What is RTI?

Basics of RTI

Reflectance Transformation Imaging (RTI), also known as Polynomial Texture Mapping (PTM), is a technique allowing the production of an interactive photograph.

Whereas a photograph represents a single possible illumination of a subject, an RTI will render the subject under any illumination.

In detail, an RTI file is a compilation of a set of photographs where light orientation and angle of incidence can be modified at will by the user.

Raw data is acquired either by hand, or using a light dome with both the subject and the camera maintained in a fixed position. In each picture, the light source orientation and angle of incidence is varied.

It makes this approach well-suited for documenting sub-planar items where subtle differences in elevation carry important information. Usual examples are imprints, planar fossils, brush strokes, artefacts.

RTI can be very useful for material description, in particular for textile. Whereas it is very difficult to grasp a fabric's aspect from a single photograph, an RTI can accurately give a great feeling of how a material behaves. In fact, light domes are also used to generate ultra-realistic textile maps for 3D renderings.

The RTI file

An RTI file provides an exhaustive visual documentation for items that would otherwise require multiple photographs for a proper descriptive account. Unlike photogrammetry, RTI does not resort to a surface modelling step. Consequently, the most subtle details are preserved. Surface rendering can also be adapted to highlight details of importance.

Additionally, RTI files have several interesting practical properties. Computation is much shorter than 3D models and require no or very little pre-processing and post-processing. Files have a moderate size by current standards. Coupled with free and open source software RTI files can be disseminated easily (see the chi website for a desktop version and vcg isti cnr website for a webGL version that allows embedding RTI on webpages, like in this example).

Altogether, RTI files are useful surrogates to the actual, visual observation of an item. Often, resorting to RTI files makes the mailing of items (hazardous) and visits to institutions (costly) both avoidable.