Channels and bit depth (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 section 280 and Adding spot colors (Photoshop) on section 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 infor- mation 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 2 or 256 possible values And a pixel with a bit depth of 24 has 2 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 section 321

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

To convert between 8 bits per channel and 16 bits per channel:

To convert to a 16-bit-per-channel image first flatten the image (See "Flattening all layers on section 295

Choose Image > Mode > 16 Bits/Channel or 8 Bits/Channel.