Real World Photoshop CS5 for Photographers: Anti-aliasing and Feathering
From the book
You can partially select pixels in Photoshop. How is this possible? Remember that Photoshop internally handles a selection as a grayscale channel, so it's a simple matter to store a partially selected pixel as a shade of gray in that channel. One of the most common partial selections is along the edges of a selection. The two most common ways of partially selecting the edges are anti-aliasing and feathering (see Figure 9-11).
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Figure 9-11 Magnified views of anti-aliasing and feathering
Anti-Aliasing. If you use the Rectangular Marquee tool, the edges of the selection are nice and crisp, which is probably how you want them. Crisp edges around an oval or irregular shape, however, are rarely a desired effect. That's because of the stair-stepping required to make a diagonal or curved line out of square pixels. What you usually want are 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. Once you've made a selection with Anti-alias turned off in the Options bar, you can't anti-alias it—though there are ways to fake it, as you'll see next.
Feathering. 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 around the edges of a selection. The border isn't just extended out; it's also extended in.
To understand what feathering does, it's important to understand the concept of the selection channel discussed 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.
There are several ways to feather a selection:
- Before selecting, specify a Feather amount in the Options bar.
- After selecting, click Refine Edge in the Options bar and adjust the Feather option.
- After selecting, choose Select > Modify > Feather.
- Apply the Gaussian Blur filter to the selection's Quick Mask.
If you use the Refine Edge dialog, 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.