By Dr Megan Kasten, OG(H)AM’s UK Postdoctoral Researcher
Last month, the OG(H)AM project digitally recorded two of the ogham stones and seven ogham-inscribed portable objects in the National Museum of Ireland’s collections with the help of Irish project partners, the Discovery Programme. The team included OG(H)AM Postdoc Researchers, Megan Kasten and Nora White, OG(H)AM intern Clara Scholz (who is in Maynooth, thanks to Erasmus+), and Discovery Programme’s Robert Shaw and Anthony Corns. While laser scanning is usually what people think of when they hear about 3D scanning, we used three different digital imaging approaches: Structured Light Scanning, Structure-from-Motion photogrammetry, and Reflectance Transformation Imaging.
Structured-Light Scanning (in this case we used a handheld white-light scanner, the Artec Leo) creates a 3D model by projecting a flashing light with a known pattern onto the surface of an object. The two cameras built into the scanner simultaneously take overlapping photos, and the scanner recognises distortions in the patterned light caused by the surface geometry of the object. The software records the geometry of the object and automatically generates a dense point cloud and mesh (the measured points and digital ‘surface’ of a 3D model). We used this method on the two ogham stones on display in the National Museum of Ireland’s Treasury – this scanner worked well with the nonreflective material and the scale of these stone pillars, but was not suited to the smaller portable objects of various materials we were set to record later that week. While I personally prefer photogrammetry, the wireless Artec Leo was very quick and easy to use.
Structure-from-Motion (SfM) photogrammetry is a flexible 3D imaging technique which only requires digital photographic equipment (though this can quickly turn into a very long list of equipment). Essentially, you move around an object with your camera (or place the object on a turntable and keep the camera in one place), take a series of photographs, aiming for each image to overlap with its neighbouring images by at least 50-60%. The photographs can then be processed through one of many photogrammetric software packages, like Reality Capture, Agisoft Metashape, or the free, open-source Meshroom. The software matches features shared between the individual photographs and calculates the position from which each photo was taken; the software then creates a point cloud from these measurements to represent the object’s geometry, which can then be processed further to generate a 3D model. This is the primary method we will be using to record stones and objects in 3D in the OG(H)AM project.
With photogrammetry, the quality of the 3D model is highly dependent on the quality of the photographs used to build the model – clear, crisp photos with flat lighting (no shadows) are ideal. Because of the reflective nature of some of the objects we were recording (i.e. the silver Ballyspellan brooch, or the Moynagh Lough antler handle which was polished from use), one of our major concerns was controlling the lighting in a way that allowed for optimal photos for photogrammetry but minimised reflections in the photos.
Another bit of kit that was useful in recording these smaller objects was the Cognisys Stackshot for focus stacking. Focus stacking has many uses, but in macro photography, the technique allows for the entire depth of the object to be in focus in a single photograph by combining several photos where different regions of the object are in focus. While it is possible to do this manually, the Stackshot does this mechanically. The user tells the software when the front of the object is in focus, then physically moves the camera down the rail until the back of the object is in focus, and finally tells the Stackshot how many photos it should take for each part of the object to be in focus. While this made the process of focus stacking more predictable, one also had to think carefully about how the final image would adhere to the requirements of photogrammetry.
Reflectance Transformation Imaging (RTI):
RTI is not a true 3D imaging technique – it has often been described as a 2.5D imaging technique. However, it requires much the same equipment as SfM photogrammetry, and is useful in examining worn or faint details on an object, so it isn’t unusual to see the two techniques used during the same object recording session. Unlike SfM photogrammetry, in this approach, the camera stays in one location, pointed directly at a single surface of the object. A reflective black or red ball is placed in the frame next to the object. For each photograph, the light source is moved to a new position (unless one has access to a specialised ‘RTI dome’, one must imagine an invisible ‘hemispherical dome’ of lighting positions and point a flashlight at the object from strategic positions, beginning at the camera lens and ending parallel with the edge of the object, all the way around the object). The reflective ball is used in processing the Polynomial Texture Map – the software calculates how each pixel of each photo interacts with a different lighting position, by tracking the light’s position in the reflective ball. The resultant .ptm file is a compilation of all of the photos, which allows researchers to ‘change the lighting position’ in the photo at will and see how the object reacts.
We captured RTI for a couple of objects where the inscription was either positioned on a very reflective surface of the object, or the inscription was so worn/faint that it wasn’t certain that it would show up clearly in the 3D model. The downside is that RTI files are not as easy to disseminate as 3D models – but they will be a great research tool for ogham specialists who cannot access the objects as easily.
Structured-from-Motion Photogrammetry: Outdoor challenges
At the weekend, Nora and I went to Co. Wicklow to re-record the Castletimon ogham stone with photogrammetry to capture the letters inscribed along the top (or ‘end’ as the stone is horizontally positioned) of the stone, which had been not been sufficiently captured in a previous attempt. When we arrived, the sun was shining directly on the stone – this is not ideal for photogrammetry, as photos with flat/consistent lighting create the best results, and any photos taken in direct sunlight would have had my own shadow feature prominently in the centre. Luckily, Nora’s jacket provided just the right amount of shade, though undoubtedly our set-up probably looked strange to passers-by!
We will keep you updated as our 3D recording and processing continues, but we wanted to introduce you to the digital imaging methods we would be utilising throughout the project.