Relative Hard Thresholding
The segmentation techniques described in the preceding sections require that you determine the appropriate value for the segmentation threshold. The most common method is trial and error. However, if the images used by your application contain image-to-image variations in brightness, then a static threshold value does not segment the variety of different images correctly. The figure below illustrates this problem.
Failure of static threshold value with changing image brightness
The Blob tool allows you to define a threshold value relative to the pixel values within an image.
A relative threshold is specified using two sets of parameters. First, you specify the percentage of the pixels within the image that you want the tool to treat as tail pixels. Tail pixels are those pixels within the image that have the lowest and the highest values. In a histogram plot of pixel values, the tail pixels appear at the left and right sides of the histogram. The Blob tool determines the pixel values above and below which the percentages of pixels that you specify lie. These pixel values are called the right tail pixel value and the left tail pixel value.
The second parameter you specify for a relative threshold is the threshold percentage. The Blob tool determines the pixel value that lies at the specified percentage of the distance between the two tail pixel values, and the tool uses this pixel value as the threshold pixel value. The figure below shows how the Blob tool computes a relative threshold.
Computing a relative threshold value
In the example shown in the figure above, the left and right tail percentages were specified as 5 percent, and the threshold percentage value was specified as 40 percent. The tool computed the values for the left tail, right tail, and threshold pixel values.
The figure below shows how using a relative threshold can produce an appropriate threshold value for each of the different brightness images.
While you could perform these steps yourself using histogram analysis and pixel mapping functions, the Blob tool provides the integrated capability to compute a relative threshold value.