The Histogram dialog shows you information about the statistical
distribution of color values in the image that is currently active. This
information is often useful when you are trying to
color balance
an image. However, the Histogram dialog is purely informational: nothing
you do with it will cause any change to the image. If you want to perform
a histogram-based color correction, use the
Levels tool.
2.5.1.
Dialog call
This is a dockable dialog; see the section on
Dialogs and Docking
for help on manipulating it. It can be activated in two ways:
From the Toolbox menu:
File->Dialogs->Histogram
From the image menu:
Dialogs->Histogram
2.5.2.
About Histograms
In GIMP, each layer of an image can be decomposed into one or more color
channels: for an RGB image, into R, G, and B channels; for a grayscale
image, into a single Value channel. Layers that support transparency
have an additional channel, the alpha channel. Each channel supports a
range of intensity levels from 0 to 255 (integer valued). Thus, a black
pixel is encoded by 0 on all color channels; a white pixel by 255 on all
color channels. A transparent pixel is encoded by 0 on the alpha
channel; an opaque pixel by 255.
For RGB images, it is convenient to define a Value "pseudochannel". This
is not a real color channel: it does not reflect any information stored
directly in the image. Instead, the Value at a pixel is given by the
equation V = max(R,G,B).
Essentially, the Value is what you would get at that pixel if you
converted the image to Grayscale mode.
For more information on channels, please consult the
Working with Images
section.
2.5.3.
Using the Histogram dialog
Channel
Figure 9.16.
Channel options for an RGB layer with alpha channel.
This allows you to select which channel to use. The possibilities
depend on the layer type of the active layer. Here are the entries
you might see, and what they mean:
Value
For RGB and Grayscale images, this shows the distribution of
brightness values across the layer. For a grayscale image,
these are read directly from the image data. For an RGB
image, they are taken from the Value pseudochannel.
For an indexed image, the "Value" channel actually shows the
distribution of frequencies for each colormap index: thus,
it is a "pseudocolor" histogram rather than a true color
histogram.
Red, Green, Blue
These only appear for layers from RGB images. They show the
distribution of intensity levels for the Red, Green, or Blue
channels respectively.
Alpha
This shows the distribution of opacity levels. If the layer
is completely opaque or completely transparent, the
histogram will consist of a single bar on the left or right
edge.
RGB
Figure 9.17.
Combined histograms of R, G, and B channels.
This entry, only available for RGB layers, shows the R, G,
and B histograms superimposed, so that you can see all of
the color distribution information in a single view.
Linear/Logarithmic buttons
Figure 9.18.
The histogram shown at the top, changed to logarithmic mode.
These buttons determine whether the histogram will be displayed
using a linear or logarithmic Y axis. For images taken from
photographs, the linear mode is most commonly useful. For images
that contain substantial areas of constant color, though, a linear
histogram will often be dominated by a single bar, and a
logarthmic histogram will often be more useful.
Range Setting
Figure 9.19.
Dialog aspect after range fixing.
You can restrict the analysis, for the statistics shown at the
bottom of the dialog, to a limited range of values if you wish.
You can set the range in one of three ways:
Click and drag the pointer across the histogram display area,
from the lowest level to the highest level of the range you
want.
Click and drag the black or white triangles on the slider
below the histogram.
Use the spinbutton entries below the slider (left entry:
bottom of range; right entry: top of range).
Statistics
At the bottom of the dialog are shown some basic statistics
describing the distribution of channel values, restricted to the
selected range. These are the mean, standard deviation, and median
of the selected histogram portion; the number of pixels in the
image; the number whose values fall within the selected range; and
the percentage whose values fall within the selected range.
Published under the terms of the GNU General Public License
