Filter to carry out Rolling Ball Threshold Filtering for selecting dark features surrounded by bright areas. This technique is typically much more efficient than simple thresholding, where variations in background can cause it to fail. A Top Hat is defined as a 3x3 array with 3 pixels attached to each of the sides (a square with the corner pixels removed). The minimum value in the central 5 pixels (3 x 3 cross) is compared with the minimum value in the rest of the kernel (the brim). If the difference between the brim and hat value maximum exceeds a threshold value, then it is retained otherwise it is discarded. The threshold value is set a 12 and an alternative value may be selected by holding down the ALT key when running the script.