Gimp - 2.4 Guía Del Usuario
GNU Image Manipulation Program
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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.
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.
Actually, both RGB and grayscale images have one additional color channel, called the alpha 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 proportional
mixture of color from the layer and color from underneath.
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 proportional
mixture of color from the layer and color from underneath.
Figure 5.2: Example of an image in RGB and Grayscale mode
(a) An image in RGB mode, with the channels cor-
responding to Red, Green and Blue.
responding to Red, Green and Blue.
(b) An image in Grayscale mode, with the channel
corresponding to Luminosity.
corresponding to Luminosity.
In GIMP, every color channel, including the alpha channel, has a range of possible values from 0 to 255; in computing terminol-
ogy, 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.
ogy, 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.
Figure 5.3: Example of an image with alpha channel
(a) Red channel
(b) Green channel
(c) Blue channel
(d)
The
Alpha
channel shows the
image area which
is transparent.
image area which
is transparent.
(e) A color image
in RGB mode with
an Alpha channel.
in RGB mode with
an Alpha channel.
The third type, indexed 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 which consumes
relatively little memory, and back in the dark ages (say, ten years ago), they were very commonly 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.)
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 which consumes
relatively little memory, and back in the dark ages (say, ten years ago), they were very commonly 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.)
Some very commonly used types of files (including
and
) 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 to save it
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 to save it
GIMP makes it easy to convert from one image type to another, using the
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.
in the other direction.