Filed under: Guest Blogger, On Location | Tags: Archaeology, knapped tools, lithics, RTI
This is the second post by our guest blogger Dr. Leszek Pawlowicz, an Associate Practitioner in the Department of Anthropology, Northern Arizona University, Flagstaff, AZ, USA. He can be contacted at firstname.lastname@example.org. Thank you again, Leszek! Even in the age of digital photography, archaeology still relies heavily on old-school hand-drawn illustrations for documenting artifacts, particularly for publication. It can be difficult, or even impossible, to get a single photograph that shows all of the artifact’s key details. In my area of interest, knapped stone tools, low relief in small-scale surface topography, high relief in large-scale topography, specular (reflective) materials, variations in material color and contrast across the surface, all conspire to make these artifacts difficult to photograph. A skilled illustrator is capable of creating a drawing that reveals far more detail than even a good standard photograph (Figure 1). But creating such a drawing requires experience, time, and money, often making it impractical.
Reflectance Transformation Imaging (RTI) is a great way to document and image knapped stone tools. The ability to interactively modify the lighting angle, as well as mathematically manipulate the perceived interaction of light and surface, allows the viewer to see details difficult to photograph. It’s almost the equivalent of having the object in your hands, and turning it at various angles to the light to reveal details of its structure and manufacture. However, one drawback is that you can’t embed an RTI view in a paper or publication unless it’s in electronic format. Also, unlike a drawing that can show all the critical details at once, you may have to move the virtual lighting angle in many different directions to reveal all details. Using data from the custom RTI systems I described in a previous post, I’ve been working on ways to create static images of knapped stone tools with detail comparable to that in a line drawing.
Figure 2 shows a series of images of a modern knapped projectile point fashioned from specular black obsidian, a particularly difficult material to photograph because of its shininess and lack of contrast. 2-A is an original digital photograph, with the lighting coming from the standard upper left direction; while a fair amount of detail is visible, shiny highlights obscure some details, and the overall convex shape of the point interferes with lighting on the right side of the point. 2-B shows an RTI view with lighting from the upper left; while glossy highlights have been eliminated, and more detail is visible on the right, other details are somewhat more subdued in this static view. Figure 2-C uses the RTI specular viewing mode, which imparts an artificially shiny character to the surface. This is actually a superior result for this mode on knapped stone tools – most of the time, it doesn’t look nearly this good (as you’ll see shortly). But this image suffers from lack of detail in some areas, probably because of the artifact’s overall convex surface shape. Figure 2-D uses the RTI Static Multilight mode, where the RTI data is analyzed to determine the optimal blend of multiple light angles to reveal details. Once again, this is a better result than I usually get for knapped stone tools, but some details are still hard to see because of lack of contrast in many areas. Overall, I have found that the recently added RTI Normals viewing mode leads to the best results. This mode color-codes the surface based on the perpendicular direction at every point in the image. Figure 2-E was generated using the Normals mode and a second-order HSH RTI file (standard Polynomial Texture Mapping or PTM) files produce markedly poorer results). Even in the raw colored state, the amount of detail visible across the entire surface is vastly superior to the other views. Using standard image enhancement techniques, one can further process this image to generate an extremely detailed view of the artifact’s surface details (Figure 2-F). While this modern artifact was made of a difficult material, the freshness of manufacture, lack of wear, and non-exposure to the elements make the surface features quite sharp and easy to see. What about an actual prehistoric point with a real history?
Figure 3 shows a series of similar photographs for the Molina Spring Clovis point, collected about 10 years ago in the Apache-Sitgreaves National Forest. Clovis points are approximately 13,500 years old; this one in particular has seen a lot of wear on the flake scars, making them difficult to make out. What’s more, the point’s material (chert) is slightly glossy and very light in color, minimizing contrast in surface details. Figures 3-A through 3-F show the same image processing sequence as Figures 2-A through 2-F; note in particular that the Specular and Static Multilight modes (3-C and 3-D) do not produce especially useful images for this point. The raw Normals image (3-E) brings up details difficult to impossible to spot in the previous images, and enhancing the Normals image (3-F) makes those details even easier to see. For Figure 3-G, a Matlab script was used generate a modified view based on slope; this brings out certain details not immediately visible in the original normals image. In my opinion, images like 3-F and 3-G are superior to standard digital photographs of knapped stone tools and could well be used instead of line drawings for publication purposes. Even in cases where line drawings are preferred, these images could provide a useful background basis for tracing artifact details, instead of drawing them freehand. Beyond static images, the normals data can be used to generate a full 3D representation of the artifact’s surface. Some examples of this are visible at my website, along with downloadable 3D files and RTI data files for several projectile points. The 3D surfaces aren’t yet fully accurate, as they can be affected by inaccuracies in the normals calculation, and error accumulations in the surface fitting. However, these results are already significantly improved from my initial efforts, and I will be working on improving them further. Even in the current form, I believe that it should be possible to extract usable information than can be used to quantitatively characterize knapped stone tools.
