Sunday, 3 September 2017

3D documentation of ancient millstones: preliminary tests

The traditional drawing of ancient millstones consists in a plan (eventually with shading to give a sense of three dimensions) and one or more cross sections of the object. This is not always easy because of the dimensions, weight and sometimes shape (mainly the Pre-Roman millstones are irregular and asymmetric) of this type of artefacts. Furthermore, the millstones are generally in museums or in storerooms: in these places, it is often difficult to move the objects or to have enough time for drawing quietly and checking well all the details. In short, drawing a millstone is not like drawing a sherd of pottery!
For these reasons, it could be useful applying a methodology based on the Structure from Motion (SfM) techniques in addition to the traditional drawing methods. In this post I’m going to present the preliminary results of a test aimed at the three-dimensional documentation of a fragment of an Iron Age millstone from Northern Italy (a so-called “Olynthus mill” or “Hopper rubber”).

The first step was the construction of a rectangular wooden frame made by 4 rods of different length (40, 60, 80 cm), so that it’s possible to build frames of different areas according to the dimensions of the millstone to be drawn. On the surface of the rods some cross marks equally spaced are signed: these marks will be use as reference points with known coordinates for the rectification of the3D point cloud, mesh and (eventually) texture (something like GCP, Ground Control Points).

Four bolts adjustable for height hold the frame together and allow to level it perfectly. Once the frame is ready, you need to enter the millstone into it, in such a way as to leave sufficient space between the stone and the rods for taking pictures.
Some recognizable markers should be placed in different points of the millstone: these are for aligning and merging the two point clouds that will be generated (see below). A simple solution is to use small spheres of coloured modelling clay visible in the point clouds.

At this stage, you start with the typical workflow of the SfM. You take an appropriate number of pictures of the upper surface first; then do the same to the lower surface, turning upside down the millstone inside the frame.
The pictures can be processed by the software you want. I used Regard3D / OpenMVG for generating two point clouds (one for the upper surface and the other for the lower) and CloudCompare for editing/cleaning the point clouds and for performing their rectification (thanks to the cross marks on the frame), alignment and merging (thanks to the coloured markers on the stone). CloudCompare and MeshLab have also been used for generating meshes and for computing other parameters, among which the measurements.

The final result is a point cloud and a mesh of the millstone.

Using MeshLab you could also obtain the texture of the object, but for my aims it’s enough a 3D model (point clouds or mesh) from which I can get a plan, some cross sections and all the measures I need. Thanks to these data, I can detail or check my handmade drawing or do it from scratch.

In conclusion, the usage of an homemade wooden frame makes easier and more precise the data acquisition for the SfM and make faster and more complete the documentation of this kind of artefacts. The method described leaves room for improvements and developments; it could become a “standard” documentation technique for the ancient millstones and for other archaeological objects with analogous drawing issues.

Thank’s to Alessandro Bezzi (Arc-Team).

Denis Francisci

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