Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 288

Figure 3.105
The selection of
the lion still can be
improved.
3.14.4 Drawing a Mask Using Paint Tools with
Various Edge Attributes
So far you have learned about select tools that are either based on shapes
(Rectangle, Ellipse, Lasso, and Paths) or select on the basis of color and color-
connected areas ( Fuzzy Select tool [Magic Wand], Select by Color tool ). The
edges of the selection are initially “sharp-edged” and this affects the entire
selection equally.
However, GIMP provides two methods to either draw or edit masks with
paint tools. Since you can determine the edge attributes with the paint tools
and the Eraser tool, a selection created in this manner can therefore have
various edge characteristics depending on the brush pointer you apply.
With the first method, you can paint a mask on a separate layer using
several different paint tools and the color black. Then you create a selection
from it using the Layer > Transparency > Alpha to Selection menu item.
In the second method, you must first create a rough selection of the
image object or section with the “usual” select tools. Then you switch by
clicking the Toggle Quick Mask button at the bottom left of the image window,
switching to masking mode . You can then use the Paintbrush and Eraser tools