Photoshop Selections, Channels And QuickMasks

Your average Photoshop document consists of three colour channels: Red, Green and Blue. However, in addition to the colour channels, Photoshop also uses channels to store information which can be used in the process of selecting and masking different pixels within an image. To distinguish them from colour channels, these channels are normally referred to as alpha channels. Alpha channels are a huge topic in Photoshop but, in this article, we will focus on two techniques which cause alpha channels to be created automatically: the Save Selection command and QuickMask mode.

If you want to work with a given portion of an image, you need to make a selection. Photoshop has a number of 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 notoriously difficult to select and may take hours. So, having 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 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 you view alpha channels in isolation, they appear black and white. However, it is also useful to look at an alpha channel together with the image to which it will be applied. When doing so, Photoshop has to find a way of representing the alpha channel which will not obscure the image. The default solution is that 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.

The 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 just press the letter “q” on the keyboard. Once 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 pretty much the same way as they are on alpha channels. In fact, 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.

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