Filed under: Equipment, Guest Blogger, Lighting, Technology | Tags: capture, guest blogger, paste print, Reflectance transformation imaging (RTI), RTIViewer, specular enhancement
Our guest blogger is Dr. Lothar Schmitt, a post-doc in the Digital Humanities Lab at University of Basel in Switzerland. Thank you, Lothar!
For some people early prints are a boring topic, but a few specialists appreciate these crude woodcuts and engravings with their stiffly rendered religious subjects. There are reasons for this unusual predilection: Beginning in about 1400, prints became an increasingly important means to make images affordable for the general public. In addition, printing images stimulated the development of several technical innovations. Among these are ways to reproduce three-dimensional surfaces and to imitate the appearance of precious materials like gold reliefs or brocade textiles.
One such technique is called “paste print.”
With only about 200 examples existing worldwide, this kind of print is rare. It consists of a layer of a slowly hardening oil-based material (Fig. 1, No. 3) that was covered with a tin foil and brushed with a yellowish glaze in order to look like leaf gold (Fig. 1, No. 4). All these layers were stuck to a sheet of paper (Fig. 1, No. 1). To produce an image, the surface of an engraved metal plate was coated with printing ink and pressed into the paste. Through this process, the printing ink was transferred as a dark background (Fig. 1, No. 5), while the cut image of the metal plate generated a relief of golden contours and hatchings. Since these layers became brittle over time, most paste prints are heavily damaged (Fig. 1, No. 2). Moreover, the subjects they show are sometimes hard to decipher.
Traditional photographs are not well suited to reproduce paste prints because it is impossible to record the interaction between the light and the barely discernible relief of the print’s surface with one single capture. To document such effects, our team, a Swiss National Science Foundation (SNSF) research group of four people at the Digital Humanities Lab in Basel, Switzerland, made the decision to try Reflectance Transformation Imaging (RTI). The benefits of RTI are ideal for revealing the material properties of the prints. However, since RTI is not able to properly reproduce the gloss of a metal surface, we were unsure about the results. The first test was very promising.
We traveled from Basel to nearby Zürich, where there is a paste print of an unidentified saint glued into a manuscript at the Zentralbibliothek Zürich (B 245, fol. 6r). The library staff, among them Rainer Walter and Henrik Rörig, were very helpful. Peter Moerkerk, head of the digitization center, even made a high-resolution scan of this print that we could use as a reference image (Fig. 2).
For capturing RTIs we constructed a Styrofoam hemisphere with a diameter of 80 cm. On the inside of the hemisphere, there are 58 evenly distributed LEDs that can be triggered in succession. The LEDs are synchronized via a simple control unit that is connected with the flash sync port of the camera. The control unit coordinates with the interval mode of the camera in order to capture a sequence of images automatically. The resulting RTI file shows the subtle surface texture and is instrumental for comprehending the relief and the layered structure of the print (Fig. 3).
As we pointed out earlier, the glossy effects of the golden parts appear too dull, but the “specular enhancement” feature of the RTIViewer helps to distinguish between the surface conditions of the different materials that were employed to make the print.
RTIs of two other paste prints in Switzerland and several others in German collections will be captured in 2015 and 2016. If you are interested in our proceedings, please see our web site: http://dhlab.unibas.ch/?research/digital-materiality.html
Filed under: Guest Blogger, Internship | Tags: cultural heritage imaging, digital humanities, history, scientific documentation
Our guest blogger, Matt Hinson, is a junior at the School of Foreign Service at Georgetown University in Washington, D.C. Welcome to CHI, Matt!
As a student majoring in International History, I’m interested in learning methods of interpreting history through the analysis of cultural heritage. Yet I’ve observed it is sometimes difficult to understand how to apply the knowledge I’ve gained through academics. As I learn more about the current threats to major historical sites and landmarks, I have come to realize that the documentation of world heritage sites is a major component of the effort to save them. And working to save the world’s historic and cultural treasures is a valuable application of historical research skills. So when I stumbled upon Cultural Heritage Imaging (CHI), an organization on the forefront of the scientific documentation of cultural treasures, I applied for a summer internship, and here I am.
As someone who would like to further pursue academic research in history, I have realized that learning how to study physical historical artifacts can add to my existing knowledge of how to analyze historical texts. Being introduced to the imaging methods that are featured here at CHI will improve my overall research skills in preparation for future projects. The emerging field of digital humanities is another area in which CHI heavily contributes. As the humanities become more and more connected with technology, I believe it is important that students of the humanities become better acquainted with such technologies. Learning about RTI, photogrammetry, and the other digital imaging methods developed at CHI is a great way to see how the field of the humanities can be transformed by such techniques.
This summer, I hope not only to learn from CHI but also to be useful to the organization in advancing its goals. CHI is working on a number of extremely interesting projects, and I would like to contribute to them as much as I can. I am not a technical expert when it comes to 3D imaging, so I intend to concentrate on expanding the cultural and historical dimensions of CHI’s work in world heritage preservation. I am very much looking forward to my time here at CHI!