is to modify the CMYK image. RGB images can use a larger RGB tone range to reproduce brighter, more saturated colors. However, after the image is separated into CMYK, all the pixels in the image are within the CMYK tone range.

The development trend of the entire printing industry filing RGB images is often met with the resistance of some experienced scanner operators and color separation specialists. These old professionals learned the separation technique when using old scanners decorated with rows of knobs. But they didn't hear RGB image files used for prepress until customers began scanning on cheap desktop CCD scanners. RGB image editing shows that desktop scanners are starting to become a threat, and some prepress technicians are contemptuously linking RGB color correction to low-quality image capture.

In addition to the RGB mode, Photoshop's Lab color mode also has good features.

RGB is based on the optical principle, while CMYK is the color pattern of the light reflected by the pigment. The advantage of Lab is that it compensates for the lack of the previous two color modes. RGB has too much excess color between blue and green, too little color between green and red, and CMYK loses more color in the editing process. Lab has compensation in these areas. Lab also consists of three channels, L denotes illuminance, which controls brightness and contrast, and a channel includes colors from dark green (bottom luminance value) to gray (middle luminance value) to bright red (high luminance value), b channel The colors included range from light blue (bottom brightness value) to gray to yellow (high brightness value). Similar to RGB mode, the mixing of colors will produce brighter colors. Only the value of the luminance channel affects the light and dark changes of the color. You can think of Lab as a two channel RGB mode plus a luminance channel mode.

Lab mode is device-independent and can be used to edit any image. And as fast as RGB mode, it is several times faster than CMYK. Lab can guarantee no loss of color in the CMYK range when performing color mode conversion. In fact, whenever you convert an RGB image to CMYK, Photoshop adds an intermediate step to Lab mode.

Of course, in order to use the color correction method of hue, saturation, and brightness (HSL), it is not necessary to convert the image to Lab. Professional image editing programs (such as Photoshop) enable RGB mode images to be calibrated by adjusting the HSL value, including HSL values ​​in the overall or specific base color or inter-color.

If you often use PhotoCD images, you can open them directly in Lab mode. Kodak's proprietary YCC mode is basically the same as Lab, so the color loss will be minimal during the conversion process.

For the above reasons, when editing the image, if possible, I use Lab or RGB as much as I can, and change to CMYK mode when I have to. Once you become a CMYK image, you shouldn't easily turn it back. If you really need it, turn it into Lab mode.