Feathering
Before making a selection you should always first decide
how much 'feathering' should surround the selection. This is
a little difficult to explain, for the feathering setting
won't make any obvious difference to the way the selected
area appears on your screen. However, when you do
anything to the selected area, such as lightening it,
moving it, copying it or whatever, the feathering will
make a difference, for the feathering blurs the edges of the
selection. Since the selection no longer has a sharp edge,
parts of the picture that appear to be just outside the
selected area will also be slightly affected by what you do.
Ultimately, you must learn not to be misled by the 'marching
ants' - they do not simply indicate the limit of the
'selected area' - rather, they indicate which parts of the
image will be affected strongly by what you do while
the selection is in effect.
You can apply feathering to a selection either before or
after making the selection - the main difference is that if
you set up feathering BEFORE making the selection then the
settings will be remembered and applied to future selections
made with the same tool. Feathering adjustments applied to
an existing selection will not be remembered.
To set feathering before making the selection,
choose a selection tool (such as the lasso tool) turn on the
feathering option in the selection tool's options window
then set the feathering radius. A radius of 1 or 2 pixels
would give a subtle effect while a radius of 10 or more
would give a very obvious effect.
To increase the feathering of an existing
selection choose Select>Feather... then set the
feather radius.
To remove feathering from an existing selection
choose Select>Sharpen.
The best way to understand feathering is to try it for
yourself. The above photograph shows a young boy chained
naked to the gates of the Paris town hall (!). Open the
picture in the Gimp and then choose the ellipse selection
tool 
. In
the tool's option dialog, make sure that feathering is
turned off, then make a circular selection around the boy's
face. You can constrain the selection to be circular (as
opposed to elliptical) by holding down the Shift key as you
use the tool. If the circle is not quite in the right place
then Alt+drag it (if you drag the circle you will move the
boy's face with it, leaving behind a circle filled with the
current background color).
Now let's make a copy of the selection to see what we've
got. Sometimes you can duplicate a selection by
Ctrl+Alt+dragging it - try it if you like - a surer way is
to copy then paste then drag the selection. Either way, you
should see that the selection has hard edges. If you look at
the layers dialog (Ctrl-L) you will see that the duplicated
selection is in a layer called 'floating selection' this is
a temporary layer which you may lose unless you click the
new layer button
in the layer dialog (do it!).
Now let's see whether it is possible to apply feathering
after a selection has been made. Make sure that the
background layer is active (i.e. dark) in the layers dialog
then make another circular selection just as before, without
any feathering. After having made the selection, choose
Select>Feather>5. Then duplicate and move the selection
as before - you should see that the duplicated selection now
has a soft edge.
Finally, let's try setting the feathering before we make
the selection. Turn on the feathering option of the
elliptical selection tool and set the feathering to 25
pixels. Make sure that the background layer is active
(i.e. dark) in the layers dialog then make another circular
selection just as before. Then duplicate and move the
selection as before - you should see that the duplicated
selection now has a very soft edge. You may now have
a picture something like this:
In this image, the three duplicated heads have
featherings of (from top to bottom) 0, 5 and 25.
Anti-aliasing
This is about the same as using feathering
set to 1 i.e. it blurs just very slightly the edge of the
selection, making it less jagged. Since only a narrow edge
is blurred, there is no loss of detail.
Adding to and subtracting from selections
If you ever want to add to an existing selection,
hold down the Shift key while you use your selection
tool (rectangular selection tool, lasso, magic wand or whatever) - a small '+'
sign will appear to confirm that you are adding. You may
realise that there is a possible problem here because the
Shift key is also used to constrain some selection tools
e.g. the ellipse tool is constrained to make circles. But
what if you want to add an ellipse, not a circle? The Gimp
programmers have thought of that - just hold down Shift as
you begin making the selection but release it before you
release the mouse button. Or, if that sounds difficult, use
the 'add to selection'
option instead of using Shift.
To
subtract from the existing selection, hold down the
Alt key while you select the area to be subtracted (look
for the small '-' sign) or choose the 'subtract from the
current selection' option
.
The fact that you can easily add to
or subtract from a selected area should be reassuring for
you, for it means that if your first attempt at selecting an
area is not quite right then you don't need to start all
over again - just make corrections to the selection by
adding or subtracting as necessary.
Now you are ready to begin learning about the magic
wand. Click NEXT below...
