Selections and Alpha Channels

CONCEPTS

What is an alpha channel?
An alpha channel is a user created grayscale channel.
Where are alpha channels located, and how do you make use of them?
Alpha Channel are displayed on, and put to work with, the Channels palette, which may be found on the Window tab.
Why would you use an alpha channel?
Because selections may be saved in alpha channels, and subsequently loaded onto an image at the user's discretion.

SAVING SELECTIONS IN ALPHA CHANNELS

1. How to save a selection to a channel using existing options settings:

Make a selection.
Click Save selection as channel. This function is represented by the "circle within a square" icon, which is located second from the left, at the bottom of the Channels palette.

2. How to save a selection to a channel with options:

Make a selection.
Select > Save Selection
Options:
A. The document menu allows saving a selection to a channel in the current document, or in a different document.
B. Name the document if you wish.
C.The channel function allows the selection to be designated as either a new channel, or mixed with an existing channel.
D. Pick an operation choice to add, subtract, or intersect the current selection with another alpha channel. See page 278 for additional details.
After selecting the desired options, click OK.

SKILLS

1. How to show or hide a channel:

Click in the eye column on the left side of the Channels palette. The eye indicates that a channel is visible.

2. How to display an alpha channel without loading it onto the image:

Click the desired alpha channel's name.
The selected zone is white, the rest of the channel is "masked" in black.
Click the composite image at the top of the Channels palette to return to the regular image.

3. How to modify channel options:

Double click beside a channel name to display channel options.
A. Name the channel if you so desire.
B. The selected zone is white, and the remainder of the selection is masked in black.
Color Indicates: Selected Areas reverses this, now the selected zone is black, while the remainder of the channel is white.

4. How to duplicate a channel:

Drag and drop the name of the desired channel to the Create new channel button, which may be found second from the right, at the bottom of the Channels palette.
or
Right click the channel name, and then use of the Duplicate Channel command.
Name the duplicate channel if you wish.
Click OK.

5. How to delete a channel:

LOADING ALPHA CHANNELS ONTO IMAGES

1. How to load a channel selection onto an image using existing options:

Ctrl-click the name of the channel you wish to load, and it will be added to the image.

2. How to load a channel selection onto an image and exercise options:

Click on the composite channel.
Select > Load Selection
Choose the alpha channel containing the desired selection.
You may add, subtract, or intersect the alpha channel with other selected areas of the image. See page 278 for additional details.
Use the Invert function to reverse the selected an unselected areas in the loaded selection.
After selecting the desired options, click OK.