Download HDevelop User`s Manual

2.3.6 Menu ’Visualization’
37
Most parts of the tool are built up similarly to the gray value histogram, which is described in detail above
(menu entry Visualization . Gray Histogram Info); in the following, we concentrate on points
specific to the feature histogram. An important point regards the “source” of the regions or XLDs: The
feature histogram is calculated for the regions or XLDs that were displayed most recently in the graphics
window. Thus, if you display an image, there are no regions or XLDs, therefore the histogram remains
“empty”. As soon as you display regions or XLDs on top of an image, the histogram is calculated. If you
display regions or XLDs without an image, you can still calculate feature histograms, but only for shape
features. Please keep in mind that only the most recently displayed regions or XLDs are the source of
the histogram, not all objects currently displayed in the graphics window!
The histogram itself is displayed with the horizontal axis corresponding to the feature values and the
vertical axis corresponding to the frequency of the values, i.e., to the number of regions or XLDs with
a certain feature value. In figure 2.20, the histogram of the orientation of the clips is displayed. When
comparing the histogram to the gray value histogram in figure 2.19 on page 34, you will note a typical
difference: Because in most cases the overall number of regions or XLDs is much smaller than the overall
number of pixels, feature histograms often consist of individual lines, most of them having the height
1 (when displaying absolute frequencies). Of course, this effect depends on the selected feature: For
features with floating-point values, e.g., the orientation, the probability that two regions or XLDs have
the same feature value is very small, in contrast to features with integer values, e.g., the number of holes.
If you click on the button Set, a dialog that lets you modify the displayed range of feature values appears.
You can influence the calculation of the histogram with the slider Tolerance. The selected value is used
to discretize the horizontal axis: Instead of determining the frequency of an “exact” feature value, regions
with feature values falling within discrete intervals are summed. Graphically speaking, the horizontal
axis is subdivided into “bins” with a width equal to the value selected with the slider Tolerance.
Automatic selection
As already noted, the region feature histogram facilitates the task of finding suitable threshold parameters
for the operators select_shape, select_gray, and select_shape_xld: Select the entry selected
in the combo box Display, choose suitable visualization parameters in the three combo boxes Color,
Draw, and Width, and then position the two vertical lines such that the desired regions are highlighted.
For example, in figure 2.20 the three clips pointing upwards and to the right were selected. The values
displayed next to the vertical lines can then directly be used for the parameters Min and Max, the feature
name selected in the combo box Feature for the parameter Features.
2.3.6.9 Visualization . Region Info
This menu item opens a tool for the convenient inspection of shape and gray value features of individual
regions. It can, for instance, be used to determine thresholds for operators that select regions based on
GUI
Figure 2.20 shows the feature histogram window together with the graphics window. Upon opening,
the tool displays the histogram of the area (default feature selection) of the regions or XLDs that were
displayed most recently in the currently active graphics window. You can select various features in the
combo box Feature; the status bar displays additional information about the select feature, e.g., which
operator is used to calculate it. Further information about region features can be found in the description
of the menu item Visualization . Region Info.