2.2 Histogram

The histogram function generates a distributional histogram (by default) for the currently displayed image data. For a multi-band image, a separate histogram graph is generated for each displayed image band (i.e. one for red, one for green, and one for blue). For a single-band image, only one histogram graph is displayed. Along with the graphs, the mean pixel value, standard deviation, minimum pixel value and maximum pixel value for each band are also calculated. The user may also obtain histogram percentage information by moving the cursor over the histogram graph.

Percentage Information:
This information is generated when the user moves the cursor over the histogram graph within the bounds of the X axis of the graph (the Y position of the cursor is not relevant). The cursor position, along the X axis, indicates a specific gray level value. The percentage value represents the percentage of pixels with a gray level value less than or equal to the selected gray level.
The image, line, and poly radio buttons allow the user to select the object the histogram is to be calculated for.

Image:
A histogram is calculated for the currently displayed image (default).

Line:
A histogram is calculated for all pixels that lie along a line.

Poly:
A histogram is calculated for all pixels that are contained within a defined polygon.
The X/Y, X/Z, and Y/Z radio buttons allow the user to select the dimension the histogram is to be calculated in.

X/Y:
A histogram is calculated for all bands of the currently displayed image.

X/Z:
A histogram is calculated for all bands along the line of the image indicated by the cursor position for all bands currently displayed. If the original radio button is selected, the histogram values will reflect the original plane data and if the mapped radio button is selected, the histogram values will reflect the plane data as it is currently displayed. Any graphics elements currently applied to the image will be included in the calculation of the mapped histogram.

Y/Z:
A histogram is calculated for all bands along the column of the image indicated by the cursor position for all bands currently displayed. If the original radio button is selected, the histogram values will reflect the original plane data and if the mapped radio button is selected, the histogram values will reflect the plane data as it is currently displayed. Any graphics elements currently applied to the image will be included in the calculation of the mapped histogram.
The distributional and cumulative radio buttons allow the user to select the type of histogram to be displayed.

Distributional:
The x-axis of the histogram represents gray level values 0 to 255; the y-axis represents the total number of pixels that have the corresponding gray level value.

Cumulative:
The x-axis of the histogram represents gray level values 0 to 255; the y-axis represents the total number of pixels that have the corresponding gray level as well as all previous gray levels.
The original and mapped radio buttons allow the user to select the type of data the histogram is to be calculated for.

Original:
Calculate a histogram for the image values as they are loaded into the image planes (i.e., for non-byte data, the unscaled values are not used).

Mapped:
Calculate a histogram for the mapped image values as they appear on the display monitor (i.e., the current mapping is applied before the histogram is calculated).
Mask Values:
Enter specific pixel values and/or ranges of values to be excluded from the histogram calculations. Specific pixel values are separated by commas, while range values are separated by a dash. The following conditions cause an error message to be displayed:

  1. The first character in the mask value specification is not numeric.
  2. Any alphabetic characters appear in the specification.
  3. Any punctuation marks other than a dash (-) or a comma (,) are used.
  4. Two consecutive punctuation marks appear in the specification.
  5. Both ends of a range are not specified (i.e. "10 - " or "- 100").
  6. A value less than 0 or greater than 255 is specified.

Update:
Calculate and update the histogram based on the currently selected options.

Dismiss:
Close the histogram panel.