a question about scan's region

if I want to control the scope of the feature point scan, like a circle，what should I do.sth about this question，1.skipRegion not works in kpmatching.cpp 2.To limit the useless of getQueryFeaturePoints in the circle 3.I don't know how to use the useless points in the non - circle, VisualDatabaseFacade:: erase can only delete image ID, but no feature points can be deleted.I'm sorry I don't know much about the c++ language