Image Histogram
Click the Image Histogram tool to open the histogram window and display the intensity histogram and statistics for the active image or ROI. A histogram shows you a frequency distribution of the intensities in your image. It describes, in graphic form, the brightness and contrast characteristics of your image. If you are not familiar with histograms, you may want to refer to the topic, Histograms.
When you select the Histogram command, the Image Histogram panel is displayed showing a graphical representation of the distribution of pixel values in the image.
By default, the histogram’s X-axis will be calibrated to your intensity unit-of-measure and the Y-axis will represent your spatial unit-of-measure.
Important - the histogram measures pixel values as they exist in the image bitmap. It does not reflect values as they are interpreted though the LUT. This allows you to make visual adjustments to your image for viewing purposes without affecting the data used for analysis. In the event that you want to have LUT intensity adjustments reflected in your histogram, you must apply the LUT to your bitmap using the Apply LUT command.
There are several controls at the top of the Histogram window. They are described below.
Reset:
Click this
to undo all LUT, Brightness, Contrast, Gamma, Black Level, and White Level
adjustment that have been made to the active image.
Best Fit:
Click on
this tool to reveal the automatic Black Level and White Level adjustment
options. Click
here to learn more about
the options on this menu.
Auto-Range:
This control
is only relevant if live video is currently active in the Image-Pro
workspace. This
control is the same as the Dynamic Auto-Range
control found in the Settings
drop-down list box of the Camera
group. It
allows you to enable or disable Image-Pro's
dynamic display range feature. When
enabled, Image-Pro
continually adjusts the display range settings for the live image, always
ensuring that the "best
fit" black level and white level values are applied even as the
scene changes over time. In
other words, if this checkbox is checked, as the scene changes, the black
level and white level values will be automatically adjusted to suit the
scene. When
Dynamic Auto-Range is unchecked,
Image-Pro
applies the dynamic range values defined through the Adjust > Display
controls to the live image. These
values are static — they do not change as the scene changes.
Collect:
Use this control to populate the measurement tables of the Data Collector with the
data currently appearing in the Histogram window. For
example, go to Measure > Data Collector
and click on Data Table to show
the Data Collector Table, then click the Collect
tool on the Image Histogram panel to populate the Data Collector Table with
the current histogram data. Click
the down arrow of the Collect tool to show the following options:
- All Frames: This option is only meaningful if the active image is an image sequence. If this option is selected, Image-Pro collects histogram data for all frames of the sequence. If
this is unchecked, only the currently active frame's histogram data will
be collected.
- Clear Collector: Select this option to have Image-Pro clear the collector of all other data before it adds the histogram data. You might want to do this, for example, if there is stale data in the Collector from previous data collections. If
left unchecked, the histogram data will simply be added to any
other data that is already in the Collector tables.
- Clear Measurements: Select this option to have Image-Pro clear the collector tool's measurement filter before collecting the histogram data. You might want to do this, for example, to ensure filters set from previous data collections do not interfere with the data you are now attempting to collect. If left unchecked, any existing measurement filters will be applied to the collection of histogram data.
The appearance of the histogram can be modified, and its data saved to a file, using the context menu options (described below).
Image Histogram Window Context Menu
When you right-click in the Image Histogram window, a context menu appears with additional options.
Histogram: Shows/hides the histogram in the Image Histogram window.
Dynamic: Shows/hides the dynamic representation of the histogram. This is a virtual histogram of the image after the black level, white level, and LUT adjustments have been applied.
LUT: Shows/hides the graphical representation of the brightness, contrast, and gamma adjustments.
BW Levels: Shows/hides the dotted vertical bars representing the black level and the white level.
View: Click on
this option to reveal the following view options:
- Axis X: Shows/hides the X axis values along the bottom of the histogram.
- Axis Y: Shows/hides the Y axis values along the left side of the histogram.
- Axis Y Log: Toggles the Y axis of the Image Histogram display between expressing the Intensity Value scale in terms of integers or logarithms of integers. The latter provides a 'stretching' effect.
- Legend: Shows/hides the Image Histogram window legend.
- Statistics: Shows/hides the Statistics table. Also, provides options for the placement of the Statistics table within the Image Histogram window.
Options: Click
on the Options tool to reveal the following sub-menu options:
- Ignore ROI: If selected, displays histogram data for the entirety of the currently active image, regardless of ROIs that have been defined.
- Active frame: This option is only relevant for sequence files. If selected, displays histogram data for only the active frame of the sequence file. Otherwise, all frames of the sequence are included in the histogram calculation.
- Mono interpretation: This option is only relevant for color images. If selected, luminance values are calculated from the R, G, and B channels for all pixels, and the luminance values are graphed in the histogram. Otherwise, the R, G, and B channels are graphed separately.
- Spatial Calibration: If selected, calculates the histogram values and statistics based on the current spatial calibration.
- Range Statistics: If selected, calculates statistics based on the selected range. NOTE: To show/hide statistics, use the context menu's View > Statistics options (described above). When you select this option, a shaded area is added to the histogram display; you can adjust the range by dragging the edges of the shaded area to desired low and high points of the range. The statistics are automatically updated as you adjust the range.
- Exclude Bins: Allows you to exclude the specified number of pixel values ("Bins") from the histogram display. For example, if you enter "10," the lowest and highest 10 pixel values are not graphed in the histogram .