A mask is a grayscale image that is used to represent either a selection within an image (called a selection mask) or the transparency of a layer (called a layer mask).
You use a selection mask to save a particular selection. This way you can maintain a collection of selections that you’ve built, ready to use when you need them. In a selection mask, pixels that are completely selected in the image are represented as white, pixels that are not selected at all are represented as black, and pixels that are partially selected are represented as intermediate shades.
Although a selection mask is an independent object, a layer mask is part of a specific layer used to specify the transparency of the layer’s pixels. The pixels themselves don’t change—in other words, their RGB components remain intact. With layer masks, you can manipulate a layer’s transparency independent of its colors.
You can use all GIMP tools on a mask: drawing tools, transformation tools, and even selection tools. This makes doing very precise work on a mask possible—and creating a selection much more complex than what you saw in Chapter 13.