[Courses][gimp] Lesson 7: Selecting Complex Shapes

[Akkana Peck]

In Lesson 4, we used some simple basic tools to select part of an
image.  In this lesson, I'll cover selection of complex shapes.
I'll use a butterfly as my sample:
http://shallowsky.com/images/gimpcourse/lesson7/butterfly.jpg

GENERAL ADVICE

When selecting images out of an image, there are two important tricks:
(1) Zoom the image as much as you can, so you can see what you're
doing, and (2) Turn on "feathering".

You may remember zooming from Lesson 1: it changes the size at
which the image is displayed.  Use the View->Zoom menu, or (in gimp
2.2) the menu at the bottom of the image window; or just hit ctrl-= or
ctrl-+ repeatedly until the image is as big as you need it to be.

Feathering means making the edges of the selection "fuzzy".
The edge will only be partially selected, so if you paste such
a selection, the edges will merge with the background below it.
This is important when you're cutting shapes out of an image,
because it helps you avoid sharp edges that make it obvious
that the image has been changed.  Most selection tools have
Feathering as an option in their Tool options.

LASSO SELECT

We touched briefly on the Lasso selection tool in the last lesson.
With it, you can draw an outline around an object in order to select
that object.  But as a few of you found out when you tried to use it
to cut out specific objects, it's tricky to use: you have to hold the
mouse down and drag out the whole shape in one complex motion.

BEZIER PATHS

That's where Bezier Paths come in!  The Bezier (pronounced
"bez-ee-ay") Path tool looks sort of like a fountain pen drawing a
thread through a wire with lumps on it. :-) The tooltip says "Create
and Edit Paths" or "Select Regions with Bezier curves".  Don't get it
confused with the "magic scissors" tool right next to it, which is
somewhat similar (I'll talk about that in a minute).

The Paths tool lets you define a selection of any shape, like the
Lasso, but with Paths, you can go step by step, stop to take a rest,
and edit the path later.

One thing that means is that you can zoom waaaay in on the image.
Make it huge, like 300 or 400% of actual size.  It's okay if you need
scrollbars.  The important thing is that it's big enough that you can
click exactly where you need to, and not have to worry about whether
your hand is accurate enough with the mouse.

Time to start!  Click-and-release the mouse button wherever you want
to start your selection.  Then click a little farther down the line.
Continue around the image, playing "connect the dots" all the way
around. Make the dots as close together or as far apart as you need to
to follow the object's contour.  If you need to scroll, go ahead and
do so.  The mousewheel works for scrolling vertically, if you don't
want to break your concentration by moving over to the scrollbar.

What if you click in the wrong place?

You can edit bezier points at any time.  In gimp 2 this is easy:
go back and mouse over the errant point (watch the mouse cursor
carefully, and make sure it switches to the crossed-arrow icon that
means "move" in gimp), then move it.  "Undo" also works in gimp 2
to remove the last point, in case you clicked in the wrong place.

In gimp 1, ctrl-click seems to move the current point.  (If you hold
down Alt you'll see the cursor change to a "move" icon, but that
moves the whole path, not just one point.)

When you finally get all the way around your object and back to the
first point, click on the first point again to "close" the path.

Look over your path.  You have a nice connect-the-dots of straight
lines framing your image.  If you made the dots close enough together,
this is probably good enough.  But if you want to make the line follow
your object's outlines even more closely, you can make the lines
curve!  In gimp 2, click on Edit Mode: Edit in the tool options.
(Gimp 1 is always in this mode.)  Now, if you click on one of your
dots and drag away a little bit, "handles" will sprout from that
point, and by dragging the handles around, you can make the lines
adjacent to that point curve rather than being straight.  Try it --
it's easier to see than it is to explain.

In truth, I don't use the bezier handles very often.  It's usually
easier to make a lot of points connected by straight lines.  But it's
worthwhile knowing that the handles are there if you ever need them.

Now you have a path, and you've adjusted your points' positions and
handles as much as you're doing to.  You've probably spent quite a lot
of time on this: making a good path is hard work, and takes a lot of
concentration.  So at this point, I often make a backup copy of the
path.  Go to the Layers dialog and select Paths from the tabs at the
top.  You'll see the path you just created there.  Select "Duplicate
Path", either by right-clicking on the path's line in the Paths
dialog, or by clicking on the Duplicate button down at the bottom of
the dialog.  You might also want to name the path, by double-clicking
on the path, which selects the name and lets you type in a new name.

At this point, I recommend saving as XCF, if you haven't already done
so.  This will save the path and the selection, so you won't lose it.

