Adobe 23101335 User Guide - Page 126
Channels and bit depth (Photoshop), About color channels, About bit depth
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116 CHAPTER 3 Working with Color To adjust RGB color display to match Photoshop color display (ImageReady): Choose View > Preview > Use Embedded Color Profile. Note: In order to use the Use Embedded Color Profile command in ImageReady, you must save the original image, with color profile embedded, in Photoshop. Channels and bit depth (Photoshop) A working knowledge of color channels and bit depth is key to understanding how Photoshop stores and displays color information in images. About color channels Every Adobe Photoshop image has one or more channels, each storing information about color elements in the image. The number of default color channels in an image depends on its color mode. For example, a CMYK image has at least four channels, one each for cyan, magenta, yellow, and black information. Think of a channel as analogous to a plate in the printing process, with a separate plate applying each layer of color. In addition to these default color channels, extra channels, called alpha channels, can be added to an image for storing and editing selections as masks, and spot color channels can be added to add spot color plates for printing. (See "Storing masks in alpha channels" on page 199 and "Adding spot colors (Photoshop)" on page 191.) An image can have up to 24 channels. By default, Bitmap-mode, grayscale, duotone, and indexedcolor images have one channel; RGB and Lab images have three; and CMYK images have four. You can add channels to all image types except Bitmap-mode images. About bit depth Bit depth-also called pixel depth or color depth- measures how much color information is available to display or print each pixel in an image. Greater bit depth (more bits of information per pixel) means more available colors and more accurate color representation in the digital image. For example, a pixel with a bit depth of 1 has two possible values: black and white. A pixel with a bit depth of 8 has 28, or 256, possible values. And a pixel with a bit depth of 24 has 224, or roughly 16 million, possible values. Common values for bit depth range from 1 to 64 bits per pixel. In most cases, Lab, RGB, grayscale, and CMYK images contain 8 bits of data per color channel. This translates to a 24-bit Lab bit depth (8 bits x 3 channels); a 24-bit RGB bit depth (8 bits x 3 channels); an 8-bit grayscale bit depth (8 bits x 1 channel); and a 32-bit CMYK bit depth (8 bits x 4 channels). Photoshop can also read and import Lab, RGB, CMYK, and grayscale images that contain 16 bits of data per color channel.