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
You may be asking, ``Why do we need another way of representing a
selection? Weren't the selection tools presented in
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
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
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.