text: The tool called gpaGUI provides an interactive interface for geometric phase analysis. It allows to generate 2D maps of various crystallographic quantities using high-resolution images.
  Since the geometric phase analysis is performed in frequency domain, the high-resolution image needs to be transformed into frequential representation using Fourier transform. Mathematically, the frequential image is a  complex  matrix with the size equal to the original image. Crystallographically, it can be seen as an artificial  diffraction pattern of the original image depicting intensity peaks corresponding to the crystallographic planes present in the original image. After performing desired calculations, the frequential representation can be transformed back to the original spatial domain using inverse Fourier transform.
  Various crystallographic analyses can be performed using the frequential image. If it is filtered so that only the information from a region close to a particular diffraction spot is used (the rest is set to zero), a filtered direct image obtained by inverse Fourier transform then depicts only the planes corresponding to the selected diffraction spot. Moreover, due to its complex nature, the frequential image can be used to calculate amplitude and  phase. Together with a vector of one crystallographic plane depicted in the image, they can be used to generate a 2D map interplanar distance of given plane. If two vectors of non-parallel planes are known, the method can be used to generate maps of  strain and  displacement.
  Graphical user interface of gpaGUI is vertically divided into two halves, each of which contains:
  * Diffractogram preview allowing to select one diffraction spot corresponding to a crystallographic plane.
  * Visualization of a selected quantity (input image, filtered image or one of the maps mentioned above) allowing to select point of interest or region of interest for further analysis.
  * Results of detailed analysis of point or region of interest. The point analysis allows the user to select any pixel of the visualized map to see exact values of the particular pixel and its closest  neighbourhood. If analysis of broader area is needed, a polygonal region can be outlined in the map allowing to enumerate its statistical details: mean, standard deviation, median, minimum, maximum and total area of the polygon.
  Since each half of the interface allows to specify one crystallographic plane, gpaGUI allows to calculate all the aforementioned crystallographic quantities including those which require two vectors. Precision and repeatability of the whole analysis relies on accuracy of the diffraction peak localization. To overcome inaccuracy of manual peak localization (with a mouse click), gpaGUI provides a possibility to process the input image with  in order to accurately localize and index the peaks.