Tutorials

The Alpha Channel

09/12/2002

A Photoshop file can have 24 channels. Some of the channels will be used by the color space. For example, RGB will take up three channels (red, green, and blue), leaving 21 channels that can be used as alpha channels.

Alpha channels are masks through which you can apply effects. The alpha channel is an 8-bit channel, which means it has 256 levels of gray from 0 (black) to 255 (white). White acts as the selected area; black acts as the protected area. The level of gray in between determines the level of exposure. For example, 50 percent gray allows for 50 percent exposure. Alpha channels are basically specialized selection processes.

When you use the selection tools, you are segregating a portion of the image to be modified. The actual selection process does nothing to the image. It's the application of a filter, adjustment control, or any other modification through that selection that will affect the image. This is the same concept behind the alpha channel. It is merely a selection to expose the image for an effect to be applied.

The beauty of these alpha channels is that they can be saved, modified, or combined to create special effects. When you use the Lasso, you make a selection. When you deselect, the selection is gone. If you Save Selection (Select > Save Selection), it is sent to an alpha channel that can be recalled as many times as you need.

You can paint right in the alpha channel with any of the tools. Filters also work in the alpha channels. This opens the door to a multitude of possible effects. Keep in mind that you are making modifications to a mask and not the image. Through that mask, modifications will later be made to the image. When you paint, you use shades of gray. There are no colors available in the alpha channel because it is not an image, or part of the image.

When you save a selection, you are given the choice to send it to a new document. This will send it to a separate file that can be accessed when you choose Load Selection (Select > Load Selection) to call up an alpha channel. Saving alpha channels to a new document lets you have thousands of alpha channels.