The twin structure of NdGaO3 and YAlO3 : Dy crystals was examined using white beam synchrotron X-ray topography. The relation of the orthorhombic lattice basis vectors has been obtained for every domain pair in the crystals with GdFeO3-type structure. A new computer program has been used to generate Laue simulation patterns and to calculate the image shifts in topographs. The proposed approach allow for the analysis of both, sample images for specified reflections and Laue patterns as a whole. Topography was used to map domain patterns, to show misorientations and to detect symmetric aspects of ferroelastic twins. Information on twin laws and the twin distribution was obtained simultaneously.