You use alpha channels to make and store selections that you make with the selection tools, such as the Lasso or Magic Wand. You can also use alpha channels to edit selections.
An alpha channel is a type of mask. Masks can be thought of as selections using some or all of 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, black pixels are unselected, 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 the channel by using 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 options 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 combination.
Replace, Add To, Subtract From, or Intersect With are the self-explanatory options.
7. Click OK.
Your alpha channel is complete and appears in the Channels palette.
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. No intermediary shades are supported.
- You can convert alpha channels to 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 (Command key+click on the Mac) the alpha channel in the Channels palette.
- Ctrl+Shift+click (Command key +Shift+click on the Mac) to add the alpha channel to an active selection.
- Ctrl+Alt+click (Command key +Option+click on the Mac) to subtract the alpha channel from an active selection.
- Ctrl+Alt+Shift+click (Command key +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 Raw. A single image can have a maximum of 24 channels.
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