Overview

Histograms are a very powerful feature when working with machine vision systems. They allow you to view data in a chart format that makes it much easier to see things like why a part isn’t found properly. Statistic calculated based on the histogram information is displayed so you can see the image feature.

KEY POINTS

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They give hints for things like how to find a medium gray blob when a dark and light gray blob can also be found within the search window.

Using Histograms

To bring up the [Histogram] dialog box, first select the vision object that you would like to see a Histogram for. (This can be done from the flow chart or the sequence tree.) The following vision objects support Histogram.
Blob, Contour, DefectFinder, Correlation, Geometric, Polar, OCR, CodeReader, ImageOp

Histograms are supported for the Correlation and Blob objects. Next run the vision object or vision sequence to make sure that the vision object you selected has valid results. Then click the [Histogram] button on the Vision Guide toolbar or select [Histogram] from the [Vision] menu. The [Histogram] dialog box will appear as shown in Histograms with Correlation Objects if the vision object is a Correlation object and as shown in Histograms with Blob Objects if the vision object is a Blob object.

The [Histogram] dialog box shows Gray Level on the horizontal axis and Pixel Count on the vertical axis. This makes it easier to see how many pixels are shown for each gray level in the current image. The left side of the [Histogram] graph (or lower gray level numbers) shows pixels that are approaching darker gray levels (black). Darker pixels are represented with a low value (approaching 0). The right side of the [Histogram] graph (or higher gray level numbers) shows pixels that are approaching lighter gray levels (white). Lighter pixels are represented with a high value (approaching 255).

The following statistic will be calculated using the results of the graph displayed in the [Histogram] dialog box.

Statistics	Description
Total	Number of the total pixels in the search window.
Min	Minimum value of Gray Level in the entire search window.
Max	Maximum value of Gray Level in the entire search window.
Avg	Average value that is acquired by calculating Gray Level in the entire search window.
Median	Median value of Gray Level in the entire search window.
Std Dev	Standard deviation that is acquired by calculating Gray Level in the entire search window.

If a vision object is Correlation, Geometric, Polar, OCR, CodeReader, or ImageOp (other than Operation: Binarize) object, the [Histogram] dialog box described in Histograms with Correlation Objects will be displayed.
If a vision object is Blob, Contour, DefectFinder, or ImageOp (Operation: Binarize) object, the [Histogram] dialog box described in Histograms with Blob Objects will be displayed.