Filed under: Commentary, Guest Blogger, Lighting, On Location, Technology | Tags: capture, guest blogger, medieval, mosaic, Preservation, PTM, Reflectance transformation imaging (RTI), tesserae
Our guest blogger is Heidrun Feldmann, a PhD student in History of Art at the University of Basel and an assistant on the research project “Digital Materiality” at the Digital Humanities Lab there. Thank you, Heidrun!
It is obvious that art historians need good reproductions of works of art to do their research. However, photographic images, which are static and two-dimensional, are not capable of reproducing the visual impression we have when we look at mosaics. Their specific materiality and surface properties make a visualization of these characteristics difficult. Besides, as ancient or medieval mosaics are usually placed on the walls of churches, they interact with those specific surroundings. The lighting conditions inside these buildings, as well as the optical impressions for a visitor moving across the room, change dynamically, which results in a unique sensory experience. This is also a reason why the designs of mosaics in such religious contexts were often attuned to the liturgy. The impressive sparkling effect is caused by the surface properties of the countless tesserae, which – when animated by light − shimmer in many different colours and shine like precious metals. Sometimes those tesserae were placed in the setting bed with a certain tilt angle. This might seem irregular to us today, but then it was done intentionally to optimize the reflectivity of the surface.
With the aid of RTI (Reflectance Transformation Imaging), we now have more options for capturing and simulating the reflection properties of a mosaic’s surface, as well as its interaction with changing light conditions. The RTIViewer software enables us to convey the impressions of this highly dynamic medium to people who cannot visit the actual mosaic in situ. RTIs also help us document the current condition of mosaics more accurately than in the past, and they support our goal to answer questions about how light was used in medieval architecture.
To test the RTI method, we visited the Bode-Museum in Berlin, where a mosaic, originating from the church of San Michele in Africisco, Ravenna, is exhibited as part of the Early Christian and Byzantine Collection (Figure 1). We thank Gabriele Mietke, curator of the department, for allowing us to take our photos. The mosaic is fitted into the architecture of the museum, where an apse was constructed to imitate the original place of its installation in the church in Ravenna, albeit without the original lighting situation.
Scholars have extensively debated the condition and state of preservation of this mosaic. Without going into all the details, we can say it is certain that the mosaic we see in the museum differs from the original of 545 AD because of its turbulent history. It has been restored and changed more than once, and some critics say that the whole mosaic is merely a copy. For us this was particularly interesting. We were wondering if the RTIs would provide further information regarding interventions, changes, or repairs.
Because of its size and form, it was impossible to take pictures that cover the whole of the apse. Therefore we captured it in twelve segments. About sixty photographs were taken of each of these segments, changing the position of the flashlight by hand for every picture. The twelve RTI files we obtained in this way show the reflection properties much better than any static photograph could do.
There are some limitations with glossy surfaces, because specular reflection cannot be adequately represented with the typical mathematical model used in Polynomial Texture Maps (the first form of RTI). However, changing the angle of the incoming light in the RTIViewer software allows us to identify areas whose structure and reflection properties differ from the others. In those areas the tesserae are of a different size or form and seem to be set in another way. All this suggests that these are the areas where the mosaic has undergone some kind of repair or restoration (Figures 2 and 3).
Having successfully tested the technique under the special conditions in the museum, we are now looking forward to the next step: capturing RTIs of medieval mosaics in situ and working on enhanced models for the visualization of gloss.
To find out more about our research project, see http://www.dhlab.unibas.ch.
Filed under: Commentary, Guest Blogger, News, On Location, Technology | Tags: ancient footprints, photogrammetry, Preservation
Sarah Duffy, PhD is a Postdoctoral Research Associate at the University of York in the Department of Archaeology. In May of 2013, after a series of storms, ancient footprints were revealed on a beach near Happisburgh (pronounced “Hays-boro”) on Britain’s east coast in Norfolk (see a 6-minute video). The footprints were fragile and washing away a little day by day. Sarah was called to the site by Dr. Nick Ashton, Curator of Palaeolithic and Mesolithic collections at the British Museum, to document it. Here is our interview with Sarah about this dramatic discovery.
