|Adobe Photoshop Basics|
Before we move on to some mask and alpha channel exercises, let's discuss alpha channels and the channels palette. (Continued below...)
The contents of the Graphics Software site are copyright © Sue Chastain and About.com. These pages may be printed for personal reference, but they may not be distributed or republished for any purpose without prior written permission. Please see the About.com User Agreement for more information.
Anytime you think you may need to reuse a selection, you can save it permanently in an Alpha Channel. To do this, make a selection, choose Select > Save Selection and a dialog box will appear.
By default the selection will be saved with the current document, but you can pull down the menu to make a new document from the selection. In most cases you will save the selection with the current document. In the Name field, you'll want to type a name for your selection so you can identify it when it's time to reload the selection.
The Operator options would be available if your document already contained an Alpha channel. You can add, subtract and intersect the selection with an existing alpha channel. Or, as usual, you can create a new channel.
An image can contain up to 24 channels, including all color channels. These alpha channels are saved even when you close and reopen an image, but only when the image is saved in a format the supports it. These include Photoshop's native format (PSD), PDF (Photoshop 6.0 only), PICT, TIFF, and RAW.
Your saved selections appear in the channels palette. To load a saved selection, you would choose Select > Load Selection (but I bet you knew that!). If your document contains more than one alpha channel you would choose the one you want from the Channel drop down menu. You can also invert the selection as you load it. If you had an existing selection already made you can use the Operation options to add, subtract, or intersect the selection. Otherwise, New will be the only available operation.
You can also work with saved selections via the channels palette. Take a minute to save a few selections so you can follow along while we discuss the channel palette. To view a channel as grayscale, just click on it in the channel palette. To view the selection as an overlay with the composite image still visible, click the eyeball next to RGB and the eyeball next to the thumbnail of the channel you want to see as an overlay.
When a channel is selected in the channels palette, any editing you perform is applied only to the channel, just as if you were in Quick Mask mode. Double clicking on the channel brings up its options and allows you to rename a channel. To the far right of the thumbnail, you'll see the keyboard shortcuts for selecting specific channels. It's handy to remember Ctrl-~/Command-~ to select the RGB composite image.
You'll also notice a row of buttons along the bottom of the channels palette. These buttons allow you to load a channel as a selection, save a selection as a channel, create a new empty channel, and delete a channel. If you want a dialog box to appear so that you can name the selection when you use the button method of saving a selection, hold down the Alt/Option key when you click on the button. To create a new alpha channel based on an existing channel, drag one of the channels to the create channel button. A duplicate channel will appear which you can edit. Again, hold down Alt/Option for a dialog box. To delete a channel, drag and drop it onto the far right button with the trash can icon.