Whether it's deleting part of an image, adding color, or perhaps applying a filter, you need to let Photoshop know which pixels you want to change. Using a painting tool (or other tool that works with brushes), you select a brush and drag. The size, shape, and hardness of the brush, combined with the course along which the tool is dragged, control what happens.
When working with adjustments, filters, and many other menu commands, you must first identify the target area. You can, of course, apply a filter or other command to an entire layer. By not isolating a section of the layer, you have, in effect, identified where you want the filter to work: the entire layer. However, often you need to apply a filter to only a section of a layer. That's when you need to use selections and masks.
Experienced Photoshop users understand that selections are critical to effective use of the program. Understanding the theory behind selections can be a key to mastering their application.
Consider a selection to be an area of an image that is activated, the part on which you're working. Pixels within the selection can be changed; those outside the selection are protected. Selections can be made with tools and commands, and you can also create selections from paths and masks.
Generally speaking, a pixel is either inside a selection or outside a selection. However, under a variety of circumstances, a pixel can be partially selected. Any filter or adjustment command applied to the selection is partially applied to any partially selected pixelsthe filter or effect is applied with less intensity. Selections that are anti-aliased or feathered can have partially selected pixels along their edges. Selections made from masks can have up to 256 variations of a selected status among the pixels.
The flashing dashed line that indicates the edge of a selection in Photoshop is technically referred to as a selection border, selection marquee, or selection edge, but you are far more likely to hear the term marching ants.
As an example, consider two selections. One has no feathering or anti-aliasing: A pixel is either inside the selection or outside. The second selection has two pixels partially selected on either side of the line of selection. When the two selections are filled with black, there are two different results (see Figure 7.1).
Figure 7.1. At the top, the black fill is consistent and stops at the edge of the selection. Below, the partially selected pixels get filled with black but are partially transparent.
When a selection is feathered, anti-aliased, or otherwise contains partially selected pixels, the selection marquee indicates which pixels are at least 50% selected. It is certainly possible to have selections that contain only pixels that are less than 50% selected. In such cases, the pixels are affected by whatever additional steps you take (fill, delete, filter, and so on), but the marching ants are not visible around those areas. Photoshop provides a warning when no selection border appears or when the feathering is so large that no selection is made at all (see Figure 7.2).
Figure 7.2. When pixels are less than 50% selected, the effect of a filter or an adjustment might be extremely subtle, but the command is executed nonetheless. If no selection is made, no pixels are changed.
When you select pixels, keep in mind that sometimes it's easier to select the pixels that you don't want and then use the Select, Inverse command to swap the selection to the portion that you do want.
Just as a painter uses masking tape to protect parts of a surface and expose others, so, too, can you use masks in Photoshop. Masks are used to create and store selections. (Until a mask is loaded and used as a selection, it has no effect on the appearance of an image.)
Masks can be created from selections or created from scratch. In either case, masks are stored in alpha channels, which are grayscale representations of an image. The shades of gray determine levels of selection for individual pixels.
If one of the warnings shown in Figure 7.2 appears and you don't know why, take a look in the Options bar. Make sure your selection tool doesn't have some unwanted feathering.
You can use painting tools, selection tools, and even adjustment commands and filters to alter masks. Virtually anything you can do with a grayscale image can be done with an alpha channel.
Photoshop also offers Quick Mask mode, which enables you to make a selection with all the flexibility of masks but without creating an additional channel.