Masks scanning process - by Martin Blazeby
Two different types of technologies have been employed in the project to capture the mask artefacts into 3D digital form, photogrammetry and laser scanning. Both offer varying degrees of scanning criteria mainly associated with cost, security, access, accuracy and portability.
Photogrammetry
Photogrammetry is a low cost, portable and non-intrusive technique of recording 3D objects by means of photographs or video. The main components consist of a slide projector, a slide containing a fine grid, a calibration box and a camera or video recording system with tripod. The process of photogrammetry is as follows: the slide is projected onto the area where the artefact will be positioned, the calibration box is placed in situ and a photograph is taken from a fixed position. This calibration photograph is crucial, as it is required to calculate the distances and angles between camera, projector and object. When this is complete the artefact can be photographed with the grid projected onto the surface, it will be necessary to rotate the object in order to record all angles covered by the projected grid. The amount of photographs will vary depending on how fine the grid system is and how complex the artefact is.
Using photogrammetry software (Shape Snatcher) the calibration photograph is loaded and the relevant photographs are imported. The software interprets the two dimensional photographs containing the grid projected on to the surface of the artefact into three-dimensions. Each segment of the artefact as photographed from different angles is then saved separately and stitched together as one 3D mass. This process can be time consuming and problems of bad data and miss alignment can occur due at the the photography stages if the camera or projector is accidentally, and without knowing, moved out of their fixed positions. If this happens then a whole session can be lost, significantly problematic if using the software sometime after the session or away from the artefact. However, the resultant data produced when successful is of sufficient quality and level of detail.
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Photogrammetry setup |
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Artefact positioned |
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Artefact with projected grid |
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Grid interpreted by software |
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3D stitching process |
Laser scanning
For the purpose of the project we have used two types of laser scanners, a fixed based Arius 3D laser scanner located at the department of Geomatic Engineering, UCL and a portable Minolta laser scanner loaned by the Archaeological Computing Research Group, University of Southampton. Both of these laser scanners are non-intrusive and measure highly accurate levels of detail, characterising each measurement point according to its colour and location. Each measurement point is recorded by three geometric values as XYZ and three reflectance values as RGB collected simultaneously from the target surface.
The Arius 3D scanner is a very large non-portable machine allowing for data capture to only take place at UCL. This requires a high level of forward planning and management as the artefact required for scanning will inevitably have to be removed from its museum location and transported off site to the scanner. Security and safety is paramount in an operation like this, usually involving the time of both the scanning staff and of museum curators who will accompany and protect the artefact at all times. The resultant scanned data using the Arius 3D is of high resolution more than sufficient for the purposes of the project.
The Minolta portable laser scanner, unlike the Arius 3D scanner allows for far greater portability, allowing scanning onsite at various museums. This however, requires permissions and unrestricted access to specific museum collections and archives. A great deal of planning is essential when dealing with European collaborators and an explanation of the scanning processes to officials is a prerequisite to those who may otherwise not be technologically understanding of the process involved. The Minolta scanner is robust for transportation and once setup and operated by its handler, the scanning process is quick and the data precise.
The scanned data created by both technologies are then processes by proprietary software to stitch component segments together similar to that used in the photogrammetry process.
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Arius 3D scanner |
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Arius 3D scanner in operation |
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Minolta scanner |
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3D stitching process |
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