CHI: Sarah, can you tell us how you got involved in the project and what you found when you arrived?
Sarah: It really came down to good timing and an opportune meeting of research colleagues. When the footprints were discovered, I had just begun working with Dr. Beccy Scott at the British Museum on a project based in Jersey called “Ice Age Island”. Beccy and my partner, Dr. Ed Blinkhorn, another early prehistoric archaeologist, have collaborated for many years, and he introduced us. During the week of the discovery, Beccy happened to be at a research meeting with Dr. Nick Ashton and suggested that he get in touch with me about the Happisburgh finding.
The next day, I received a call from Nick who asked if I would be able to take a trip to Norfolk to record some intertidal Pleistocene deposits at Happisburgh. Having just completed my PhD thesis (a “dissertation” in US currency), I was up for an adventure and ready to take on the challenge, and so I was on a train to the southeast coast two days later.
My first plan of action was to figure out how on earth you pronounced the name of the site! The next step was to figure out what equipment I would need to have available when I arrived. This proved somewhat challenging, as I had never visited the Norfolk coast, and it seems quite humorous in hindsight that one of the pieces of kit I requested was a ladder.
All of this said, the urgency to get the site photographed became clear when I showed up one very rainy afternoon in March. Standing on the shore, I felt very privileged to have been invited to record such an important set of features, which disappeared within only weeks of their discovery by Dr. Martin Bates.
CHI: What were your goals in the project and why did you choose to shoot images for photogrammetry? Can you tell us a little bit about your approach?
Sarah: Since I wasn’t sure what to expect when I reached the site, I took both photogrammetric and RTI kit materials with me. I intended to capture the 3D geometry of the prints with photogrammetry and subtle surface relief with RTI. However, when I arrived, both the weather and tidal restrictions limited the time we were able dedicate to recording. I therefore focused my efforts on photogrammetry, which proved a flexible and robust enough technique that we were able to get the kit down the cliff side in extremely challenging conditions and capture images that were used later to generate 3D models.
Based on the looming return of the tide and the amount of time required to prepare the site, our window of access was quite small. While I took the images, aided by Craig Williams and an umbrella, the rest of the team battled the rain and tide by carefully sponging water from the base of the features. As mentioned, I originally intended to capture images from above the site using a ladder. However, as the ladder immediately sank into the wet sand, I was forced to find other means of overhead capture: namely Live View, an outstretched arm, and umbrella. There was just enough time to photograph the prints, loosely divided into two sections, using this recording approach before we had to retreat back up to the top of the cliff (and to a very warm pub for a much deserved fireside pint!).
CHI: When you got back to your office, how long did it take you to process the images, and what software did you use?
Sarah: Originally, I used the Standard Edition of PhotoScan by Agisoft, later returning to the image set in order to reprocess it with their Professional Edition. PhotoScan’s processing workflow is relatively straightforward, and the time required to generate geometry is somewhat dependent on the hardware one has access to. The post-processing of the images was by far the most time-consuming component of the processing sequence. Since the software looks for patterns of features, there was a substantial amount of image preparation that needed to be completed first, before models could be produced. For example, rain droplets on the laminated surface that contained the prints needed to be masked out, as well as the contemporary boot prints that accumulated in the sand that surrounded the site throughout the image sequence.
CHI: How were the 3D models you produced used by the other archaeologists involved with the site?
Sarah: Once the models had been generated, the rest of the team, including Nick Ashton, Simon Lewis, Isabelle De Groote, Martin Bates, Richard Bates, Peter Hoare, Mark Lewis, Simon Parfitt, Sylvia Peglar, Craig Williams, and Chris Stringer, wrote the paper on the results. Nick Ashton and Isabelle De Groote closely analyzed the models of the prints in order to study size, movement, direction, and possible age of the early humans who might have created these features. Isabelle later worked with the 3D printing department at Liverpool John Moores University in order to have one of the digital models printed.
