Getting Started With Alpha Channels And QuickMask Mode In Adobe Photoshop CS4

There are three colour channels in typical Photoshop file: 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.

In order to manipulate any part of an image, you need to make a selection. Photoshop has a number of different tools and techniques for making selections and the time taken will depend on the complexity of the subject. Certain photographic elements, such as people's hair, are notoriously difficult to select and may take hours. So, having made a selection, it is not unreasonable that the user should want to save the selection for later reuse. The Save Selection command, which is found in the Select menu, is one of the easiest ways of doing this.

When you save a selection Photoshop creates an alpha channel and prompts you to give it a name. You can view the alpha channel by going to the Channel window and clicking on its name. Alpha channels are simple 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 actual fact, the reverse can be true since the user can set his or her preferences by double-clicking the channel thumbnail in the Channel window. The different levels of grey within an image represent different levels of selection. This makes alpha channels ideal for saving selections with feathering and fades.

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.