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<h3 class="title"><a id="gimp-using-concepts"></a>3.5. Basic Gimp Concepts</h3>
</div>
</div>
</div>
<a id="id3308023" class="indexterm"></a>
<p>
This section is intended to give you a brief introduction to the
basic concepts and terminology you will need to understand in
order to make sense to the rest of the documentation. Everything
here is explained in much greater depth elsewhere. With a few
exceptions, we have avoided cluttering this section with a lot of
links and cross-references: everything mentioned here is so
high-level that you should easily be able to locate it in the
index.
</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">Overview</span>
</dt>
<dd>
<p>
The Gimp is an
image manipulation program. At the most sweeping level, using
Gimp involves three basic
steps: (1) opening images or creating new ones; (2) altering
those images; (3) saving the results.
</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">Opening Images</span>
</dt>
<dd>
<p>
Depending on how Gimp was started, there may already be
one or more images open when you begin. You can open new
images from files using the <a href="ch05s02s02.html" title="2.2. Open">Open</a> command from the File
menu. Gimp is capable of opening a large variety of
graphics file formats; see <a href="ch02s05.html" title="5. Files">Files</a> for more information.
Depending on how your system is set up, you may also be
able to open images by clicking on icons in a file
manager, or by drag-and-drop from other programs. If you
aren't sure whether you can do this, just try it. The
worst thing that can happen is that your computer could
explode.
</p>
</dd>
<dt>
<span class="term">Altering Images</span>
</dt>
<dd>
<p>
Gimp provides you with an enormous number of ways of
acting on images: painting tools, color manipulation
tools, transformation tools, filters, etc. The bulk of
this manual is devoted to describing these tools and how
to work with them.
</p>
</dd>
<dt>
<span class="term">Saving Images</span>
</dt>
<dd>
<p>
When you are finished working with an image, you will want
to save the results. (In fact, it is often a good idea to
save at intermediate stages too: Gimp is a pretty robust
program, but we have heard rumors, possibly apocryphal,
that it may have been known on rare and mysterious
occasions to crash.) Most of the file formats that Gimp
can open, can also be used for saving. There is one file
format that is special, though: XCF is Gimp's native
format, and is useful because it stores
<span class="emphasis"><em>everything</em></span> about an image (well,
almost everything; it does not store "undo" information).
Thus, the XCF format is especially suitable for saving
intermediate results, and for saving images to be
re-opened later in Gimp. XCF files are not readable by
most other programs that display images, so once you have
finished, you will probably also want to save the image in
a more widely used format, such as JPEG, PNG, TIFF, etc.
</p>
</dd>
</dl>
</div>
</dd>
<dt>
<span class="term">Images</span>
</dt>
<dd>
<p>
Images are the basic entities that Gimp works with. Roughly
speaking, an "image" corresponds to a single file, such as a TIFF
or JPEG file. You can also think of an image as corresponding
to a single display window, but this is not quite correct: it
is possible to have multiple windows all displaying the same
image. It is not possible to have a single window display more
than one image, though, or for an image to have no window
displaying it.
</p>
<p>
A Gimp image may be quite a complicated thing. Instead of
thinking of it as something like a sheet of paper with a picture
on it, you should think of it as more like a book, whose pages
are called "layers". In addition to a stack of layers, a Gimp
image may contain a selection mask, a set of channels, and a set
of paths. In fact, Gimp provides a mechanism for attaching
arbitrary pieces of data to an image, as what are called
"parasites".
</p>
<p>
In Gimp, it is possible to have many images open at the same
time. If they are large, each image may use many megabytes of
memory, but Gimp uses a sophisticated tile-based memory
management system that allows it to handle even very large
images gracefully. There are, however, limits, and it is
usually beneficial when working with images to put as much
memory into your system as possible.
</p>
</dd>
<dt>
<span class="term">Layers</span>
</dt>
<dd>
<p>
If an image is like a book, then a layer is like a page within
the book. The simplest images only contain a single layer, and
can be treated like single sheets of paper, but sophisticated
Gimp users often deal with images containing many layers, even
dozens of them. Layers need not be opaque, and they need not
cover the entire extent of an image, so when you look at an
image's display, you may see more than just the top layer: you
may see elements of many layers.
</p>
</dd>
<dt>
<span class="term">Channels</span>
</dt>
<dd>
<p>
TO BE WRITTEN
</p>
</dd>
<dt>
<span class="term">Selections</span>
</dt>
<dd>
<p>
Often when you do something to an image, you only want a part of
it to be affected. The "selection" mechanism makes this
possible. Each image has its own selection, which you normally
see as an moving dashed line separating the selected parts
from the unselected parts (the so-called "marching ants").
Actually this is a bit misleading: selection in Gimp is really
graded, not all-or-nothing, and really the selection is
represented by a full-fledged grayscale channel. The dashed
line that you normally see is simply a contour line at the
50%-selected level. At any time, though, you can visualize the
selection channel in all its glorious detail by toggling the
<a href="ch04s03s05.html" title="3.5. Quick Mask">QuickMask</a>
button.
</p>
<p>
A large component of learning how to use Gimp effectively is
acquiring the art of making good selections---selections that
contain exactly what you need and nothing more. Because
selection-handling is so centrally important, Gimp gives you a
large number of tools for doing it: an assortment of
selection-making tools, a menu of selection operations, and the
ability to switch to Quick Mask mode, in which you can treat the
selection channel as though it were a color channel, thereby
"painting the selection".
</p>
</dd>
<dt>
<span class="term">Undoing</span>
</dt>
<dd>
<p>
When you make mistakes, you can undo them. Nearly
everything you can do to an image is undoable. In fact, you can
usually undo a substantial number of the most recent things you
did, if you decide that they were misguided. Gimp makes this
possible by keeping a history of your actions. This history
consumes memory, though, so undoability is not infinite. Some
actions use very little undo memory, so that you can do dozens
of them before the earliest ones are deleted from this history;
other types of actions require massive amounts of undo memory.
You can configure the amount of memory Gimp allows for the undo
history of each image, but in any situation, you should always
be able to undo at least your 2-3 most recent actions. (The
most important action that is not undoable is closing an image.
For this reason, Gimp asks you to confirm that you really want
to close the image if you have made any changes to it.)
</p>
</dd>
<dt>
<span class="term">Plug-ins</span>
</dt>
<dd>
<p>
Many, probably most, of the things you do to an image in Gimp
are done by the Gimp application itself. However, Gimp also
makes extensive use of "plug-ins", which are external programs
that interact very closely with Gimp, and are capable of
manipulating images and other Gimp objects in very sophisticated
ways. Many important plug-ins come packaged together with Gimp,
but there are also many available by other means. In fact, the
ability to write plug-ins (and scripts) is the easiest way for
people not on the Gimp development team to add new capabilities
to Gimp.
</p>
<p>
All of the commands in the Filters menu, and a substantial
number of commands in other menus, are actually implemented by
plug-ins.
</p>
</dd>
<dt>
<span class="term">Scripts</span>
</dt>
<dd>
<p>
In addition to plug-ins, which are programs written in the C
language, Gimp can also make use of scripts. The largest number
of existing scripts are written in a language called Script-Fu,
which is special to Gimp (for those who care, it is a dialect of
the Lisp-like language called Scheme). It is also possible to
write Gimp scripts in Python or Perl. These languages are more
flexible and powerful than Script-Fu; their disadvantage is that
they depend on software that does not automatically come
packaged with Gimp, so they are not guaranteed to work correctly
in every Gimp installation.
</p>
</dd>
</dl>
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