Working with Masks and Alpha Channels
About masks and alpha channels
When you select part of an image, the area that is not selected is “masked” or protected from editing. So, when you create a mask, you isolate and protect areas of an image as you apply color changes, filters, or other effects to the rest of the image. You can also use masks for complex image editing such as gradually applying color or filter effects to an image.

Examples of masks

A. Opaque mask used to protect the background and edit the butterfly B. Opaque mask used to protect the butterfly and color the background C. Semitransparent mask used to color the background and part of the butterfly

In Photoshop, masks are stored in alpha channels. Masks and channels are grayscale images, so you can edit them like any other image. With masks and channels, areas painted black are protected, and areas painted white are editable.

To save a mask selection
First you have to define the mask that you want to use. Do that with the Selection tool.
You can save any selection as a mask in a new or existing alpha channel.
Select the area or areas of the image that you want to isolate.
Click the Save Selection button at the bottom of the Channels palette. A new channel appears, named according to the sequence in which it was created.

To load a saved selection into an image
Choose Select > Load Selection.
Specify the Source options in the Load Selection dialog box:
Document Chooses the active file as the source.
Channel Chooses the channel containing the selection you want to load.
Invert Makes the nonselected areas selected.
Select an Operation option to specify how to combine the selections if the image already has a selection:
New Selection Adds the loaded selection.
Add To Selection Adds the loaded selection to any existing selections in the image.
Subtract From Selection Subtracts the loaded selection from existing selections in the image.
Intersect With Selection Saves a selection from an area intersected by the loaded selection and existing selections in the image.


Photoshop lets you create masks in the following ways:
Quick Mask mode
Lets you edit any selection as a mask. The advantage of editing your selection as a mask is that you can use almost any Photoshop tool or filter to modify the mask. For example, if you create a rectangular selection with the Marquee tool, you can enter Quick Mask mode and use the Paintbrush tool to expand or decrease the selection, or you can use a filter to distort the edges of the selection. You can also use selection tools, because the quick mask is not a selection. You can also save and load selections you make using Quick Mask mode in alpha channels.

Alpha channels
Alpha Channels let you save and load selections. You can edit alpha channels using any of the editing tools. When a channel is selected in the Channels palette, foreground and background colors appear as grayscale values. Storing selections as alpha channels creates more permanent masks than the temporary masks of Quick Mask mode. You can reuse stored selections or even load them into another image.

Selection saved as an alpha channel in Channels palette
Note: In Photoshop and ImageReady, it’s possible to create layer masks to isolate and protect areas of an image. Layer masks, and also vector masks in Photoshop, let you produce a mix of soft and hard masking edges on the same layer. By making changes to the layer mask or the vector masks, you can apply a variety of special effects. Because ImageReady doesn’t let you work with channels, its layer masks aren’t stored as alpha channels.

SOURCE: PHOTOSHOP ONLINE HELP

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