Advanced Methods for Image Registration Applied to JET Videos

The last years have witnessed on JET a significant increase in the use of digital cameras, which are routinely applied for imaging in the IR and visible spectral regions. One of the main technical difficulties, in interpreting the data of camera based diagnostics, is the presence of movements of the field of view. Small movements occur due to machine shaking during normal pulses while large ones may arise during disruptions. Some cameras show a correlation of image movement with change of magnetic field strength. For deriving unaltered information from the videos and for allowing correct interpretation, an image registration method, based on highly distinctive Scale Invariant Feature Transform (SIFT) descriptors and on the Coherent Point Drift (CPD) points set registration technique, has been developed. The method ensures a reliable determination of the alignment parameters and their corresponding uncertainty. The algorithm incorporates a complex procedure for rejecting outliers. The method has been applied for vibrations correction to a representative set of videos collected by the JET wide angle infrared camera. It proved to provide reliable image vibration correction when the camera is operated in full view. Another application has been developed for the JET fast visible camera, where the image rotation artefacts occur due to the sensitivity of the image intensifier to the magnetic field (above a certain threshold). The method has proved to be able to deal with the images provided by this camera frequently characterized by low contrast and a high level of blurring and noise. It has also the advantage of using directly the input images as they are provided by the cameras without any pre-processing step. The parameters of the algorithm are the same for all videos in the database.