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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
"http://www.docbook.org/xml/4.3/docbookx.dtd";>
<!-- section history:
2009-03-20 j.h: fixed bug #557343
2008-06-03 prokoudine: yet another shot at Russian content
2007-02-27 prokoudine: fixes to Russian translation
2007-02-27 lexa: reorganized concepts
2006-05-02 Dust: added Korean translation
2006-02-27 kolbjÃrn: added norwegian
2006-01-07 HdJ: Added quote and acronym tags, added english and dutch
version of layers explanation
2005-12-18 Lexa: reviewed and added de translation
-->
<sect1 xmlns:xi="http://www.w3.org/2001/XInclude"; id="gimp-concepts-basic">
<title>Basic Concepts</title>
<indexterm>
<primary>Concepts</primary>
</indexterm>
<figure>
<title>Wilber, the GIMP mascot</title>
<mediaobject>
<imageobject>
<imagedata format="PNG" fileref="images/using/wilber.png" />
</imageobject>
<caption>
<para>
The Wilber_Construction_Kit (in src/images/) allows you to give the
mascot a different appearance. It is the work of Tuomas Kuosmanen
(tigertATgimp.org).
</para>
</caption>
</mediaobject>
</figure>
<para>
This section provides a brief introduction to the basic concepts and
terminology used in <acronym>GIMP</acronym>. The concepts presented here
are 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 can easily
locate it in the index.
</para>
<variablelist>
<varlistentry>
<term>Images</term>
<listitem>
<para>
Images are the basic entities used by <acronym>GIMP</acronym>.
Roughly speaking, an <quote>image</quote> 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 (although in truth 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.
</para>
<para>
A <acronym>GIMP</acronym> image may be quite a complicated thing.
Instead of thinking of it as a sheet of paper with a
picture on it, think of it as more like a stack of sheets, called
<quote>layers</quote>. In addition to a stack of layers, a
<acronym>GIMP</acronym> image may contain a selection mask, a set of
channels, and a set of paths. In fact, <acronym>GIMP</acronym> provides
a mechanism for attaching arbitrary pieces of data, called
<quote>parasites</quote>, to an image.
</para>
<para>
In <acronym>GIMP</acronym>, it is possible to have many images open
at the same time. Although large images may use many megabytes of
memory, <acronym>GIMP</acronym> uses a sophisticated
tile-based memory management system that allows
<acronym>GIMP</acronym> to handle very large images gracefully.
There are limits, however, and having more memory available
may improve system performance.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Layers</term>
<listitem>
<para>
If a simple image can be compared to a single sheet of paper, an image
with layers is likened to a sheaf of transparent papers stacked one on
top of the other. You can draw on each paper, but still see the contant
of the other sheets through the transparent areas. You can also move one
sheet in relation to the others.
Sophisticated <acronym>GIMP</acronym> 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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Resolution</term>
<listitem>
<para>
Digital images comprise of a grid of square elements of varying colors,
called pixels. Each image has a pixel size, such as 900 pixels wide by
600 pixels high. But pixels don't have a set size in physical space. To
set up an image for printing, we use a value called resolution, defined
as the ratio between an image's size in pixels and its physical size
(usualy in inches) when it is printed on paper. Most file formats (but
not all) can save this value, which is expressed as ppi - pixels per
inch. When printing a file, the resolution value determines the size the
image will have on paper, and as a result, the physucal size of the
pixels. The same 900X600 pixel image may be printed as a small 2X3" card
with barely noticable pixels - or as a large poster with large, chunky
pixels. Images imported from cameras and mobile devices tend to have a
resolution value attached to the file. The value is usually 72 or 96ppi.
It is important to realize that this value is arbitrary and was chosen
for historic reasons. You can always change the resolution value inside
<acronym>GIMP</acronym> - this has no effect on the actual image pixels.
Furthermore, for uses such as displaying images on line, on mobile
devices, television or video games - in short, any use that is not print
- the reslution value is meaningless and is ignored, and instead the
image is usually displayed so that each image pixel conforms to one
screen pixel.
