Reflection transformation imaging on larger objects: an alternative method for virtual representation

The term Reflection Transformation Imaging has been originally used by Tom Malzbender of the HP Laboratories to describe an image-based method to acquire the reflectance properties of objects’ surface. This technique, as other image-based relighting techniques, creates a “special” image of the object which encodes a reflectance function per-pixel. These images are called Polynomial Texture Maps (PTMs) (Malzbender et al. 2001). PTMs are generated starting from a set of photos of the object of interest taken under controlled illumination. Thanks to the reflectance functions encoded in the PTMs it is possible to generate new images of the object under different lighting conditions. Due to the easiness and effectiveness of this method PTMs have been used in several applications in the field of Cultural Heritage in recent years. One of the main motivation to use PTMs to represent artifacts is that this kind of visualization allows the user to change interactively the effects of the illumination on the object’s surface, and hence the exploration of the object is performed in a new interesting way. In particular, the use of contrast enhancement mechanisms related to the PTM viewing technology (specular enhancement, diffuse gain) have proven to be very useful not only in term of documentation, but also in terms of analysis of the surface details hard to reveal under classical viewing systems. Another reason is that PTMs may be an interesting alternative to the other well-known 3D acquisition techniques, like 3D scanning, which are “expensive” in terms of equipment, acquisition time and post-processing. Moreover, visualization of detailed 3D models is problematic in online environment while PTMs can be a good representation to make data more accessible to the public. Finally, PTMs seem to be a better solution for the visualization of certain objects, like bas-relieves, where the information provided by re-illumination is more important than the one provided by geometry. Most of the existing work about PTMs acquisition regard small size objects. In this paper we present a new low-cost system for producing high quality PTMs of medium-large objects, from 60-70 cm of width by 50-60 cm of height up to 2m of width by 1m of height. During the description of our system we also underline some differences with respect to the classic 3D scanning pipeline. We also present an analysis of quality of the PTMs acquired by our system and a novel software to browse huge resolution PTMs. After an overview of the application of PTMs in Cultural Heritage in Section 2, we analyze our method for PTMs acquisition in all its aspects in Section 3. In Section 4 we propose an analysis of quality of the PTMs obtained in comparison with 3D scanning, and a study on quality degradation relative to the number of shots used to calculate the reflectance function. This analysis is not limited to our system and it is a good starting point to optimize PTMs acquisition techniques. Some examples of Cultural Heritage objects acquired with our system are presented in Section 5. Conclusions and future work are outlined in Section 6.