If you've ever been in a minor car accident and later talked to an insurance adjuster, you've probably been confronted with their idea that you may not be fully blameless or at fault in the accident. And, just as you can be 25-percent or 50-percent at fault, you can partially select pixels in Photoshop. One of the most common partial selections is around the edges of a selection. And the two most common ways of partially selecting the edges are anti-aliasing and feathering.
If you use the Marquee tool to select a rectangle, the edges of the selection are nice and crisp, which is probably how you want them. Crisp edges around an oval or nonregular shape, however, are rarely a desired effect. That's because of the stair-stepping required to make a diagonal or curve out of square pixels. What you really want (usually) is partially selected pixels in the notches between the fully selected pixels. This technique is called anti-aliasing.
Every selection in Photoshop is automatically anti-aliased for you, unless you turn this feature off in the selection tool's Options bar. Unfortunately, you can't see the anti-aliased nature of the selection unless you're in Quick Mask mode, because anti-aliased (partially selected) pixels are often less than 50-percent selected. Note that once you've made a selection with Anti-aliased turned off on the Options bar, you can't anti-alias itthough there are ways to fake it (see below).
Anti-aliasing simply smooths out the edges of a selection, adjusting the amounts that the edge pixels are selected in order to appear smooth. But it's often (too often) the case that you need a larger transition area between what is and isn't selected. That's where feathering comes in. Feathering is a way to expand the border area around the edges of a selection. The border isn't just extended out; it's also extended in (see Figure 8-11).
Figure 8-11. Feathering
To understand what feathering does, it's important to understand the concept of the selection channel that we talked about earlier in the chapter. That is, when you make a selection, Photoshop is really "seeing" the selection as a grayscale channel behind the scenes. The black areas are totally unselected, the white areas are fully selected, and the gray areas are partially selected.
When you feather a selection, Photoshop is essentially applying a Gaussian Blur to the grayscale selection channel. (We say "essentially" because in some circumstanceslike when you set a feather radius of over 120 pixelsyou get a slightly different effect; however, there's usually so little difference that it's not worth bothering with. For those technoids out there who really care, Adobe tells us that a Gaussian Blur of the quick mask channel is a tiny bit more accurate and "true" than a feather.)
There are three ways to feather a selection.
Tip: Tiny Feathers
You don't need to use whole numbers when feathering. We find we often need a value of only .5 or .7 to get the effect we're looking for (a nice, subtle blend from what is selected to what's not). Let's say that after you've spent five minutes making a complicated selection, you realize that Anti-aliased was turned off on the Options bar. You can fake the anti-aliasing by feathering the selection by a small amount, like .5. This blurs the selection slightly (the edges contain partially selected pixels), giving an anti-aliased look.
Tip: Feathering a Portion of a Selection
When you choose Feather from the Select menu, your entire selection is feathered. Sometimes, however, you want to feather only a portion of the selection. Maybe you want a hard edge on one half of the selection and a soft edge on the other. You can do this by switching to Quick Mask mode, selecting what you want feathered with any of the selection tools, and applying a Gaussian Blur to it. When you flip out of Quick Mask mode, the "feathering" is included in the selection.
Note that if you want a nice, soft feather between what is feathered and what isn't, you first have to feather the selection you make while you're in Quick Mask mode (see Figure 8-12).
Figure 8-12. Feathering part of a selection