Using Channels and Masks

Using Channels and Masks

About channels

Channels are grayscale images that store different types of information:

•Color information channels are created automatically when you open a new image. The image’s color mode determines the number of color channels created. For example, an RGB image has four default channels: one for each of the red, green, and blue colors plus a composite channel used for editing the image.

•You can create alpha channels to store selections as 8-bit grayscale images. You

use alpha channels to create and store masks, which let you manipulate, isolate, and protect specific parts of an image. In addition to supporting alpha channels from Photoshop, you can save, load, and delete selections as alpha channels in ImageReady 7.0.

•You can create spot color channels to specify additional plates for printing with spot color inks.

An image can have up to 24 channels. The file size required for a channel depends on the pixel information in the channel. Certain file formats, including TIFF and Photoshop formats, compress channel information and can save space. The uncompressed size of a file, including alpha channels and layers, appears as the rightmost value in the status bar at the bottom of the window when Document Sizes is chosen from the pop-up menu.

Note: As long as you save a file in a format supporting the image’s color mode, the color channels are preserved. Alpha channels are preserved only when you save a file in Adobe Photoshop, PDF, PICT, Pixar, TIFF, or Raw formats. DCS 2.0 format only preserves spot channels. Saving in other formats may cause channel information to be discarded.

(See “About file formats” on page 453.)

Using the Channels palette (Photoshop)

The Channels palette lets you create and manage channels and monitor the effects of editing. The palette lists all channels in the image—composite channel first (for RGB, CMYK, and Lab images), then individual color channels, spot color channels, and finally alpha channels. A thumbnail of the channel’s contents appears to the left of the channel name; the thumbnail automatically updates as you edit the channel.

Viewing channels

You can use the palette to view any combination of individual channels. For example, you can view an alpha channel and the composite channel together to see how changes made in the alpha channel relate to the entire image. By default, individual channels are displayed in grayscale.

To display the Channels palette:

1Choose Windows > Channels, or click the Channels palette tab.

2Use the scroll bars or resize the palette to see additional channels.

When a channel is visible in the image, an eye icon appears to its left in the palette.

To show or hide a channel:

Click in the eye column next to the channel to show or hide that channel. (Click the composite channel to view all default color channels. The composite channel is displayed whenever all the color channels are visible.)

To show or hide multiple channels, drag through the eye column in the Channels palette.

These guidelines apply to channels you display:

•In RGB, CMYK, or Lab images, you can view the individual channels in color. (In Lab images, only the a and b channels appear in color.)

•If more than one channel is active, the channels always appear in color.

•In alpha channels, selected pixels appear as white; unselected pixels appear as black (partially transparent or selected pixels appear as gray). These are the channel default options.

•If you display an alpha channel at the same time as color channels, the alpha channel appears as a transparent color overlay, analogous to a printer’s rubylith or a sheet of acetate. To change the color of this overlay or set other alpha channel options,

see “Creating alpha channels (Photoshop)” on page 280.

Changing the display of the palette

You can show the individual color channels in color (rather than grayscale) in the Channels palette and specify the size of the thumbnails. Using thumbnails is the most convenient way of tracking channel contents; however, turning off the display of thumbnails can improve performance.

To show color channels in color:

1 Do one of the following:

• In Windows and Mac OS 9.x, choose Edit > Preferences > Display & Cursors.

• In Mac OS X, choose Photoshop > Preferences > Display & Cursors.

2 Select Color Channels in Color, and click OK.

To resize or hide channel thumbnails:

1Choose Palette Options from the Channels palette menu.

2Select a display option:

•Click a thumbnail size. Smaller thumbnails reduce the space required by the palette— helpful when you’re working on smaller monitors.

•Click None to turn off the display of thumbnails.