Masks are powerful GIMP tools, and it would be quite difficult to do many things without them. What are masks? They are selections! Actually, they are grayscale images that represent selections. In masks, the white regions represent selected pixels, the black regions unselected pixels, and the gray regions partially selected pixels.
You may be asking, ``Why do we need another way of representing a selection? Weren't the selection tools presented in Chapter 3 good enough?'' The selection tools are good, but masks provide a whole new set of options for creating and editing selections. This chapter demonstrates how the GIMP's painting tools, gradients, and filters can be used with masks to get selection results that would be impossible with the selection tools alone. Masks are complementary to the selection tools, and this chapter shows how the two can be used together to produce the most effective results.
In addition to the new capabilities for editing and creating selections that masks provide, they also have another very useful feature. A mask can be stored and used more than once. Selections created with the selection tools are more ephemeral. When created, they exist only until another selection is made or until they are canceled. Moreover, while a selection is present in the image window, it only allows operations to be applied to the pixels in the selected region. This means that active selections can impede the work flow because pixels outside the selected region cannot be processed. Thus, there is a need for selections that can be stored and reused. Masks provide this capability, and, as will be seen shortly, it is easy to convert selections to masks and vice versa.
There are two types of masks in the GIMP: channel masks and layer masks. A channel mask is an independent entity and can be applied to any image layer. Alternatively, a layer mask is associated with a single layer, on which it is totally dependent. Every layer in an image can have a layer mask, but each layer mask is specific to its own layer. In addition, the layer mask is directly linked to its layer's alpha channel. As was discussed in Section 2.2, the alpha channel controls the opacity of the layer. As will be developed more in this chapter, the alpha channel is just a special mask and, as a mask, it is just a special type of selection.
Selections, channel masks, layer masks, and alpha channels. They are the same, they are different, and it is all explained in this chapter. Read on!