I was recently asked for a quick way to
draw a border three pixels wide around the inside edge of a rectangular selection on top of an in-progress graphic design. Two methods immediately came to mind and a third occurred to me later that afternoon. I discovered a fourth method while writing this article.
Method 1
The most obvious method to use is
Select > Border [1], which opens the Border Selection
dialog [2]. This dialog contains three adjustments.
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| Border Selection Dialog |
The first adjustment is the
size-determinant of the border [3], which can be stated in the usual units (pixels, inches, etc.). I call it the size-determinant because it's not the actual size. The actual size is (size-determinant * 2) +
1, for values greater than or equal to 2. For a value of 1, the actual border size is 1. This, of course, means that you can't create a border 3 pixels in size with this function, nor can you create
borders of an even width. Also, other than for a border size of 1 (which creates an inner border), the border selection is centered over the original selection outline. There is no option to create either an inner border or an outer border.
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| Border size 5, solid fill, automatic corner rounding |
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| Border size 5, feathered, automatic corner rounding |
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| Lock selection is OFF |
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| Lock selection is ON |
Once you have chosen your options and
hit the OK button, a new selection area is drawn for the
border. You can then fill the area with whatever color or pattern you
prefer. Note that the outer edges of the corners of a rectangular
selection are automatically rounded. There is no control over that.
If you want square corners, you have to use a painting tool to paint
in the corners manually or otherwise fill in the missed corners
yourself.
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| Pattern fill |
Method 2
The second method is what I call
Fill-Shrink-Delete. Due to all of the restrictions of the
Border Selection function, this method turned out to be the easiest
way to accomplish creation of a 3-pixel inner border. It's best to do
this on a separate layer, then merge it down when you're done. After
creating a new transparent layer, fill the entire selection box with
the desired border color. Then, choose Select > Shrink [4]
and select a size [5] that is equal to your border width. Note that
if the selection is touching the edge of the image, then you must
tick Shrink from image border [6] to cause the selection to
shrink away from the edge.
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| Shrink Selection |
Alternatively, you could open the
selection tool's Select options dialog [7], tick Expand
from center [8], and reduce the Size [9] width and height
fields by twice the size of the border. Finally, hit the Delete
key to remove the inside of the selection. This results in a solidly
filled border with sharp corners.
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| Select Options |
If you want a feathered border, it's a
bit more complicated. After making the first selection, but before
filling it, choose Select > Feather [10] and choose a
feathering amount [11], then fill the selection. Before you can
shrink the selection, you have to left-click inside the selection
area to re-activate the selection box. You'll see that it shows as
being slightly larger than your original selection in order to
accommodate the feathering. This makes it a little tricky to figure
out by how much to reduce the selection area size. Shrink the
selection using one of the aforementioned methods. You may wish to
subtract a few extra pixels to account for the next feathering.
Choose Select > Feather and feather this selection, then
hit the Delete key.
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| Feather Selection |
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| Feather = 3, Shrink = 7 |
Method 3
The third method is pretty quick to do
and involves the use of a painting tool, but it's useful only for
rectangular selections. Choose the Pencil or Paintbrush
tool and set the Brush size to the desired width of the border
and the Brush style to the desired hardness or softness. Zoom
in if necessary and left-click the brush exactly in one corner.
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| Start painting a line |
Scroll to an adjacent corner, hold down
the Shift key, and left-click in that corner to draw a line
along the edge from the previous corner. Note that when you hold down
the Shift key you'll see a line [12] connecting the previously
clicked location to the current cursor position. This indicates the
path that will be affected when you left-click again.
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| Continue painting a line |
Repeat these steps in order around the
rectangle. If your brush is not a square, you won't be able to
touch the exact corner, so you'll have to touch up the corners
afterward. If you want the outside edges of the lines to be
feathered, choose Select > Feather before using the brush.
Note, however, that the inner edges of the lines will not be
feathered. Alternatively, for full feathering, you can use the
Paintbrush tool with a fuzzy brush.
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| Feather 5, Hard pencil |
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Hardness 75 paintbrush
|
Method 4
The fourth method involves using the
Stroke Selection dialog. This dialog is hidden in the
Selection Editor, which is accessed via Select >
Selection Editor [13]. In the lower right-hand corner is an icon
that looks like a square with a paintbrush [14]. Clicking it opens
the Stroke Selection dialog [15]. There are two main options:
Stroke line and Stroke
with a paint tool.
 |
| Stroke Selection Dialog |
The controls for Stroke line are
Solid color, Pattern, Antialiasing, Line
width, and Line Style.
Solid color and Pattern are mutually exclusive and
determine how the border is filled. Solid color fills with the
current foreground color. Pattern fills with the current
pattern, which can be seen by opening the Patterns dialog via
Windows > Dockable Dialogs > Patterns (Ctrl+Shift+P).
If Antialiasing is ticked, the border will be slightly
feathered; otherwise, it will be solidly filled. Line width
controls the width of the border. An even width is drawn centered
over the selection outline. An odd width is drawn with the extra
pixel on the outside of the selection. Line
Style can be opened to allow a choice of Cap style,
Join style, Miter limit, Dash pattern, and Dash
preset. It is here where you can, for example, choose to have
square or rounded corners.
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| Solid color solid line, line width 7, hard corners |
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| Pattern with antialiasing, line width 10, rounded corners |
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| Solid color dotted line, line width 6, rounded corners |
The controls for Stroke with a paint
tool are Paint tool
and Emulate brush dynamics. In GIMP 2.8.6, the Paint
tool options are Pencil, Paintbrush (default), Eraser, Airbrush, Ink, Clone,
Heal, Perspective Clone, Convolve, Smudge, and Dodge/Burn. For the paint tools that are painting a color, the foreground color is used.
 |
| Paintbrush, Dynamics off in tool options, Emulate brush dynamics ticked |
Ticking
Emulate brush dynamics theoretically enables the use of
the type of Dynamics that is set for the chosen paint tool in
that tool's options dialog. In actuality, the brush dynamics are used
whether this field is ticked or not.
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| Paintbrush, Dynamics on in tool options, Emulate brush dynamics not ticked |
The Stroke Selection dialog is
pretty cool and can be used to create interesting borders quickly;
however, I see room for improvement in two areas. It would be nice to
be able to control how the border is drawn around the selection —
whether inside, outside, or straddling. It would also be nice if the
dialog stayed open after hitting the Stroke button. As it is
now, if you're experimenting with borders, you have to keep
re-opening this dialog.
Bonus Method 5 (21 May 2014)
Rob A. on GimpChat suggested a fifth alternative: Turn the
selection into a path and stroke the path. He says this is especially
helpful to eliminate jaggies on non-orthagonal sections.
After creating a
selection, choose Select > To Path. Open the Paths
dialog and turn on visibility of the path you just created [16].
Right-click on the path in the Paths dialog and choose Stroke Path
[17]. This will open the Stroke Path dialog, which has the
same options as the Stroke Selection dialog described under Method 4.
When you click the Stroke button, a nice smooth stroke is made
along the path.
In the following
image, the upper red line was created using Method 4 on a selection
and the lower red line was created using Method 5 on a path. I had
the same options set for both strokes. Notice how nice the path
stroke looks compared to the selection stroke. (Also notice how the
selection stroke is not anti-aliased even though I had that option
ticked. I guess that's a bug.)
Conclusion
As can be seen here, and as is not
uncommon, there are multiple ways to accomplish a given task in GIMP.
The method you choose depends on what you're trying to achieve. I
hope you learned something useful from this article. I certainly did
while writing it. If you know another method for creating a border
around a selection area, please share it so we can all benefit.
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