Adobe Photoshop CS4 is a bitmapped image editor which treats each image as a continuous space consisting of a mosaic of tiny squares called pixels. Selecting pixels in a bitmapped image can be a tricky business. That’s why Adobe Photoshop CS4 offers so many ways of making selections. There are selection tools, modifier icons and keyboard shortcuts. Once you’ve made a selection, you will often want to preserve it so that you can use the same selection in the future; especially if it’s taken you a lot of effort. There are two ways of saving a selection: as a path or as an alpha channel.
To save a selection as an alpha channel, use the Save Selection command, which is found in the Select menu. When you save a selection in this way, Photoshop creates an alpha channel, a special type of channel which can be viewed by going to the Channel window and 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; or vice versa, 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, making alpha channels ideal for saving selections with feathering and fades. Paths on the other hand are incapable of representing different degrees of masking and selection. Paths are vector shapes which can be manipulated using a series of tools which Adobe have simply imported from Illustrator, their vector-based drawing program.
To covert a selection into a path, choose Make Work Path from the Paths panel menu. You will be asked to enter a number representing the tolerance setting that Photoshop should use in creating the vector path. The permitted range is from 0.5 to 10. Lower numbers lead to a very detailed trace resulting in shorter line segments and many points. Higher numbers lead to a less precise trace with fewer points. A number somewhere between 1 and 2 normally yields good results for most selections. Having converted the selection to a work path, it is important to save it since a work path is only temporary. Simply choose Save Path from the Paths panel menu.
With the use of the imported Adobe Illustrator vector-based editing tools, paths can be modified to very precisely correspond to edges around elements within the image. This makes them ideal for creating cut-outs of products and other subjects within photographs.
The Author is a developer and trainer for Macresource Computer Training, an independent training company offering Adobe Photoshop training in London and throughout the UK.
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