Now turn the path into a selection: "Path to Selection", either from
the button at the bottom of the dialog, the right-click menu on the
path, or (in gimp 2) "Create selection from path" in tool options,
back in the toolbox window.

My selection looks like this:
http://shallowsky.com/images/gimpcourse/lesson7/butterfly-selected.jpg

FEATHERING A PATH-BASED SELECTION

Voila!  You have a beautiful selection.  But what about feathering?

Gimp 1.2 has a Feathering checkbox in the Bezier Selection Tool
Options dialog, like most selection tools.  But that option is gone
from Gimp 2!  In gimp 2, you'll have to use the menus after your
selection is made: Select->Feather...
You can use that on any selection: sometimes it's useful if you've
made a rectangular or lasso selection and decide later that it needs
the edges feathered.

INTELLIGENT SCISSORS

GIMP has a second path tool: the so-called "intelligent scissors", the
scissors icon right next to the path tool (with the tooltip "Select
Shapes from Image").  The Scissors tool creates a path, just like the
regular path tool; but each time you click to create a new point, it
tries to guess where you really wanted it, and warp it there,
following whatever it can find that looks like an outline or sharp
boundary.  Some people like it; others don't.  Try it and see if it
works for you.  But most people end up using the regular path tool
instead of the scissors.

RUNNING FILTERS ON SELECTIONS

You've made your selection.  What can you do with it?

Of course, you can copy, then paste into another image: 
http://shallowsky.com/images/gimpcourse/lesson7/butterfly-on-white.jpg

But in addition, most gimp operations will operate only on the
selection, if a selection exists.  For example, in this image, the
background is a bit busy.  All the leaves detract a little from the
butterfly.  If I make them darker compared to the butterfly, the
butterfly itself will stand out better.

INVERTING THE SELECTION

Since I want to darken the background, not brighten the butterfly,
what I really want is a selection that includes everything *except*
the butterfly: the exact opposite of the selection I have now.
That's easy: from the menus, Select->Invert.

Now I bring up my any brightness tool from Lesson 2 (I'll use Curves)
and fiddle with the controls, and watch the background change.

HIDING THE SELECTION BOUNDARY

The only problem is that I can't see whether the boundary looks too
obvious, because now those darned "marching ants" are getting in the way.

But I can make the marching ants disappear temporarily:
View->Show Selection, or ctrl-T ("toggle selection").

WARNING: toggling selection visibility off is somewhat dangerous,
because the next time you use selection, you'll try to make a
selection and no marching ants will appear and you'll get all
confused.  Or you'll try to draw something, and it will only draw
in part of the image (where the selection is).  Or at least, I get
confused by that.  So try to remember to toggle selection
back on immediately after you're finished working on a selection.

With the marching ants gone, I can see what I'm doing, and I turn the
background's brightness down to enhance the butterfly, which reamains bright:
http://shallowsky.com/images/gimpcourse/lesson7/butterfly-darkbg.jpg

You can use this technique with most gimp filters: you can sharpen
an object while leaving the background unchanged, or even blur the
background.

BLURRING

You may have noticed all those blur options in gimp's menus, and
wondered, "Why would anyone want to make a picture more blurry
than it already is?"

Blurring is actually surprisingly useful.  For instance, those drop
shadows we've been using so much are made by making a copy of your
text, changing the color to black, then blurring the result.

Blurring can also be useful to de-emphasize parts of an image that
are distracting.  In this case, I'll blur the leaves, which will
make the butterfly look sharper in comparison.

Blur is a submenu, Filters->Blur, and there are lots of types.
I usually use Gaussian Blur, and in gimp 1.2, where there are two
types of Gaussian Blur, just pick one -- they're very similar.
But do experiment with some of the others, like Motion Blur --
they can be fun when you want special effects.

Experiment with blur radius.  Gimp 2 has a preview in the blur dialog,
so you can see exactly what the effect will be.  With gimp 1, just try
something, and undo if you decide you want more or less blur.

Here's my butterfly with a blur radius of 20 (looks unrealistic):
http://shallowsky.com/images/gimpcourse/lesson7/butterfly-blur.jpg
and with a blur radius of 7 (more subtle and, IMO, better):
http://shallowsky.com/images/gimpcourse/lesson7/butterfly-lessblur.jpg

Homework: Select a complex shape from an image, using path or
intelligent scissors, and apply any gimp filter (or combination of
filters) to either the shape or its background.  Or, if you prefer,
just paste the shape into another image.

Next lesson: How to get really cool looking text.