2.5.  Histogram dialog

Revision History
Revision $Revision: 2416 $ 2007-09-30 j.h

The Histogram dialog shows you information about the statistical distribution of color values in the active layer or selection. 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.

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.

  • From the image menu: Colors->Info->Histogram.

The active layer name is shown at the top of the dialog.

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:

Linear/Logarithmic buttons

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 logarithmic histogram will often be more useful.

Range Setting

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 some basic statistics are shown describing the distribution of channel values, restricted to the selected range:

  • Mean : the mean value of the interval in the selected channel.

  • Std Dev : Standard deviation. Gives an idea about how homogeneous the distribution of values in the interval is.

  • Median : For example, the value of the fiftieth peak in a 100 peaks interval.

  • Pixels : The number of pixels in the active layer or selection.

  • Count : The number of pixels in a peak (when you click on the histogram) or in the interval.

  • Percentile : The ratio between the number of pixels in the interval and the total number of pixels in the active layer or selection.