Several color modes of digital images (medium)

Second, color design and processing software in the color mode

Color mode refers to the different ways the image defines colors when displayed and printed. In the Photoshop image processing software, there are 8 color modes such as bitmap, grayscale, two-tone, index, RGB, Lab, CMYK, and multi-channel, and there are certain specific connections between them. Sometimes there is a need to switch from one mode to another in order to output a print file or need special handling of an image.

(a) RGB mode

The RGB mode of Photoshop uses the RGB model to assign an intensity value ranging from 0 (black) to 255 (white) to the RGB component of each pixel in the color image. For example, a bright red may have an R value of 246, a G value of 20, and a B value of 50.

The RGB image uses only three colors and reproduces up to 16.7 million colors on the screen. The RGB image is a three-channel image, so each pixel contains 24 bits (8×3). The default mode for new Photoshop images is RGB, and computer monitors always use the RGB model to display colors. This means that when working in a non-RGB color mode (such as CMYK), Photoshop will temporarily convert the data to RGB data and display it on the screen.

Working in RGB mode has the following advantages: 1. It saves memory and improves performance. 2, with greater device independence. Because the RGB color space does not depend on the display or ink. Whether you are using a monitor, computer, or an output device, corrections to the image are preserved.

(B) Lab Model

Lab mode uses the Lab color model. In the Photoshop Lab mode, the psychological brightness component (L) can range from 0 to 100, and the a component (green-red axis) and b component (blue-yellow axis) can range from -120 to +120. The Lab image is a 24-channel (8 x 3) three-channel image per pixel.

Lab color is an internal color mode that Photoshop uses when converting between different color modes.

(c) CMYK model

The CMYK mode uses the CMYK color model. In Photoshop's CMYK mode, each print ink for each pixel is assigned a percentage value. The brightest (highlight) color distribution has a lower percentage of printing ink color, and the darker (dark) color assigns a higher percentage value. For example, bright red may contain 2% cyan, 93% magenta, 90% yellow, and 0% black. In the CMYK image, pure white is generated when the values ​​of all four components are 0%.

To print an image, use CMYK mode to convert RGB, indexed colors, or Lab images to CMYK images. You can also use the CMYK mode to directly process CMYK images that have been scanned or entered from a high-end system. The CMYK image consists of four colors for printing separations. It is a four-channel image and each pixel contains 32 (8 x 4) pixels.

Note the following when converting images to CMYK mode:

1. Be sure to store backups of RGB or indexed color images in case you want to reconvert the images.
2. When converting from one mode to another, Photoshop uses the Lab color mode, which ensures that the color does not change significantly during the conversion process. For example, when converting an RGB image to CMYK, Photoshop uses the information in the RGB Settings dialog box to first convert the RGB color values ​​to Lab mode. After the image is in CMYK mode, Photoshop converts the CMYK value back to RGB and displays the image on the RGB display. The conversion of CMYK to RGB on the screen does not affect the actual data in the file. The conversion is performed on the backup of the data.

3. Although all hue and color corrections can be performed in both RGB and CMYK modes, they should be carefully selected. Under normal circumstances, multiple conversions between different modes should be avoided. Because each conversion requires a recalculation of the color value, it will be lost due to trade-offs. If the RGB image is to be used on the screen, do not convert it to CMYK mode. Conversely, if CMYK scanning is to separate and print, do not perform correction in RGB mode. In RGB mode, you can use the CMYK Preview command to simulate the effect of the change without actually changing the image data.

(4) Special color modes provided by Photoshop

1, bitmap mode. Use two color values ​​(black and white) to represent the pattern of pixels in the image. The bitmap mode image is also called a black and white image, or a bit image because its bit depth is 1. Bitmap mode pixels are represented by a binary bit, so this image file takes up the least amount of disk space.

2, gray mode. Each pixel of the grayscale image has a luminance value between 0 (black) and 255 (white) for a total of 256 gray levels. The gray value can also be expressed as a percentage of black ink coverage (0% equals white, 100% equals black). Images produced using black and white or grayscale scanners are often displayed in grayscale mode. Bitmap mode and color image can be converted to grayscale mode.

To convert a color image to a high-quality black and white image, Photoshop throws away all the color information in the original image. The gray level of the converted pixel represents the brightness of the original pixel. You can use the Channel Mixer command to mix color channel information to create a custom grayscale channel. When converting from grayscale to RGB, the pixel's color value is based on the previous grayscale value. Grayscale images can also be converted to CMYK images (for creating four-tone print colors without having to convert to "two-tone" mode) or Lab color images.

3, dual tone mode. Create two-tone (two-color), three-tone (three-color), and four-tone (four-color) grayscale images using two to four color inks. Photoshop allows you to create single-tone, two-tone, three-tone, and four-tone images. Monotone is a grayscale image printed with a single, non-black ink. Two-tone, three-tone, and four-tone are grayscale images printed with two, three, and four inks. In these types of images, color inks are used to reproduce light gray shades rather than reproduce different colors.

4, index color mode. Up to 256 colors can be used when processing images. When converting to an indexed color, Photoshop builds a color lookup table (CLUT) that stores and indexes the colors in the image. If a color in the original image does not appear in the lookup table, the program will select the closest color in the existing color or use the existing color to simulate the color.

By limiting the color palette, index colors can reduce file size while maintaining visual quality. For example, for multimedia animations or web pages. Only limited editing is available in this mode. If you want to edit further, you should temporarily convert to RGB mode.

5, multi-channel mode. Use 256 gray levels in each channel. Multi-channel images are very useful for special printing. For example, convert the duotone for printing in the Scitex CT format.