X-RAY PHASE-CONTRAST TOMOGRAPHY APPLIED TO VIRTUAL UNFOLDING OF HERCULANEUM PAPYRI

Abstract

In 1750 a large villa on a hill overlooking Herculaneum and the Bay of Naples was discovered. This villa probably once belonged to the wealthy Roman aristocrat Lucius Calpurnius Piso Caesoninus, a well-known Roman politician, who was a patron of Epicurean philosophers. The villa was lost from history in A.D. 79 in the catastrophic eruption of Mount Vesuvius when it was buried by tons of thick volcanic mud which gradually hardened to a concrete-like consistency. During villa excavations, workers discovered many important works of art and an impressive library of some 1800 papyri rolls - thus the name Villa of the Papyri. The library consists of a remarkable collection of Epicurean philosophical texts written in Greek, and a lesser number of Latin texts. Due to the intense heat of the volcanic flow and the pressure exerted by the weight of mud, lava, pumice, and rubble, these rolls are extremely fragile, lost more or less the original cylindrical shape, and the individual sheets stuck together tenaciously. Since their discovery, numerous efforts have been made to open the carbonized-Herculaneumpapyri and read the priceless information contained in them employing several manual and destructive techniques. My Ph.D. research work concerns the study of Herculaneum papyri using a non-destructive X-ray technique and a new set of numerical algorithms for ‘virtual-unrolling’. In particular, I used X-ray-micro-computed tomography (μX-CT) and X-ray Phase Contrast Tomography (XPCT) as powerful non-destructive testing techniques for the full-volume inspection of the papyri, able to give morphological and physical information on the inner structure of the investigated sample. I analyzed three Herculaneum papyri fragments: PHerc.1103 and PHerc.1105 belonged to the scroll exterior part, scorza, and PHerc.110 belonged to a papyrus partially unrolled in 1867. The main purpose of my research work was building a computational platform for the virtual 3D investigation of the Herculaneum papyri fragments. Due to the poor conditions of the fragments, virtual operations on the 3D tomographic images, such as segmentation and flattening of the papyrus sheets, result complicated and require an approach developed specifically for this particular task. In this thesis, I describe in detail this computational platform, which is the composite result of 3D data digitisation, segmentation, modelling, flattening and texture mapping of papyrus sheets. Following 3D data digitisation, the segmentation step is presented, thanks to which I can digitally identify the single papyrus sheet within the volume. After the sheet surface is modeled using triangular meshing, preferred to other kinds of mesh because it simplifies the math and allows for faster operations. Finally, I focus on the parameterization methods for unfolding sheet with minimum distortion. Once the good parametrization method has been identified, the next step is texturing, i.e., assignment of 3D color information to each point on a 2D mesh. The resulting texture reveals the features of the flattened sheet. Besides, investigations on different phase retrieval approaches have been performed. I implemented and tested three of the widespread methods to find the best one. Another challenging problem, due to the ring artifacts present in the reconstruction that disturbs the image impression, was resolved through algorithms adapted to this particular case. The artifacts removal process allowed the visualization of clean slices and helped to reveal traces compatible with writing. It has so been demonstrated the benefit of applying the XPCT technique to trace handwriting. Some of the results of this research are reported in the following paper has been accepted for publication: Stabile S., et al., Computational Platform for the virtual unfolding of Herculaneum Papyri, Sci. Reports.