a novel framework for fast and continuous registration between two imaging modalities is disclosed . the approach makes it possible to completely determine the rigid transformation between multiple sources at real - time or near real - time frame - rates in order to localize the cameras and register the two sources . a disclosed example includes computing or capturing a set of reference images within a known environment , complete with corresponding depth maps and image gradients . the collection of these images and depth maps constitutes the reference source . the second source is a real - time or near - real time source which may include a live video feed . given one frame from this video feed , and starting from an initial guess of viewpoint , the real - time video frame is warped to the nearest viewing site of the reference source . an image difference is computed between the warped video frame and the reference image . the viewpoint is updated via a gauss - newton parameter update and certain of the steps are repeated for each frame until the viewpoint converges or the next video frame becomes available . the final viewpoint gives an estimate of the relative rotation and translation between the camera at that particular video frame and the reference source . the invention has far - reaching applications , particularly in the field of assisted endoscopy , including bronchoscopy and colonoscopy . other applications include aerial and ground - based navigation .