CHI: Since the footprints were washed away, your images are the best record of the site that exists. Are the 3D models accessible? What will you do to preserve this material?
Sarah: Coverage of the footprints, including excerpts of the digital models that I generated and the 3D printout, can be viewed at the Natural History Museum exhibit in London, Britain: One Million Years of the Human Story, which closes September 28th. Findings from the analysis have also been published in PLOS ONE, an open-access, peer-reviewed scientific journal.
As mentioned, when I visited the site last March, I had hoped to undertake a RTI survey. Although conditions on the day of recording did not permit multi-light capture, I have since been able to generate virtual RTI models that reveal the subtle topography of the prints. An excerpt of one of these models can be viewed on my website.
Additionally, the research team, in collaboration with the Institute of Ageing and Chronic Disease at Liverpool, are currently working on extracting further information from the image set. Findings from this work will be made available in the future. Once analysis is complete, the images and resulting models will be archived with the British Museum.
CHI: Thank you for your time, Sarah, and what a great story!
Sarah Duffy has been collaborating with Cultural Heritage Imaging (CHI), including taking CHI’s training in Reflectance Transformation Imaging (RTI) and working with the technique since 2007 while she was a graduate student in Historic Preservation at the University of Texas at Austin. Sarah authored a set of guidelines for English Heritage on RTI. During her doctoral work, she also began to apply her imaging skills in the area of photogrammetry.
We often get asked why we set up CHI as a nonprofit. I can understand the question, because we are doing some pretty high-tech projects, and we work with a number of famous institutions whose names people recognize, perhaps making us seem grander and better endowed than we really are.
My background is in computer science, and I worked in software product development for years. At some point I had decided I wanted to apply my skills to “make the world a better place.”
Mark had similar leanings. His background was in philosophy and studio arts, primarily sculpture. He began looking into 3D modeling and laser scanning in the late ’80s, and by the mid-1990s he was teaching the subject. I had a minor in sculpture and ceramics. We both loved history, art, and archaeology. We had met in 1983 and married in 1989. By the late ‘90s, digital cameras were coming into play, and structured light scanning technology was becoming available for 3D capture.
Mark and I got fired up. We started seeking out people who worked in archaeology or museums to better understand their needs. Our first questions were: What did they wish to do that they couldn’t do in the field? Could the emerging imaging technologies help them in research and creating access to more cultural material?
Over time, and as we learned more, the seeds of CHI took root in us. By 2002 we began to imagine how existing and emerging technologies could be used to create robust, powerful, low-cost tools to document cultural heritage objects and collections. And so we formed Cultural Heritage Imaging.
Today, well over 10 years after we started our nonprofit, we remain committed to fostering the improvement, availability and adoption of these documentary tools. We see them as “democratizing technology,” because our vision is founded on making cultural and natural science techniques and materials available to people all over the world.
Many of our collaborations are only possible because we are a nonprofit. “Pulling on the same oar” for humanity’s benefit is a powerful reward. Our nonprofit status is attractive to top researchers and organizations who are drawn to work on and contribute to saving history. These experts are sometimes willing to help for very little money, and occasionally they even raise their own grant funding. Our open source approach is inclusive and allows others to add new features to the tools, moving the whole community forward.
The downside of this commitment to openness is there is a constant need to raise money, and much of the money we get is earmarked for specific purposes. It’s great to get funding for a project we want to do, like the National Endowment for the Humanities Start-up grant we recently received. However, many of the requirements of running the organization and fostering community growth are not covered by the grant funds.
Funding is critical! We get a lot of volunteer support, we work with students and professors, we get discount rates from many professionals. We are extremely grateful for this help and it makes an enormous difference. At the end of the day, we rely on the good graces of our donors to keep us, and the community, going.
