Reflectance Transformation Imaging (RTI) is a Computational Photography assisted technique, which uses multi-lighting conditions to capture a set of images, from a fixed camera position, with the aim of virtually and interactively revealing the characteristics of an imaged surface.
Images are created from multiple photographs of a subject, where each shot is taken with light projected from a different direction to produce varying highlights and shadows. These are then combined so the changing interplay of light and dark discloses fine details of the surface. During image acquisition the camera remains static, but the illumination is different for each picture. RTI processing software, most commonly RTIBuilder, is used to make 2.5D models from the images, i.e., 2D graphical projections that uses height information to create the illusion of 3D viewing surfaces or artefacts. Unlike 3D models, the user is restricted to seeing the models from the singular angle from which they were photographed. To separate these from other types of 2.5D models, such as Digital Terrain Models (DTM) and Digital Elevation Models (DEM), they will be referred to here as RTI models. Another RTI software, most commonly RTIViewer, is used to view and manipulate the models by changing light direction and/or rendering mode. The rendering modes display and enhance the surface elevations of the RTI models in various ways. RTI is an improvement of the traditional raking light method of photographing graffiti, i.e., image acquisition with illumination from one sharp angle. While this technique illuminates one side of the surface it often obstructs others. More importantly, the models also contain estimated information regarding the shapes and textures of the recorded surfaces.
There are two main techniques of RTI image acquisition, Highlight RTI (H-RTI) and Rigged Light RTI (RL-RTI). When using H-RTI, a light source is moved for each picture taken and the angle of the light source is captured in the reflection of one or more spheres situated somewhere within the images. The pictures should be taken with lighting from an even spread of angles and with the same distance to the surface. This distance should be about two to four times the length of the documented surface. RTIBuilder uses the reflections in the spheres to calculate the angles the pictures were taken from. An optional RTI kit, including reflective spheres in different sizes and equipment to rig these for photo sessions is available from www.culturalheritageimaging.org. Since the method is based on manipulating lights and shadows the pictures are normally taken in dark or dim conditions.
When RTI is required
RTI tecniques has been employed in several projects for the virtual examination and study of the Cultural Heritage artefacts (high relief fossils, ancient stone tools, oil paintings, Cuneiform tablets, numismatic collection, the Antikythera mechanism). In this field the way in which the light interacts with the object is very important because the features of the material, reflectance behaviour, and texture can offer major perceptual and cognitive hints for the study of the artwork. In many cases the ability to interactively play with the light is more useful than the manipulation of an accurately sampled 3D shape, that is hardly able to capture all the interesting aspects of the artwork.
RTI technologies present several advantages:
inexpensive and widely available hardware (in many cases, just a digital camera and a light source)
scale well to both large and very small objects
a sampling density and a precision that most current 3D scanners are unable to reach, even under optimal acquisition conditions
For those reasons, RTI techniques are widely used in the Cultural Heritage field for documentation tools, giving a precious instrument to the specialists in the analysis and interpretation process. The mobile equipment enables working in a variety of indoor and outdoor settings, including remote areas and even underwater settings. Virtually any material can be captured, with object sizes ranging from the microscopic up to larger objects or even to complete sites. The samples of the RTI Systems for Ancient Documentary Artefacts project (AHRC RTISAD) include materials such as ceramic, wood, metal, stone or bone with surface finishes ranging from matte to shiny. The different rendering methods provide extended tools for analysis beyond what is visible with the unaided eye or difficult to capture using traditional photographic methods, while still clearly representing 3D shape characteristics. Finally, RTI-files provide a perfectly lit and location-independent means for the study of an object’s surface, thus providing “access to colour and texture in a way hitherto largely restricted to those in direct contact with the material culture”.
Like every documentation method, besides its advantages, RTI also has disadvantages that may limit its applicability to archaeology, largely dictated by the shape, size, or location of the object to be documented. Ideally, the surface being photographed should be as flat as possible. Curved objects, for example, the interior of a vessel, can be difficult or impossible to photograph using RTI due to extensive self-shadowing. Objects with a less pronounced curvature might still be viable for RTI if the camera’s depth of field can be adjusted accordingly, as was the case for this analysis. Curved objects or objects with multiple sides can be documented, but photographing the entire object requires multiple image stacks taken from different angles. 3D scanning may be a more viable option for these kinds of objects. Although the process is portable, sufficient space around the object being documented is needed. The object should be positioned in the centre of the real or virtual dome, which itself should have a radius of at least two times the diameter of the object. Therefore, particularly large objects require an accordingly large surrounding space. The viability of RTI can also be limited in confined spaces such as a tomb, a cave, or a museum storeroom. The equipment itself can also pose challenges, for example, the legs of a tripod, which can cast shadows over the object when using the H-RTI method. Ambient light and especially daylight must also be considered, as the artificial light source should ideally be the only light source in the captured images. However, it is possible to conduct RTI outdoors in a shaded location provided that the light conditions are consistent and the exposure time and aperture are adjusted to reduce natural ambient light in the digital images.