The Library of Congress’ Geography and Map Division is home to a large collection of Pre-Columbian archaeological artifacts donated by the collector Jay I. Kislak, many of which are on display as part of the Exploring the Early Americas Exhibit in the Thomas Jefferson Building here in Washington, DC. The artifacts that make up the collection range in dates from the Olmec culture around 1000 BCE, to the classic period Maya (300-900 CE) and Aztec civilizations, including many objects that date from the period just before contact with the Spanish in the late 15th century.
As the Curator of the Jay I. Kislak Collection, I am always looking for new and innovative ways to make this group of archaeological artifacts more accessible to scholars and educators around the world who, for whatever reason, cannot make the trip to Washington, DC. Those that can make the trip are always welcome to use the Kislak Study Collection, located in the Geography and Map Division, where the artifacts not currently on display in the gallery are stored.
One way that we are attempting to make the collection more available is through the use of three-dimensional imaging. In the case of material artifacts, two-dimensional images, while helpful, do not allow for the complete examination of an object and, moreover, can often distort its dimensionality and structure. In order to make proper attributions and comparisons with similar objects in other collections, it is critical for scholars who cannot examine an artifact in person to have realistic views “in the round” of what they are studying. To this end we have embarked on a series of experiments here at the Library of Congress that uses three-dimensional structure from motion imaging to reconstruct scaled and true to life models of the artifacts in our collection.
Structure from motion imaging is a complex technique that allows for the extraction of three-dimensional information not only from single objects, but also from the architectural features of buildings and ruins, or from landscapes, all derived from a series of two-dimensional images. The technique was developed for computer and robot vision and is the digital equivalent of the task that the brain and eye perform as we humans move through a three-dimensional world using two-dimensional projections (for more on this see the books by Richard Hartley and Richard Szeliski).
In principle the calculations required to do this kind of imaging are algorithmically complicated, and are related to photogrammetric techniques involved in sorting out the difficult geometry of remote sensing images of the earth and other planetary bodies taken from satellites.
To make a three-dimensional model of an archaeological object a group of two-dimensional images are taken from a variety of vantage points and are processed through a pipeline of computer programs that create a three-dimensional point-cloud representing all the various surfaces that make up an artifact.
The actual process of making three-dimensional models using structure from motion consists of two parts. During the first, the computer examines the two-dimensional photographs and finds matching points in multiple images. The points are then used to calculate the actual position in space where each of the images was taken (see the photo cluster diagram below). Once the positions of each of images is known, the location of the points from all the images are plotted in space, yielding a dense reconstruction of the shape of the object that was photographed. The result is a point-cloud that is very similar to the kind of data one would extract from a laser scan of an object.
The surface generated by the point-cloud can then be further analyzed to make an interactive scaled and true-to-form model of the object by overlaying a polygonal mesh, representing mathematically the artifact’s form. The polygonal mesh might employ many shapes but are most commonly triangular, and depend on the smoothness of the surface being imaged.
Besides the density of the triangular mesh, additional features are used to visualize the smoothness and texture of the surface. Techniques like specular shading, the use of lines of reflection, and what are called isophytes, or lines of constant illumination across the surface, help accentuate the three-dimensional data derived from the two-dimensional photographs . In some cases additional algorithms might be used to smooth the surface and de-noise the photographic data in order to fill holes or blend the surface curvature to improve the visualization of the artifact.
In technical terms these meshes are actually non-directed large graphs with many vertices and faces. The model above for example contains more than 400,000 nodes at medium resolution and is a truly complex and discrete mathematical object. The underlying mathematics of this re-construction relies on the geometry of digital spaces which has been developed, over the last decade or two, for the creation of realistic virtual and augmented reality experiences and for computer gaming applications.
The main difficulty associated with structure from motion imaging centers on solving a problem in projective and epipolar geometry. The solution relates all of the images taken of a particular object to each other by using common points and reference lines. This so-called, “geometry of multiple image problem,” is an active area of research in computer vision and is being applied increasingly to archaeological contexts. Constructing these geometries allows for the scaling and reconstruction of models that can be measured and compared to other like archaeological artifacts.
Here in the Geography and Map Division we are just beginning our experiments using this technique with the hope that soon we shall be able to make three-dimensional, dynamic, and interactive models of the Kislak Collection available to scholars around the world who are interested in applying this exciting new technology to their research.
…and finally, for all those of you who don’t know, Alfred Maudslay was one of the first explorers to visit some of the most important archaeological sites of the ancient Maya and to bring a camera in order to record the state of the ruins and the inscriptions…. a true 19th century pioneer in the imaging of archaeology.