I have had the good fortune to attend a few recent events that allowed me to see some really useful work other folks are doing in our field. I thought it worth a blog post to mention a few with some links. I’ll note that I have seen even more cool stuff, but if there wasn’t a paper or a page I could link to, I decided not to include it here.
First, Mark Mudge and I were at Computer Applications and Quantitative Methods in Archaeology (CAA) in late April in Paris. Yes, I know, it is a hard-knock life having to present your work in Paris in April. I always love CAA because it is a great conference for sharing information and real-life experiences. People are super helpful, and everyone wants to see everyone else succeed. This isn’t the norm for a lot of conferences.
I want to point out 2 projects from there. The first is Eleni Kotoula’s RTI on Papyrus case study. New here is Eleni’s use of multispectral imaging and also her experiment with transmitted RTI. Eleni was interested in information for conservation about the state of this papyrus, which is mounted on cloth and under glass. The second project at CAA isn’t an RTI project, but it’s very cool and worth mentioning anyway. It is the work of Adam Rabinowitz et al. on PeriodO. As folks who follow our efforts know, we at CHI are big fans of metadata, process-history tracking, and related topics. Figuring out how to talk about periods in archaeology and art is really hard, and Adam and team have an idea for an interesting approach. This project recently won National Endowment for the Humanities support, so it will be able to go forward. You can and should contribute to this effort yourself!
Next up is the American Institute for Artistic and Historic Works (AIC) conference, which took place at the end of May, right here in San Francisco. It was great for us to have so many folks we have worked with here in SF. One of the things I noticed at this conference was that Reflectance Transformation Imaging (RTI) showed up in a number of talks, though that wasn’t the focus of the talk. RTI is another tool to look at objects people are studying, and RTIs are being shown right alongside IR images and X-rays and the like. YAY! My takeaway is that RTI is an accepted part of the practice for art conservators. We are super happy to see that.
Finally, I was able to attend a 3-day workshop on RTI current practice and future directions, hosted by the Cyprus Institute in Nicosia, Cyprus in early June. This was “All RTI, All the Time!” and it was very productive to be part of a dedicated meeting like this. There were many noteworthy projects presented there, and I’ll point out a few. Up first, the work of Todd Hanneken and others integrating spectral imaging and RTI. While many of us have done spectral imaging and RTI going back several years, what is new here is the use of a MegaVision monochrome camera system, and taking 11 spectral bands. RTI was shot in the visible and used for luminance data to generate the RTIs, and the color data collected via the spectral bands was then applied to the images shot in visible light. The team also tested shooting full RTIs in each spectral band for comparison. In addition, work from the Visual Computing Lab in Pisa for a WebGL-based RTI viewer, released in January of this year, was discussed. At the moment, this work is limited to streaming large RTI files and then allowing the user to pan, zoom, and relight, but some possible future directions were outlined. It is open source, so you can try it out yourself. There is also a project at The University of Southampton to develop a web-based RTI viewer, and we received an update on that project, but I don’t have a link where I can send you just yet.
As part of the discussion on where RTI is going, there was a lot of interest in quantitative uses of RGB and normals data. There are a variety of people working in this area, though they didn’t present directly at any of these meetings. Several of these folks are working with us at CHI, and we are very excited by this direction. First up, the work of Dale Kronkright, Greg Bearman, and several others to look at tracking changes through normals, and also to quantify normal calibration. You can find both papers here. Additionally, there is great work going on at Simon Fraser University under Professor Mark Drew to improve the accuracy of surface normals calculated from RTI data sets. They are also working on improving the appearance of RTI data in the viewing environment. There are a number of papers on this topic, most recently the masters thesis of Mingjing Zhang.
It is an exciting time to be working with RTI data! There is much more work going on than I could include here, and more that hasn’t been published yet. I want to close with a plug for the CHIForums where many topics like this are discussed, and there is a Projects Forum where folks can put up links to their work, as well as get comments and feedback.
