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 280 and “Adding spot colors (Photoshop)” on page 272.)

An image can have up to 24 channels. By default, Bitmap-mode, grayscale, duotone, and indexed-color 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.

Converting between bit depths

A 16-bit-per-channel image provides finer distinctions in color, but it can have twice the file size of an 8-bit-per-channel image. In addition, only the following Photoshop tools and commands are available for 16-bit-per-channel images:

•The marquee, lasso, crop, measure, zoom, hand, pen, eyedropper, history brush, slice, color sampler, clone stamp tools, healing brush tool, and patch tool, as well as the pen and shape tools (for drawing work paths only).

•The Duplicate, Feather, Modify, Levels, Auto Levels, Auto Contrast, Auto Color, Curves, Histogram, Hue/Saturation, Brightness/Contrast, Color Balance, Equalize, Invert, Channel Mixer, Gradient Map, Image Size, Canvas Size, Transform Selection, and Rotate Canvas commands, and a limited set of filters. (See “Using filters” on page 321.)

To take full advantage of Photoshop features, you can convert a 16-bit-per-channel image to an 8-bit-per-channel image.