  A Reflectance Transformation Imaging technique (RTI) realized by multi-rotor
Unmanned Aerial Vehicles (UAVs) with a focus on deployment in difficult to
access buildings is presented in this letter. RTI is a computational
photographic method that captures a surface shape and color of a subject and
enables its interactive re-lighting from any direction in a software viewer,
revealing details that are not visible with the naked eye. The input of RTI is
a set of images captured by a static camera, each one under illumination from a
different known direction. We present an innovative approach applying two
multi-rotor UAVs to perform this scanning procedure in locations that are
hardly accessible or even inaccessible for people. The proposed system is
designed for its safe deployment within real-world scenarios in historical
buildings with priceless historical value.
