Case Study 6: 3D Recording of an Anglo-Saxon Cross, Prestbury
- 1st January, 2008 |
- 12:00 pm #
In the churchyard of Prestbury Church stands an important Anglo-Saxon Cross, thought to mark the arrival of Christianity in the North West of England. The sandstone cross measures 940 mm by 400 mm by 240 mm, the surface is weathered and some green moss obscures the upper east face. Anglo-Saxon Cross, Prestbury The original location of the cross is unknown, however it was previously sited inside the church. The cross is highly decorated with intricate patterns. There are clearly sections missing, particularly between the three sections it is now in, discernable by gaps in the pattern, either side of fills holding the piece together. Prestbury Parochial Church Council wanted the cross accurately recorded prior to conservation work and possible re-siting.
Introduction In the churchyard of Prestbury Church stands an important Anglo-Saxon Cross, thought to mark the arrival of Christianity in the North West of England. The sandstone cross measures 940 mm by 400 mm by 240 mm, the surface is weathered and some green moss obscures the upper east face. The original location of the cross is unknown, however it was previously sited inside the church. The cross is highly decorated with intricate patterns. There are clearly sections missing, particularly between the three sections it is now in, discernable by gaps in the pattern, either side of fills holding the piece together. Prestbury Parochial Church Council wanted the cross accurately recorded prior to conservation work and possible re-siting.
Instruments and software A Minolta VI 900 laser scanning system was used for data capture. The instrument was mounted on a tripod, and set to fine mode. A middle lens was used throughout data capture. Sensor-object separation was maintained at approximately 1000 mm. The exception to this scanning offset was the very top of the cross, which had to be recorded from a distance of 2000 mm. The calibration of the system was checked using a 100 mm calibration board, prior to scanning and again on completion of data capture. The scanner was working to within the manufacturer’s specification. A tent was erected over the scanning area to reduce the ambient light levels. This ensured we obtained the best data possible. Scanning took a total of 6.5 hours and we collected 121 frames. The frames were saved directly as meshes to a flash card connected to the scanner. We undertook rough registration on- site to ensure that we had covered the whole surface, and to ensure that the data recorded was of a high quality. Rapidform2004 PP2 (INUS Technologies, Inc.) was used to register and merge the individual frames into a coherent model. The average shell-shell deviation for this process was 0.3 mm. large areas of overlapping data were deleted prior to merging, with the best data being chosen wherever possible. Rapidform 2004 SP2 was employed for the post-processing of the data, which entailed cleaning polygons and filling small holes manually. A small amount of localised smoothing was required in areas where the data was noisy. This was in the most part where there are very dense moss patches, approx. 2 cm2 in area. The data was decimated by 50% on bringing the individual frames into Rapidform 2004 SP2 prior to registering and merging, and the final model was decimated again by 50% at the end of the post-processing procedure. The final model contained approximately 3.5 million polygons. Production of 3D flythroughs in AVI format were undertaken in 3D Studio Max (AutoDesk Media and Entertainment). Photographic documentation was captured using a Minolta Dimage 5 3.3Mega pixel digital camera at a resolution of 1600x1200 pixels, mounted on a tripod. Video footage of the cross in its current location (including the immediate surroundings and the scanning process) was recorded using a Sony 3CCD DVCAM.
Why was scanning selected? A highly accurate record of the surface of the cross was required prior to possible dismantlement, conservation and re-siting. Taking a mould of the object was not an option in this case due to the friable nature of the sandstone surface. An accurate 3D model was also required in case the it is decided at a future date to create a highly accurate replica of the cross – laser scanning is an ideal technique for this.
What problems were encountered? The presence of moss in some localised areas meant that we could not record the stone surface underneath as accurately as the most of the surface. However these areas are small, and the data clearly shows the form of the pattern in these areas.
What were the final deliverables? All raw data was supplied in CDM format. This comprises the 121 frames and was provided with a data log that provides information about each frame. The completed data was supplied in STL format. Photographic documentation was provided in JPEG and TIFF formats. In addition, screenshots of the completed data in JPEG format, and AVI flythoughs of the 3D cross both textured and un-textured were supplied in AVI format.