How to use RGB and CMYK in editing images

When you open an image, it may be in RGB format or CMYK format. In the case of a CMYK image, the reason for converting to an RGB image is insufficient. Because, in the dot matrix image editing software, each time the image color space is converted, part of the original image details will be lost. If you convert an image to RGB for a while and then to CMYK for a while, the loss of detail in the image will be large. Therefore, professional designers do not easily convert the color space of the image back and forth. The question now is: If your image input process allows you to choose RGB format or CMYK format, what choice would you make? And when you first select the RGB format and make various adjustments to the image, when is it most appropriate to convert the image to CMYK format?

RGB is the color scheme used by all optical-based devices. For example, the display works in RGB mode. 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 darker when the color mode is converted 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 with RGB is much higher than that of CMYK. When Photoshop works in CMYK mode, one color channel is one more than RGB, and it also uses RGB. The display mode simulates the CMYK display effect, and the CMYK operation method is completely different from the optical-based RGB principle. Therefore, the efficiency of editing images by CMYK mode is naturally much lower.

Another reason to edit images in RGB is that some filters don't support CMYK mode. You can turn the image into CMYK mode in Photoshop and look at the filter menu. Also, the editing of images often goes through many subtle processes, such as the possibility of combining the contents of several images together, and since the original tones of the components cannot be the same, they need to be adjusted, or they may The parts are combined in some way, processed by filters, and so on. Regardless of the process, you will definitely want to produce and retain as many subtle effects as possible, making the picture as rich as possible. Who likes the flat and lifeless picture? As I said earlier, 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, and these effects, if used well, can be largely preserved. Although I still have to turn into CMYK and there is no doubt that there will be color loss, why bother to "loss" it at the beginning?

By the way, whether it is converting RGB images to CMYK or CMYKPreview to simulate CMYK display effects, it is the color separation parameter that plays a decisive role behind the program. The adjustment of the 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 discuss it in other articles.

Currently, 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 throughout the color correction and revision process, and after all adjustments are completed, convert to CMYK. It is precisely because of these RGB data that has been corrected and corrected that the professional prepress department can store valid documents for a long time. This allows images retrieved from the archive 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 system-level color management or image conversion in Photoshop.

The effect of copying the same image by various printers, digital proofing devices, or computer monitors should be exactly the same. This is possible when performing separate color separation for each device. Because each replication system can produce slightly different blends between cyan, magenta, yellow, and black to produce a similar appearance, separate color separations make the images look the same on different devices.

The primary method of observing (and measuring) the difference in color reproduced by these devices is to measure the amount of cyan, magenta, and yellow required to produce neutral ash, which we call the gray balance of the replication system. If the image is converted to CMYK, then reusing a different output device, the image requires adjusting the highlights, midtones, and shadows of the CMYK image and changing the overall gray balance and color saturation. The amount of black in the image is difficult to change without impairing the image quality, but if the image is printed without correcting the black data, a bad result is produced.

For example, CMYK images that were originally color-separated for high-quality, on-line, sheet-fed presses can cause smudging if printed on a cold-set web press. The compromise is to fix the CMYK image. RGB images can use a larger RGB tonal range to reproduce brighter, more saturated colors. However, after the image is color separated into CMYK, all pixels in the image are within the CMYK tonal range.

The entire printing industry archives the trend of preserving RGB images, often encountering the resistance of some experienced scanner operators and color separation specialists. These old professionals learned the technique of color separation when using an old-fashioned scanner decorated with rows of knobs. But they didn't hear about RGB image files for prepress until the customer started scanning on a cheap desktop CCD scanner. RGB image editing shows that desktop scanners are starting to become a threat, and some prepress technicians despise RGB color correction and low-quality image capture.

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

RGB is based on optical principles, while CMYK is the color mode in which the pigment reflects light. The advantage of Lab is that it makes up for the shortcomings of the previous two color modes. RGB has too much excessive color between blue and green, too little between green and red, and CMYK loses more color during editing. Lab has compensation in these areas. Lab also consists of three channels, L for illuminance, which controls brightness and contrast, and a channel includes colors from dark green (bottom brightness value) to gray (middle brightness value) to bright red (high brightness value), b channel The colors included range from bright blue (bottom brightness value) to gray to burnt yellow (high brightness value). Similar to the RGB mode, a mixture of colors will produce a brighter color. Only the value of the illumination channel affects the light and dark variations of color. You can think of Lab as a mode of two channels of RGB mode plus one brightness channel.

The Lab mode is device independent and can be used to edit any image. And it's as fast as RGB mode, several times faster than CMYK. Lab can guarantee that there is 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 turn it into Lab mode.

Of course, in order to utilize 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 adjusted by adjusting the HSL value, including the HSL value in the overall or specific base or inter-color.

If you use PhotoCD images frequently, 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 conditions permit, I will use Lab or RGB as much as possible, and only convert to CMYK mode when I have to. Once you become a CMYK image, don't switch back easily. If you need it, switch to Lab mode.

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