Polynomial Texture Mapping (PTM) is a type of RTI imaging application (computational photography), which was developed by Hewlett-Packard Imaging Labs and enhanced by the West Semitic Project at USC, addresses the difficulties in the photography of objects whose surfaces feature inscriptions or relief designs. They can be found on an array of archaeological objects and works of art such as inscriptions in stone or clay, plaques, coins, paintings, mosaics, relief sculpture, jewelry and other minor objects. It consists of a dome with a hole at the apex, and thirty-six Halogen lights embedded at randomly fixed intervals around the dome. An artifact is placed at the base of the dome, while a camera is positioned looking downward focusing, through the hole at the top, on the aforementioned object.
Thirty-six photographic images are sequentially taken, each with a single light shining on the artifact, thus creating thirty-images with different light angles. Then, the PTM algorithm synthesizes the data from these images to create a single image that can be examined on a PTM viewer. The viewer allows the user to move the light angle in real time, so that the combination of light and shadow representing the relief features of the object’s surface can be freely altered.
The second method concerns a domeless PTM/RTI technique that can be used to create RTI images of larger artefacts that cannot fit under the dome. This technique is known as “highlight RTI” or “H-RTI”. The successful application of this technique requires a well-trained team that will be able to emulate the lighting effect of the previously described hemispherical dome with the adoption of photographic flashes, or any other light source, a reflective sphere and a fixed-position camera on a tripod. During post-processing the reflection of the light source on the spheres enables the light tracer() algorithm to calculate the lighting angle for that image and the PTM/RTI algorithm to map the digital surface in to an accurate lighting environment. This technique allows great flexibility in subject size and location.
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