Polynomial texture mapping, also known as Reflectance Transformation Imaging (RTI), is a technique of imaging and interactively displaying objects under varying lighting conditions to reveal surface phenomena.
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Origins
The method was originally developed by Tom Malzbender of HP Labs in order to generate enhanced 3D computer graphics and it has since been adopted for cultural heritage applications.
Methodology
A series of images is captured in a darkened room with the camera in a fixed position and the object lit from different angles. These images are then processed and combined to enable a virtual light source to be controlled by the user inspecting the object. The virtual light source may be manipulated to simulate light from different angles and of different intensity or wavelengths to illuminate the surface of artefacts and reveal details.
Applications
Polynomial texture mapping may be used for detailed recording and documentation, 3D modelling, edge detection, and to aid the study of inscriptions and other artefacts. It has been applied to hundreds of the Vindolanda tablets by the Centre for the Study of Ancient Documents at the University of Oxford in conjunction with the British Museum. It has also been deployed, by Ben Altshuler of the Institute for Digital Archaeology, to scan the Philae obelisk at Kingston Lacy, and the Parian Chronicle at the Ashmolean Museum; in both cases scans revealed significant, previously illegible text.
A 'dome' supporting twenty-four lights has been used to image paintings in the National Gallery and produce polynomial texture maps, providing information on condition phenomena for conservation purposes. Studies of the technique at the National Gallery and Tate concluded that it is an effective tool for documenting changes in the condition of paintings, more easily repeatable than raking light photography, and therefore could be used to assess paintings during structural treatment and before and after loan.
The technique is now also finding uses in the field of forensic science, for example in imaging footprints, tyre marks and indented writing.