[Courses][gimp] Drawing a circle

Akkana Peck akkana at shallowsky.com
Tue Apr 12 03:22:26 EST 2005

Patricia Peck writes:
> After that, I fiddled with bump map on the same image.  Never did get 
> what I was aiming for, but this is kinda interesting:
> http://webpages.charter.net/patpeck/maskbump.jpg

That's a neat effect!  It's from a bump map?  It looks like it
must have been from a combination of a couple different text layers,
like the text and its shadow, or the text and a slightly larger
version of the same text.

Isn't it fun how you can get play around randomly and get
interesting new effects?  It was that way when I wrote the
lesson on layer masks: most of the effects were really boring,
then every now and then I'd go "Ooh, that's worth saving."
But the ones worth saving might have been quite different if
I'd started with different colors.  There are so many combinations!

Regarding drawing a circle:

GIMP doesn't have shape-drawing tools, but it does have shape
selection tools.  So the trick is to make a circular selection,
then turn that into a line.

The ellipse selection tool, of course, can make a circle, but it's
hard, just dragging it out, to make it a perfect circle with width
and height the same.  But pressing the shift key while you drag will
constrain the ellipse selection to be exactly circular.

There's a catch, though: shift has two meanings.  It can mean "Add
to current selection", and it can mean "constrain to a circle".
If you press shift first, before you start dragging, you'll see a
little "+" gets added to the mouse cursor: that means "whatever you
select now will be added to the current selection, instead of
replacing it."  But in addition, the selection will be constrained
to be circular.  (What if you want to add a non-circular selection?
I'll tell you that in a minute.)

However, if you start dragging first, and then, while the mouse is
still down, press shift, then the selection is constrained to a
circle, but will still replace any existing selection.

This works in the rectangular selection tool, too, for making squares.

I think in earlier versions of gimp, pressing shift to get a
circular selection also shifts the selection to be centered around
the point where you started, rather than using that as the upper
left point.  That was somewhat jarring, and gimp 2.2 doesn't do that
any more.  Remember, you can move a selection after you've made it
with alt-drag (if your windowmanager doesn't block that) or
alt-shift-drag (which works in most windowmanagers).

Okay, that's how to get a circular selection.  How do you draw a
line where the selection is?

Edit->Stroke Selection... will do that.  It brings up a dialog
where you can select the stroke width, or stroke with a paint tool
if you want to use a particular brush.

For some reason, I can never remember where this is (why is it in
Edit?), so sometimes I use another method: Select->Border...
replaces the selection with its own outline, and you can use
the bucket fill tool to fill that.  If you just want a circle,
Stroke Selection is better, but it's good to know that Border is
there too.  Explore that Select... menu: you might find other
functions there that will be useful to you.


I mentioned earlier that I'd tell you how to add a non-circular
selection to the current selection.  With the ellipse selection tool
active (or another selection tool, like rectangular or lasso), look
at the tool options.  At the top of the options dialog is a Mode
selector with several obscure buttons.  This lets you change whether
the selection you make will replace the current selection, add to
it, subtract from it, or intersect with it.

Why would you want to do this?  Actually it's extremely useful.
For example, remember the lesson on selection of complex objects,
where I selected a butterfly?  Suppose I made that selection,
including the butterfly's antenna, but then later decided that I
didn't want to enhance the antenna because they looked weird.
I could choose the Lasso tool, click the "Subtract from current
selection" button, then draw a big lasso around just the antenna
(very easy: I wouldn't have to outline them carefully, just make
sure that my big lasso includes all the parts of the selection which
I want to exclude).

"Add to current selection" can also be useful because it allows you
to combine several different selection tools into one selection.
You could use select by color or select similar regions to select
a complex shape, then use rectangular or elliptical selection in
"Add" mode to add a box or oval sticking off one side.

Often you can accomplish the same thing using layer masks or quickmask
(we haven't talked about the quickmask, but it's somewhat similar
to layer masks), but there are times where you specifically need a
selection with a complex shape, and add/subtract modes can be a
real time-saver then.

That ended up being almost a lesson.  But there's still no homework. :-)


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