Procedure of Restoration
A procedure of restoration is shown in Figure 2. The procedure is mainly divided into three parts: (1) measurement, (2) data processing for each result and (3) integration.
Heterogeneous Measurement Methods
At present, a variety of tools and techniques exist for gathering 3D data of archaeological relics (Addison, 2000 and Barceló, 2000) and a combination of methods is important to obtain a precise, detailed 3D model (Nagano, 1998). The authors integrate heterogeneous measurement methods, which are a total station SET6E (Sokkia), a non-contact 3D digitizer VIVID 700 (Minolta) and a stereo camera with 35mm film.
A total station, which measures a distance to a target prism by a time-of-flight method, can cover an entire area where Kamegata-Ishi stands. However, speed of the measurement is so slow that recorded points are distributed sparsely. Moreover, no texture image can be obtained.
Therefore, to obtain detailed local shapes and texture images, the authors employ a non-contact 3D digitizer and a stereo camera. A non-contact 3D digitizer obtains local shapes, which are represented as a group of dense polygons and a texture image of the surface. A stereo camera also obtains local shapes. Although the shapes are less accurate, these texture images are more vivid than ones from non-contact 3D digitizing.
A measurement with a single device to obtain local shapes and texture images may produce an inaccurate reconstructed model because of accumulation of errors. The proposed method utilizes the data obtained by the total station as a skeleton. Consequently, accumulation of errors can be avoided in advance.
Measurement
A measurement of Kamegata-Ishi was executed on March 27, 2000.
Putting Markers: First of all, markers were put on Kamegata-Ishi. Markers were used for mainly two purposes:
- Matching between a left image and a right image in stereo imaging
- Matching between a skeleton of an entire shape and local shapes for integration
Acquisition of Color and Shape: After putting markers, three measurement methods were executed simultaneously.
 |
|
(a) |
(b) |
Figure 3: Obtained shapes (a) From optical surveying (b) From stereo imaging
The total station measured 135 points and the result was stored in a note PC connected to the total station via RS-232C. The non-contact 3D digitizer obtained 41 local shapes. Each shape consists of 200 by 200 points and 400 by 400 color texture image. The stereo camera obtained 31 pairs of stereo photographs. Figure 3 shows the obtained shapes from each measurement.
Integration
To obtain a detailed shape of the entire stone, local shapes have to be placed on precise position in a skeleton of an entire shape. This registration is achieved through the process that matches each marker of the local shape to the corresponding marker of the skeleton.
Once the correspondence between markers is given, a transformation matrix, which represents the transformation from local coordinates to global coordinates, is calculated automatically by simulated annealing. The simulated annealing can decrease the sum of differences between the marker of the skeleton and the transformed marker of the local shape into global minimum.