Filed under: Commentary, News | Tags: Anna Ressman, cultural heritage imaging, Digital Imaging, Oriental Institute Museum, Reflectance Transformation Imaging, RTI, Technology, University of Chicago, visualization
Recently Anna R. Ressman, Head of Photography at the Oriental Institute Museum, University of Chicago, shared a compelling article with me, and now I’m sharing it with you.
Here is a link to the Oriental Institute newsletter (PDF), which contains the article entitled, “Behind the Scenes: Museum Photography at the Oriental Institute.”
Anna describes the process in which five very different artifacts are documented, each with a unique challenge. And yes, you guessed it, one of those artifacts was documented using the RTI highlight method.
Documentation of the Egyptian stele “was photographed with a method of computational photography called Reflectance Transformation Imaging (RTI).”
Anna concludes the section on RTI with these insights: “RTI files can be created in such a manner that pixel data is analyzed to show specular information rather than color data, which can reveal more information about the surface of the object than color data alone (figs. 3–4). As you can see, the inscriptions on the stele are much clearer in the specular-enhancement PTM image (fig. 3), even though the studio photograph (fig. 4) was taken using a macro lens under controlled studio lighting. The former may not be as aesthetically pleasing as the latter, but it reveals much more information than would normally be seen — and that is just a single image out of a series of forty-five.”
Be sure to download the complete article and check out the rest of the newsletter as well.
Anna R. Ressman is Head of Photography at the Oriental Institute Museum, University of Chicago, USA. Anna is also a freelance photographer and a fine artist.
[Photos by Anna R. Ressman/Courtesy Oriental Institute Museum, University of Chicago]
Filed under: Commentary, News, Technology | Tags: Antikythera Mechanism, PTM, RTI
Tom is a long-time friend of CHI and has been an adviser and collaborator on many projects. Tom is best known as the co-inventor (with Dan Gelb) of Polynomial Texture Mapping (PTM) in 2001 while he was at HP Labs. PTM is the first form of Reflectance Transformation Imaging (RTI).
Tom has had a long research career in the fields of computer graphics and computer vision as demonstrated by his many published papers. He has frequently applied this work to cultural heritage material, most notably as part of the team that deciphered the Antikythera Mechanism. This groundbreaking work was featured in the NOVA documentary “Ancient Computer”
Filed under: Commentary, News, Training | Tags: glossary of terms, photographic terms, technical terms
Over the years we have received a lot of requests for a glossary of terms used in RTI, and we are happy to announce that a new “Glossary of Photographic and Technical Terms for RTI” is available on our website! It includes photographic terms you need to know for RTI, like “Depth of Field,” “Color Temperature,” and “Aperture.” Also included are technical terms from computer graphics and computer vision like “BRDF,” “Fitting Algorithm,” and “Phong Lighting Model.” We have included terms for file formats like DNG, XMP and TIFF, along with basics in multi-spectral imaging such as “Infrared” and “Ultraviolet-induced Visible Fluorescence Photography.” We also included terms related to keeping good process history in your RTI work, including “Digital Lab Notebook,” “ICOM-CIDOC,” and “Empirical Provenance.” We did our best to adapt the definitions for RTI users, and we also included a few notes and recommendations on photographic settings.
As always with our work at CHI, this project was a collaboration. Lots of folks offered terms they wanted to see defined, and some provided definitions. We especially want to thank Tom Malzbender for definitions for many of the technical terms; Yosi R-Pozeilov for sharing his extensive glossary of photographic terms; and technical writer Judy Bogart for pulling it all together. And finally, we had a wee bit of funding for this work from the Institute of Museum and Library Services, as part of a larger grant project in their 21st Century Museum Professionals grants. Much of the work was done through volunteer efforts.
If you value this kind of documentation, along with the free open source RTI software, please consider making a donation to help support it.