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<h2 class="title" style="clear: both"><a id="gimp-using-images"></a>4. Working with Images</h2>
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<h3 class="title"><a id="id3307365"></a>4.1. Image types</h3>
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<p>
It is tempting to think of an <span class="emphasis"><em>image</em></span> as
something that corresponds a single display window, or to a single
file such as a <a href="go01.html#file-jpeg-load">JPEG</a> file,
but really a Gimp image is a rather complicated structure,
containing a stack of layers plus several other types of objects:
a selection mask, a set of channels, a set of paths, an "undo"
history, etc. In this section we are going to take a detailed
look at all of the components of an image, and the things you can
do with them.
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<p>
The most basic property of an image is its
<span class="emphasis"><em>mode</em></span>. There are three possible modes: RGB,
grayscale, and indexed. RGB stands
for Red-Green-Blue, and indicates that each point in the image
is represented by a "red" level, a "green" level, and a "blue"
level. Because every humanly distinguishable color can be
represented as a combination of red, green, and blue, RGB images
are full-color. Each color channel has 256 possible intensity
levels. More details in <a href="go01.html#glossary-colormodel">
Color Models</a>
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<p>
In a grayscale image, each point is represented by a brightness
value, ranging from 0 (black) to 255 (white), with intermediate
values representing different levels of gray.
</p>
<p>
Essentially the difference between a grayscale image and an RGB
image is the number of "color channels": a grayscale image has
one; an RGB image has three. An RGB image can be thought of as
three superimposed grayscale images, one colored red, one green,
and one blue.
</p>
<p>
Actually, both RGB and grayscale images have one additional
color channel, called the <span class="emphasis"><em>alpha</em></span> channel,
representing opacity. When the alpha value at a given location
in a given layer is zero, the layer is completely transparent,
and the color at that location is determined by what lies
underneath. When alpha is maximal, the layer is opaque, and the
color is determined by the color of the layer. Intermediate
alpha values correspond to varying degrees of translucency: the
color at the location is a proportionl mixture of color from the
layer and color from underneath.
</p>
<p>
In Gimp, every color channel, including the alpha channel, has a
range of possible values from 0 to 255; in computing
terminology, a depth of 8 bits. Some digital cameras can
produce image files with a depth of 16 bits per color channel.
Gimp cannot load such a file without losing resolution. In most
cases the effects are too subtle to be detected by the human
eye, but in some cases, mainly where there are large areas with
slowly varying color gradients, the difference may be
perceptible.
</p>
<p>
The third type, <span class="emphasis"><em>indexed</em></span> images, is a bit
more complicated to understand. In an
indexed image, only a limited set of discrete colors are used,
usually 256 or less. These colors form the "colormap" of the
image, and each point in the image is assigned a color from the
colormap. Indexed images have the advantage that they can be
represented inside a computer in a way that consumes relatively
little memory, and back in the dark ages (say, ten years ago),
they were very commmonly used. As time goes on, they are used
less and less, but they are still important enough to be worth
supporting in Gimp. (Also, there are a few important kinds of
image manipulation that are easier to implement with indexed
images than with continuous-color RGB images.)
</p>
<p>
Some very commonly used types of files (including <a href="go01.html#file-gif-load">GIF</a>) produce
indexed images when they are opened in Gimp. Many of Gimp's
tools don't work very well on indexed images–and many filters
don't work at all–because of the limited number of colors
available. Because of this, it is usually best to convert an
image to RGB mode before working on it. If necessary, you can
convert it back to indexed mode when you are ready of save it
</p>
<p>
Gimp makes it easy to convert from one image type to another,
using the <a href="ch05s06s02.html" title="6.2. Mode">Mode</a> command in
the Image menu. Some types of conversions, of course (RGB to
grayscale or indexed, for example) lose information that cannot
be regained by converting back in the other direction.
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<th align="left">Note</th>
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<p>
If you are trying to use a filter on an image, and it appears
grayed out in the menu, usually the cause is that the image (or,
more specifically, the layer) you are working on is the wrong
type. Many filters can't be used on indexed images. Some can
be used only on RGB images, or only on grayscale images. Some
also require the presence or absence of an alpha channel.
Usually the fix is to convert the image to a different type,
most commonly RGB.
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