by Andrew Whiteman

A typical Photoshop file consists of three colour channels: Red, Green and Blue. However, in addition to the colour channels, Photoshop can also use channels to store information used in the process of selecting and masking different parts of an image. To distinguish them from the colour channels, these channels are normally referred to as alpha channels. Alpha channels are a huge topic in Photoshop since they can be used in all sorts of very creative ways. However, in this article, we will focus on just two techniques both of which cause alpha channels to be created automatically: the Save Selection command and QuickMask mode.

Before you can work with a given section of an image, you need to make a selection. Photoshop has several tools and techniques for making selections and the time taken will depend on the complexity of the subject. Certain photographic elements, such as hair, are extremely difficult to select and may take hours. So, having painstakingly made a selection, it is not unreasonable for the user to want to save the selection for later reuse. The Save Selection command, which is found in the Select menu, is one of the simplest ways of doing this.

Saving a selection causes Photoshop to create an alpha channel, prompting the user to enter a suitable name. This channel can be viewed in the Channesl window by clicking on its name. Alpha channels are just greyscale images which use a visual code to represent selections: the black areas on the channel represent areas which will not be highlighted when the channel is loaded as a selection and white areas represent those areas which will be highlighted. In fact, the reverse can also be true since the user can set his or her preferences by double-clicking the channel thumbnail in the Channel window. The different tones of grey within an image represent different degrees of selection, making alpha channels suitable for saving selections with fades and feathering.

When looking at an alpha channel in isolation, it appears black and white. However, it is also useful to look at an alpha channel at the same time as the image to which it will be applied. This means that Photoshop has to find a way of representing the alpha channel which will not obscure the image. By default, the alpha channel changes to a red overlay with an opacity of 50%: the user can change both the colour and opacity of the overlay by double-clicking the thumbnail of the alpha channel in the Channels window.

This same technique of representing selection and masking as an overlayed colour is also used in Photoshop's QuickMask mode. To enter QuickMask mode, click on the QuickMask icon on the toolbar or simple press the letter "q" on the keyboard. When in QuickMask mode, all of Photoshop's paint tools, such as the paintbrush and gradient, can be used to make selections. Selections are represented in exactly the same way as they are on alpha channels. Furthermore, if you look at the Channels window when in QuickMask mode, you will notice that Photoshop has created a temporary alpha channel called "QuickMask" which will disappear as soon as you exit QuickMask mode.

About the Author:
The author is a training consultant with Macresource Computer Solutions, a UK IT training company offering Adobe Photoshop Classes at their central London training centre.

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