Working with Black-and-White and Color Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 323

4.3.2 Using the Threshold Function to
Extract Hair—the Task
The following example focuses on increasing existing contrast levels so you
can achieve the most exact selection of fine structures possible. Though this
solution is not perfect, it may serve as a brainteaser for developing your own
solutions.
First you'll want to choose Colors > Threshold so that you can create a
mask layer with high contrast. You can employ painting tools to touch up this
layer and subsequently use it to create selections that will help you to edit the
actual image.
Figure 4.16
The mask layer is being
created; it will serve to
extract the hair.