Adding and Subtracting Selections

There are several ways to add pixels—parts of an image—to a selection or subtract them. Photoshop provides keystroke modifiers that allow you to add or subtract pixels when selecting with certain tools, such as the Magic Wand, the lasso tools, and the marquee tools. You can even use these key commands in the Extract Filter dialog box. Menu commands are also available that enable you to expand or contract a selection path.

Keystroke Modifiers
The quickest and easiest way to clean up messy selections is to apply the Shift or Option/Alt keys as you work with the selection tools. Pressing and holding Shift or Option/Alt as you drag or click with the tool enables you to add or subtract pixels, respectively, from an existing selection path. (Shift allows you to add; Option/Alt allows you to subtract.) This can be an extremely useful way to fine-tune selection paths. For example, you’ve seen that clicking once with the Magic Wand can result in an incomplete selection. If the Tolerance setting is too low, some pixels may get left behind. If it is too high, you may wind up selecting more pixels than you need. When this happens, rather than applying the Undo command (F/Ctrl+Z) and reselecting, it may be  easier to adjust the Tolerance level and add or subtract the desired pixels by using the Shift or Option/Alt key modifiers. The Shift or Option/Alt key modifiers also come in handy when fine-tuning selections made with the free-form Lasso tool or the Polygonal Lasso tool, which doesn’t let you create curves.

The Expand/Contract Commands
The Expand and Contract commands allow you to increase or decrease, respectively, the area of an enclosed selection path such as an ellipse or rectangle. To apply the commands, all you need to do is make your initial selection and then choose Select --> Modify --> Expand Or Contract. In the dialog box that appears, enter a pixel amount and click OK.

Adding or subtracting pixels in this way can be an extremely useful way to fine-tune selection paths made with the Rectangular or Elliptical Marquee tools. As you now know, lining the selection path up to the exact edge of an object can be rather difficult with  these tools. If your path winds up too far inside or outside an object, you can use the Expand or Contract command to add or subtract pixels around the entire selection.

The Spacebar Modifier
When selecting with the Rectangular or Elliptical Marquee tools, you can move a selection path as you draw it by holding down the spacebar. This allows for greater precision when selecting geometric objects and can eliminate the need for modifying with the Expand and Contract commands.

The Grow/Similar Commands
You can also expand an existing selection by applying either the Grow or the Similar command. These commands can be applied to any type of selection, but they are best used to expand those you’ve made with the Magic Wand tool. Each command adds pixels to your selection based on the current Tolerance setting. Choose Select --> Grow to add only contiguous (adjacent) pixels to an existing selection. To add both contiguous and noncontiguous pixels to a selection, choose Select --> Similar. The difference between the two commands is comparable to making a Magic Wand selection with the Contiguous option turned on or off.

For example, if you click in the sky area of the photo with the Magic Wand Tolerance level set to the default 32, you wind up selecting some of the sky but not all of it. Without changing the Tolerance level, you can expand the range of the Magic Wand by applying the Grow or Similar command. If the Grow or Similar command does not complete the sky selection, try raising the Tolerance level and applying the command again.