Adobe Photoshop CS5 for Photographers - Martin Evening [200]
As I say, the business of using vector masks and layer masks is covered in more detail later on, but basically, a layer mask is an alpha channel applied to a layer that defines what is shown and hidden on the associated layer and a vector mask is a pen path converted to a vector mask that defines what is shown and hidden on the layer. There are good reasons for having these different ways of working and I'll be providing some practical examples in this chapter for when it is most appropriate to use either of these two main methods to mask an image.
Converting vectors to pixels
In Figure 9.7 I mention that some of the conversion processes will incur a loss of data. This is because when you convert vector data to become a pixel-based selection, what you end up with is not truly reversible. Drawing a pen path and converting the path to a selection is a very convenient way of making an accurate selection. However, if you attempt to convert the selection back into a pen path again, you will not end up with an identical path to the one that you started with. Basically, converting vectors to pixels is a one-way process. Converting a vector path into a pixel-based selection is a good thing to do, but you should be aware that converting a pixel-based selection into a vector path will potentially incur some loss of data. More specifically, a selection or mask can contain shades of gray, whereas a pen path merely describes an outline where everything is either selected or not.
Anti-aliasing
Bitmapped images consist of a grid of square pixels. Without anti-aliasing, non-straight lines would be represented by a jagged sawtooth of pixels. Photoshop gets round this problem by antialiasing the edges, which means filling the gaps with in-between tonal values so that non-vertical/horizontal sharp edges are rendered smoother by the anti-aliasing process (Figure 9.8). Wherever you encounter anti-aliasing options, these are normally switched on by default and there are only a few occasions where you might find it useful to turn this option off.
Figure 9.8 The above illustration shows a graphic where the left half is rendered without anti-aliasing and the right half uses antialiasing to produce smoother edges.
If you have an alpha channel where the edges are too sharp and you wish to smooth them, the best way to do this is to apply a Gaussian Blur filter using a Radius of 1 pixel. Or, you can paint using the blur tool to gently soften the edges in the mask that need the most softening.
Feathering
When you are doing any type of photographic retouching it is important to always keep your selections soft. If the edges of a picture element are defined too sharply, it will be more obvious to the viewer that a photograph has been retouched or montaged. The secret of good compositing is to avoid creating hard edges and to keep the edges of your picture elements soft so that they blend together more smoothly.
There are two ways to soften the edges of a selection. You can go to the Select menu and choose Modify Feather ( ) and adjust the Feather Radius setting (Figure 9.9). Or, if you have applied the selection as a layer mask, you can use the Feather slider (as shown on pages 455 and 457) to feather the mask ‘in situ’. A low Feather Radius of between 1 or 2 pixels should be enough to gently soften the edge of a selection outline, but there are times when it is useful to select a much higher Radius amount. For example, earlier on pages 360–361 I used the elliptical marquee tool to define an elliptical selection, applied this as a Curves adjustment layer mask and feathered the selection by 100 pixels via the Masks panel. This allowed me to create a smooth vignette that darkened the outer edges of the photograph.
Figure