Image Acquisitionand Proces

Image Analysis 187
7.4.4 HISTOGRAMS AND HISTOGRAPHS
A histogram can be a very useful tool for determining the intensity distribution of
an image. The formal deÞnition of a histogram is
…a bar graph of a frequency distribution in which the widths of the bars are proportional
to the classes into which the variable has been divided and the heights of the bars are
proportional to the class frequencies.
A more practical explanation is to consider a histogram as a bar chart of intensity
against intensity occurrence, with uniform intensity bandwidths (Figure 7.30).
The Vision Toolkit has two VIs that compute the histogram of an image:
IMAQ Histograph and IMAQ Histogram. IMAQ Histograph returns a cluster
containing the calculated histogram suitable for wiring directly into a front panel
graph. It also returns the histogram’s mean value and standard deviation (Figure
7.31).
FIGURE 7.31 Histograph example — wiring diagram.
FIGURE 7.32 Histogram example.
IMAQ Histogram returns a cluster of data that includes the same information
as IMAQ Histograph, but also contains the raw histogram as a one-dimensional
array, the minimum and maximum values, and the histogram’s area (in pixels)
(Figure 7.32 and Figure 7.33).