Active vision applications to cultural heritage acquisition and monitoring

This paper presents features and performances of a portable stereo active vision system (AVS), designed and built for data acquisition and monitoring of medium-scale monuments and environments of interest for cultural heritage. The research activity was partially carried out in the framework of PARNASO-SIINDA project1, a national project aimed at the development of new techniques for cultural heritage preservation, and the case-of-study of the activity was the Roman Theatre of Aosta (IT). Reliable data acquisition and monitoring of the surface alterations related to the monument degrade was a project demand that motivated the stereo vision approach because of its high spatial sampling rate, high accuracy and close correlation of shape and color measurements. The AVS system architecture is designed around three CCD cameras mounted on three independent pan-tilt stages under computer control. The cameras are equipped with different focal lens objectives, allowing the system to acquire both geometric and colorimetric information of an architectural surface (monuments, frescoes...) at different scales over areas of tens of square meters. Two cameras form a stereo pair and are equipped with long-focal objectives to sample a limited portion of the scene at high spatial resolution, while the third one is equipped with a wide-angle objective and it is intended for surveying the whole scene at low-resolution. Geometric measurements are carried out both by forward triangulation of the optical axes of the stereo pair during fixation of an object and by three-dimensional (3D) object reconstruction from the images of the stereo pair. The fixation process is performed by a PC-controlled imagebased servo loop. The fixation approach supplies sparse but accurate 3D measurement in the AVS reference system, while the stereo acquisition of image pairs allows a dense 3D local reconstruction of the surface around the fixation point. Colorimetric measurements are performed at pixel level by processing the images of one of the stereo cameras, which is equipped with tristimulus filters. Results are supplied in terms of CIE-Lab components. A user-friendly interface for the system management allows performing several measurements automatically. On-site data processing allows the user to decide for further investigations immediately, if necessary, without expensive delays. The features of the AVS system, namely the working range of depths (2-10 m), the high spatial sampling rate, the accuracy and the close correlation of shape and color measurements, makes the system competitive with respect to other available instruments and techniques for data acquisition and monitoring of small and medium-scale works of art. In the following the AVS architecture is presented, and laboratory tests made to asses its performances are described. Results of the tests and of the acquisition campaign, made on the Roman Theatre of Aosta (IT), are reported and discussed in comparison with other techniques commonly employed in cultural heritage surveys.