Channel basics

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 a channel for each color (red, green, and blue) plus a composite channel used for editing the image.

• Alpha channels store selections as grayscale images. You can add alpha channels to create and store masks, which let you manipulate or protect parts of an image. (See About masks and alpha channels.)

• Spot color channels specify additional plates for printing with spot color inks. (See About spot colors.)

  An image can have up to 56 channels. All new channels have the same dimensions and number of pixels as the original image.

  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 size of an uncompressed file, including alpha channels and layers, appears as the right-most value in the status bar at the bottom of the window when you choose Document Sizes from the pop‑up menu.

The Channels panel lists all channels in the image—composite channel first (for RGB, CMYK, and Lab images). A thumbnail of the channel’s contents appears to the left of the channel name; the thumbnail is automatically updated as you edit the channel.

You can use the Channels panel to view any combination of channels in the document window. 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.

Individual channels are displayed in grayscale. 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.

You can change the default to show the individual color channels in color. When a channel is visible in the image, an eye icon  appears to its left in the panel.

You can select one or more channels in the Channels panel. The names of all selected, or active, channels are highlighted.

• To select a channel, click the channel name. Shift-click to select (or deselect) multiple channels.
• To edit a channel, select it and then use a painting or editing tool to paint in the image. You can paint on only one channel at a time. 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.

You can move alpha or spot channels above the default color channels only if the image is in Multichannel mode (Image > Mode > Multichannel). For information about that mode’s limitations, see Multichannel mode.

• To change the order of alpha or spot channels, drag the channel up or down in the Channels panel. When a line appears in the position you want, release the mouse button.

• To rename an alpha or spot channel, double-click the channel’s name in the Channels panel, and enter a new name.

For more information, see Create a new spot channel.

You may want to delete spot or alpha channels you no longer need before saving an image. Complex alpha channels can substantially increase the disk space required for an image.