Blob Analyzer Thresholding Thresholds are used to segment the image into two types of pixels: blob pixels and back- ground pixels. Depending on the segmentation mode selected, either one or two thresholds can be used. There are two types of thresholding functions: Hard Thresholding Hard Thresholding (also referred to as binary thresholding), segregates pixels into one of two states: 0 for background pixels, and 1 for blob pixels. The result is a binary image (for example, a white blob on a black background). Hard thresholding assumes that changes in data values occur at the boundary between pixels, without allowing for a variation in grey- level values across blob boundaries.
Because this is rarely the case, soft thresholding (described in the next section) is more often used for applications. Soft Thresholding Soft Thresholding provides flexibility in treating pixels that border blob regions. A soft threshold is sloped and covers a range of pixel values. Processed pixels within the threshold range are output as weighted pixels. Weighted pixels are used to calculate blob results in a proportion corresponding to their value in the threshold range. l The range of values within a soft threshold is user-defined and corresponds to the dif- ference between a maximum and a minimum threshold value. l All weighted pixels appear in the image.
However, they contribute to the property res- ults in proportion to their weight in the threshold range. l The Pixel Weight Image displays the weighted pixels in corresponding levels of grey. Dynamic Thresholding Dynamic Thresholding provides an adaptive thresholding mode, which is useful when there are lighting variations from one instance to another. A dynamic threshold sets a percentage of the pixel distribution (histogram) to be considered as blob pixels. A dynamic threshold can be either a soft or hard threshold. It can be set to apply light, dark, inside, or outside segmentation.
Light Segmentation The Light Segmentation algorithm outputs all pixels with values higher than the threshold value as blob pixels. Pixels with values below the threshold become background pixels. This mode is used for light blobs on a dark background. ACEUser's Guide, 3.6.x, 18316-000 Rev A Page 401