Computer Graphics and Image File Formats (3)

The special color mode 1, bitmap mode provided in Photoshop uses 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 bitmap because its bit depth is 1.

2. Grayscale Modes Each pixel of a 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 and 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 printing colors without having to convert to "two-tone" mode) or Lab color images.

3. Duotone mode creates two-tone (two colors), three-tone (three colors), and four-tone (four colors) grayscale images using two to four color inks. Photoshop allows the creation of 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.

The term "duplex" in this section refers to duotone, monotone, tritone, and tetratone.

4. The index color mode uses up to 256 colors. 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 look-up 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 uses 256 gray levels in each channel. Multi-channel images are very useful for special printing. For example, convert tones are used to print in the Scitex CT format.

The following guidelines apply to converting images to "multi-channel" mode:
. Any image synthesized by more than one channel can be converted to a multi-channel image, and the original channel is 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 automatically converts the image to multichannel mode.

It is often not possible to print color composite images in "multi-channel" mode, and most output file formats do not support multi-channel mode images, but can output such files in Photoshop DCS 2.0 format.