1. Introduction

This work aims at a virtual recovery of excavated archaeological finds in cyberspace for ancient relic preservation, archaeology research, and multimedia contents generation. First, we develop an imaging device to digitize damaged pieces in form of 3-D shape and surface texture. Then we build an interface for connecting broken fragments in a virtual space so that the original model can be visually recovered. The idea of virtual recovery provides a new opportunity and flexibility for archaeologists to examine complex damaged relics. Moreover, the virtually recovered objects can be directly displayed in a multimedia format. Experiment has been made at an UNESCO world heritage in Xian, China.

2. 3D Digitizing of Unearthed Relics

Sensing 3-D shape and surface texture of excavated relics is the first step towards our goal. Shape and texture are combined to yield a 3-D graphics model. The device we use is a laser range finder. Various laser range finders have already been developed. At an excavation site, however, we need a portable laser range finder for the following reasons. First, the unearthed pieces may not be allowed to move from their excavated positions temporally or permanently, and their sizes may have a big variation. The excavation site may only have a limited space for sensing. Second, surface texture is important in addition to the shape information in the recovery and exhibition. Further, the 3-D measure changes from position to position in an excavation site. It is crucial if a device has a weight tolerable for frequent moves; each measure takes few minutes.

According to these requirements, we constructed a laser range finder. The laser producing a planar rays and a digital video camera are bound together. By moving the laser-camera linearly, the ray plane scans the entire object surface and a depth map of the object can be obtained. Image sequences are recorded in a tape that is later read into computer frame by frame for processing. Because off-line image processing is carried out, the structure of the device becomes very simple. As the laser-camera set moves along the rail direction at a constant speed, surface points at different heights are measured and a depth map is generated.

3. Recovery of Relics using Virtual Reality

For thousands of years sleeping in ancient tombs, archaeological finds have suffered from a certain degree of damage when they are excavated. Many of them are arts and artifacts with high archaeological and culture value. Recovery of these elaborated objects to a certain degree is of significance for understanding early civilization. In excavation and recovery processes, a wealth of knowledge and experiences are required. Moreover, a tremendous manpower has to be employed. Archaeologists spend much time in restoring broken pieces before an unearthed object can be displayed to audiences. They need to search damaged pieces, imagine the original shape, and connect fragments to return the appearance of damaged finds in past times. Being supported by some special frame, those pieces are usually placed in a 3D space for augmenting spatial imagination in the reconstruction. Finally, those fixed pieces are glued together, which is almost irreversible and requires full certainty in correctness. Many fragments such as those from a statue might be very heavy and assembly of them is a hard work. One can imagine how difficult it would be to play such a 3-D jigsaw puzzle with some incomplete and lost pieces. The difficulties archaeologists normally face in the recovery process are:

• Ambiguity in determining fragments combinations,
• Weight of fragments in a free assembling test by trial and error,
• Irreversibility after fragments have been adhered in the recovery.
• At the same time, an unearthed relic may have lost its original pigment painted on its surface. It is interested by archaeologists to reproduce faded or dropped ancient color.

Virtual Recovery is to restore damaged finds in a virtual space for guiding adherence of real pieces. An interface is developed for flexible manipulation of graphics objects; 3-D aspect views of a figure or 3-D models of different pieces will be combined to produce a complete model. Faded colors of objects can also be recovered by rendering the remaining color samples to the object surfaces where the ancient pigments have dropped.

4. Multimedia Representation

Virtual Exhibition: producing multimedia sources from the recovered objects for display either in a virtual museum accessed on internet or on a video kiosk at an excavation site. The excavated artifacts hence can be reserved for exhibition in their unearthed states.

The advantages of such new concepts lie in the following.

• The measured 3-D shapes of unearthed relics are in a fadeless digital format. The excavated finds can remain as fragments, which is good for preservation and archaeology study even in the next generation.
• Virtual recovery is a non-adhesive and repeatable process that can prevent fragments from miss-connection. We can manipulate objects in a weightless space by using various position sensors. Archaeologists are able to do recovery even at a remote site without moving real heavy fragments.
• Virtual exhibits can be produced directly in forms of image and 3-D graphics model. We can omit the recovery of real damaged relics. This may even change the conventional exhibition style.

People
Dr. Zheng, and his students Yamaguchi, Uehara, Murata, Sawada in Kyushu Institute of Technology, Yin Lan Zhang, Wei Xin Zhang, Zheng Zhao, and other museum staffs participate the project directly. Prof. Zhong Li Zhang, vice director of the museum, and Prof. Abe in Kyutech gave many supports to the project.