Working with Black-and-White and Color Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 324

The prerequisite is that the selected image object should stand out to
some extent from the rest of the image.
• Open the girl.png image in the SampleImages folder on the DVD.
• Save it as extractinghair.xcf in the layer-enabled XCF format.
• Select the Image > Mode menu item and make sure the image is in RGB
mode ; if it isn't, change the mode.
• Duplicate the Background layer (in the Layers dialog). Name
the new layer mask .
• Make sure the layer has an alpha channel (transparency
attributes) by right-clicking in the Layers dialog and
choosing Add Alpha Channel from the context menu, thus
adding transparency to the layer.
• Use the Threshold function ( Color > Threshold ) to set the
mask layer so that the hair strands are fully displayed—
there should be an adequate amount of contrast between
the hair and the background of the image. Be aware that a
solitary hair is extremely difficult to capture, even with this
wonderful tool.
• When the hair is deined by contrast, choose Select > By Color
(or use the Select by Color tool ) to create a selection on the
white image areas on the mask layer. So that you can work as
accurately as possible, the selection should be sharp edged,
i.e., no feathering.
• Choose Edit > Clear to delete the white image areas. Check
the result.
• Create a selection across the image areas surrounded by the
hair contour. In this case, that would be the face.
• Choose the Edit > Fill with FG Color menu item to fill the
selection on the mask layer with black.
• If you have done everything right, the contour of the head
with hair should be selected. If not, delete the old selection
and create a new one. Right-click on the mask layer in the
Layers dialog to open the context menu of this layer. Select
the Alpha to Selection option.
• In the image window, click the bottom-left icon to activate the Toggle
Quick Mask . You can also select it from within the Select menu.
• Before you proceed, make the mask layer invisible by clicking the layer's
eye icon in the Layers dialog.
• You are now ready to do some touchup work on the reddish mask.
Select the Paintbrush tool with a thin, soft brush pointer (depending on
the image and its resolution; in this case, use 5 to 9 pixels). Touch up the
incomplete hair strands in the example image. You may have to paint or
erase the reddish mask to correct the hair jutting out. Remember, black
color paints a mask and white erases it.

Figure 4.17
Touched-up hair strands on the mask with the
Background layer visible from underneath