Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 287

Figure 3.104
The extracted lion.
Now you can make
corrections with the
tool's paintbrush.
Again, the tool switches and turns into a paintbrush. With it you can
mark the foreground color of the object you want to extract. First, I chose
a medium-sized brush to paint over the lion, and then I painted within the
contours and finally tried to cover all the various shades of the lion's fur. For
the tail and the tassel, I reduced the size of the brush. Take care not to paint
into the background.
If you want to make any changes because there are still surfaces to select
or that have been overselected, this is the time to do it. If you have selected too
little, you can apply the paintbrush a second time. If you selected too much of
the image, you can switch to Mark background under Interactive refinement in
the tool settings. You can then erase the sections that you don't want.
When you are satisfied with the result, confirm your selection with the
Enter key. The blue mask disappears and a selection is made around the lion.
The result is pretty good, but altogether the contour is a bit too jagged
and there are still selection islands visible in the lion. In the next section, I will
show you how you can subsequently improve the result and how to make the
preselections with “conventional” select tools.