Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 285

• Mark foreground : This option is the default setting. The paintbrush
paints with the foreground color when creating a selection. The
colors that have been painted over mark the object that should be
extracted.
• Mark background : This option can be selected when it is easier, due
to the colors, to select the background rather than the foreground.
Moreover, you can switch to this option while working by holding
down the Ctrl key. With the Mark background option selected, the
program paints in the selected background color (in the Toolbox).
You can also subtract colors from the selection, for instance, if you
have overdrawn the boundaries of the area you want to select. Colors
within the images that have been marked as background (with the
background color) are not selected.
• Small brush - Large brush : The slider lets you select the size of the
paintbrush. A small brush lets you work with fine details.
• Smoothing : This slider lets you determine the sharpness of the edges of
the selection border. If you smoothen the selection with the brush, you
can remove or cover up small holes.
• Preview color : Lets you select the color of the overlay mask, which covers
up the background in the image.
• Color Sensitivity : This feature works with the LAB color model. If
your selection has a color with a variety of tones, you can increase the
sensitivity of the selection for this color.
Let's have a look at an image object that because of its characteristics is
more difficult to extract. Have a look at the image lion.png in the SampleImages
folder on the DVD. The lion in the picture has a color that's similar to the
background. It is difficult to extract the fringes and the strands of hair in the
lion's mane.
You can tag along again in this exercise. The result can be used for
exercises later in the topic.
• Open the lion.png image in the SampleImages folder on the DVD.
• First, perform an automatic color level adjustment ( Colors > Levels >
Auto ) on the image. The contrast of the background colors will come out
stronger.
• Select the Foreground Select tool . Circumnavigate the lion and his mane,
applying the tool's lasso with as little distance to the object as possible.
If your result isn't to your satisfaction, you can adjust the Color Sensitivity
setting. Set the values of L , a , and b up to around 500 to enhance the
contrast sensitivity of the tool.