Using Alpha Channels in Photoshop

You use alpha channels to make and store selections that you make with the selection tools, such as the Lasso or Magic Wand tool. You can also use alpha channels to edit selections. An alpha channel is a type of mask. You can think of masks as selections using some or all the 256 available levels of gray, instead of a selection outline. Alpha channels define a selection in black, white, and varying shades of gray pixels. In other words, alpha channels store selections as 8-bit grayscale images. In alpha channels, selected pixels are white, unselected pixels are black, and anything in between is partially selected or partially unselected, depending on whether you think the glass is half full or half empty.

You can create a mask by duplicating a color channel and then editing that channel with painting and editing tools and filters. Most graphics geeks refer to this as a channel mask. If you create an alpha channel from a saved selection, the channel most likely consists of just black or white pixels. If you have a feathered or even an anti-aliased selection, you may have a few gray pixels. If you create an alpha channel by duplicating and editing a color channel, your alpha channel most likely contains varying shades of gray pixels as well.

Modifying an alpha channel’s options
To modify the options for an alpha channel, follow these steps:

1. Select the channel and then choose Channel Options from the palette pop-up menu.
2. Enter a name for the alpha channel.
3. Specify the color and opacity of the overlay.
4. Specify whether you want the masked area (what the overlay will cover) to include the Masked Area (the unselected area), Selected Area, or Spot Color.
5. Click OK to close the Channel Options dialog box.
You can also simply double-click the channel thumbnail to bring up the Channel Options dialog box. Then follow Steps 2 through 5.

Saving a selection as an alpha channel
One of the great things about alpha channels is that you can save them and then retrieve them time and time again. This can be especially handy if you’ve taken a lot of time and effort to create the selection. Why reinvent the wheel if you want to select the element again in the future? Sure, you can create a mask using Quick Mask Mode and Color Range, but those masks are only temporary.

The hardest part about creating an alpha channel is making the initial selection. After that, it’s a piece of cake. Follow these steps to create an alpha channel:

1. Make a selection in your image.
2. Choose Select-->Save Selection.
You can also click the Save Selection as Channel button (a circle on a square icon) at the bottom of the Channels palette. A new channel appears with the default name of Alpha 1 and bypasses Steps 3 through 6.
3. Choose a destination image in the Document pop-up menu.
You can choose your current image or any other open image with the same pixel dimensions.
4. Choose a destination channel from the Channel pop-up menu.
You can choose a new channel or any existing channel or layer mask.
5. If you choose New, name the channel.
6. If you choose an existing channel or layer mask, select your desired operation.
Replace, Add To, Subtract From, or Intersect With are the options. Replace substitutes your existing alpha channel with the new selection. Add To, Subtract From, or Intersect With, of course, adds to, subtracts from, or intersects your current selection with the existing alpha channel.
7. Click OK.
Your alpha channel is complete and appears in the Channels palette.

Alpha channel tips and tricks
Here are a few closing alpha channel tidbits:
- Try showing the alpha channel along with the composite channel. That way you can see how clean your mask is.
Note that when you display them both, the alpha channel appears as a semitransparent color overlay.
- You cannot add channels to images that are in Bitmap mode. That’s because all pixels in a Bitmap mode file are either black or white. Photoshop does not support intermediary shades.
- You can convert alpha channels to spot channels. For more on spot channels.

Loading an alpha channel
No doubt if you’ve gone through the trouble of creating an alpha channel, it’s because you want to be able to easily load, or access, the selection again and again. To load an alpha channel, use any one of these many methods:
- Choose Select-->Load Selection. Select your document and channel. Click Invert to swap selected and unselected areas. If your image has an active selection, choose how you want to combine the selections.
- Select the alpha channel in the Channels palette, click the Load Channel as Selection button at the bottom of the palette, and then click the composite channel.
- Drag the channel to the Load Channel as Selection icon.
- Ctrl+click (Ô+click on the Mac) the alpha channel in the Channels palette.
- Ctrl+Shift+click (Ô+Shift+click on the Mac) to add the alpha channel to an active selection.
- Ctrl+Alt+click (Ô+Option+click on the Mac) to subtract the alpha channel from an active selection.
- Ctrl+Alt+Shift+click (Ô+Option+Shift+click on the Mac) to intersect the alpha channel with an active selection.

Adding channels can start to bloat your file size, so use them, but use them judiciously. The Photoshop native format and TIFF format compress channel information and therefore are good file formats to use when working with a lot of channels. The only formats that preserve alpha channels are Photoshop, TIFF, PDF, PICT, Pixar, or Photoshop Raw. A single image can now have a maximum of 56 channels.