Filter to carry out Rolling Ball Filtering for efficient noise removal. This works on black pixels (values 0). The top hat filter performs the equivalent function on white noise (pixel values of 255). A Top Hat is defined as a 3x3 array with - a square with the corner pixels removed).The maximum values the central 5 pixels (3 x 3 cross) are compared with the maximum values of the rest of the kernel (the brim). If the central minimum brim-minimum hat value maximum exceeds a threshold value, then it is replaced with the median value of the brim otherwise it is retained. Hold down the ALT key to set the threshold - default value is 12.