Dave Mitchell's DigitalMicrograph™ Scripting Website

This is more of an example script than anything. It contains a number of functions which may be of general use in fitting algorithms. As its input, the script takes an image of a blob. This could be an excised region containing a diffraction spot or an atomic column. The script will locate the centroid without knowing where the approximate centre is. However, as the Monte Carlo approach can be quite time consuming, it may be computationally quicker to use something like a centre of gravity measurement as a rapid way to approximate the centre, then to ask this script method to start looking for the centroid using the centre of gravity location as its starting point. This script starts from the centre provided - but if none is supplied then it will use the geometric centre. The script first approximates a Gaussian fit using the supplied/guessed centre and by assuming a sigma value for the Gaussian of 1/4 of the image width. Thereafter it varies the sigma with a random number generator, retaining sigma values which improve the fit. Once that iteration is complete, it varies the centre with a random number generator. It then repeats the sigma refinement and the centre refinement a number of times, until the chi squared value of the fit no longer improves. The method is quite robust, although median filtering is included to reduce any sensitivity to noise. Once complete, the script reports the centroid with sub-pixel precision.