The distinction between bitmaps and vectors is one that often confuses users new to computer graphics. Fundamentally, bitmaps describe an image as a mosaic of tiny squares called pixels, each with its own colour value. Vectors, by contrast consist of independent elements or objects each with its own attributes. Photoshop, a key component of the Adobe CS4 is a bitmapped graphic environment, it does offer users a number of tools for creating and editing vector graphics. These tools are used to create vector paths which allow very precise selection of a given portion of an image.
Selecting pixels in a bitmapped image can occasionally be a challenge. That’s why Photoshop offers so many ways of making selections. There are selection tools, modifier icons and keyboard shortcuts. Having selected the required pixels, you will want to ensure that you don’t have to repeat the laborious process of selection again. You have two methods of saving your selection: either as a vector path or a bitmapped alpha channel.
Should you with to save your 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 merely greyscale images which employ a kind of code to represent which parts of the image will be selected whenever the mask is used to make a selection. Basically, the darker the area, the weaker the selection will be; the lighter the area, the stronger the selection. White areas therefore indicate full selection and black areas, no selection at all.
The different levels of grey within an image represent different strengths of selection, making alpha channels ideal for saving selections with feathered edges and fade-outs. Paths on the other hand cannot record different amounts of selection. Paths are vector objects which can be modified with the use of a series of tools which are essentially imports from Adobe Illustrator.
If you wish to save a selection as a path, choose Make Work Path from the Paths panel menu. Photoshop will prompt you 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. Smaller figures result in a very detailed trace resulting in shorter line segments and many points. Higher numbers will create a path with fewer anchor points. As a general rule, figures in the range 1 to 2 yield the best results.
Having created a workpath, you should always remember to save it, by choosing Save Path from the panel menu. Workpaths are temporary and will simply disappear when the file is closed.
Using the borrowed 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 writer of this article works for an independent computer training company offering Adobe Photoshop CS4 training in London and UK wide.