</para>
</listitem>
</varlistentry>
<varlistentry id="gimp-concepts-channels" xreflabel="Channels">
<term>
<phrase>Channels</phrase>
<indexterm>
<primary>Channel</primary>
</indexterm>
</term>
<listitem>
<!--TRANSLATORS: this is the modified text from glossary.xml, so
you should check po/LANG/glossary.po for an old translation-->
<para>
A Channel is a single component of a pixel's color. For a colored
pixel in <acronym>GIMP</acronym>, these components are usually Red,
Green, Blue and sometimes transparency (Alpha). For a
<link linkend="glossary-graylevel">Grayscale</link> image, they are
Gray and Alpha and for an
<link linkend="glossary-indexedcolors">Indexed</link> color image,
they are Indexed and Alpha.
</para>
<para>
The entire rectangular array of any one of the color components for
all of the pixels in an image is also referred to as a Channel. You
can see these color channels with the
<link linkend="gimp-channel-dialog">Channels dialog</link>.
</para>
<para>
When the image is displayed, <acronym>GIMP</acronym> puts these
components together to form the pixel colors for the screen,
printer, or other output device. Some output devices may use
different channels from Red, Green and Blue. If they do,
<acronym>GIMP</acronym>'s channels are converted into the
appropriate ones for the device when the image is displayed.
</para>
<para>
Channels can be useful when you are working on an image which needs
adjustment in one particular color. For example, if you want to
remove <quote>red eye</quote> from a photograph, you might work on
the Red channel.
</para>
<para>
You can look at channels as masks which allow or restrict the output
of the color that the channel represents. By using Filters on the
channel information, you can create many varied and subtle effects
on an image. A simple example of using a Filter on the color
channels is the
<link linkend="plug-in-colors-channel-mixer">Channel Mixer</link>
filter.
</para>
<para>
In addition to these channels, <acronym>GIMP</acronym> also allows
you to create other channels (or more correctly, Channel Masks),
which are displayed in the lower part of the Channels dialog. You
can create a <link linkend="gimp-channel-new">New Channel</link> or
save a <link linkend="gimp-selection-to-channel">selection to a
channel (mask)</link>.
See the glossary entry on
<link linkend="glossary-masks">Masks</link> for more information
about Channel Masks.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Selections</term>
<listitem>
<para>
Often when modify an image, you only want a part of the image
to be affected. The <quote>selection</quote>
mechanism makes this possible. Each image has its own selection,
which you normally see as a moving dashed line separating the
selected parts from the unselected parts (the so-called
<quote>marching ants</quote> ). Actually this is a bit misleading:
selection in <acronym>GIMP</acronym>
is 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
<link linkend="gimp-image-window-qmask-button">QuickMask</link>
button.
</para>
<para>
A large component of learning how to use <acronym>GIMP</acronym>
effectively is acquiring the art of making good
selectionsâselections that contain exactly what you need and nothing
more. Because selection-handling is so centrally important,
<acronym>GIMP</acronym> provides many 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 <quote>painting the selection</quote>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Undoing</term>
<listitem>
<para>
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. <acronym>GIMP</acronym> 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 <acronym>GIMP</acronym> 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, <acronym>GIMP</acronym> asks you to confirm that you really
want to close the image if you have made any changes to it.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Plug-ins</term>
<listitem>
<para>
Many, probably most, of the things that you do to an image in
<acronym>GIMP</acronym> are done by the <acronym>GIMP</acronym>
application itself. However, <acronym>GIMP</acronym> also makes
extensive use of <quote>plug-ins</quote>, which are external programs
that interact very closely with <acronym>GIMP</acronym>, and are
capable of manipulating images and other <acronym>GIMP</acronym>
objects in very sophisticated ways. Many important plug-ins are
bundled with <acronym>GIMP</acronym>, but there are also
many available by other means. In fact, writing
plug-ins (and scripts) is the easiest way for people not on the
<acronym>GIMP</acronym> development team to add new capabilities to
<acronym>GIMP</acronym>.
</para>
<para>
All of the commands in the Filters menu, and a substantial number of
commands in other menus, are actually implemented as plug-ins.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Scripts</term>
<listitem>
<para>
In addition to plug-ins, which are programs written in the C
language, <acronym>GIMP</acronym> can also make use of scripts. The
largest number of
existing scripts are written in a language called Script-Fu, which
is unique to <acronym>GIMP</acronym> (for those who care, it is a
dialect of the
Lisp-like language called Scheme). It is also possible to write
<acronym>GIMP</acronym>
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
<acronym>GIMP</acronym>, so
they are not guaranteed to work correctly in every
<acronym>GIMP</acronym>
installation.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
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