getting started with photoshop - Channels & its features

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.

Using the Channels palette

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:

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

2. Use 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.

Selecting and editing channels

You can select one or more channels in the Channels palette. The names of all selected, or active, channels are highlighted. Any editing changes you make apply to the active channels.

To select a channel:

Click the channel name. Shift-click to select (or deselect) multiple channels.

To edit a channel:

Use a painting or editing tool to paint in the image. Paint with white to add the selected channel's color at 100% intensity. Paint with a value of gray to add the channel's color at a lower intensity. Paint with black to fully remove the channel's color.

Managing channels

You can rearrange channels, duplicate a channel within or between images, split a channel into separate images, merge channels from separate images into one new image, and delete alpha and spot channels when you're finished with them.

Rearranging and renaming channels

The default color channels normally appear at the top of the Channels palette, followed by the spot color channels, and then the alpha channels. You cannot move or rename the default channels, but you can rearrange and rename spot and alpha channels to suit the way you work.

Spot colors are overprinted in the order they appear in the Channels palette.

To change the order of alpha or spot channels:

Drag the channel up or down. When a line appears in the position you want, release the mouse button.

Note: You can move alpha or spot channels above the default color channels only if the image is in Multichannel mode.

To rename an alpha or spot channel:

Double-click the channel's name in the Channels palette, and enter a new name.