See More RTI Examples

Find more examples of RTI, including the Smithsonian Institution's Squeeze Project online, with nearly 400 interactive examples of RTI (in an earlier form of RTI called Polynomial Texture Mapping, PTM). These PTM files were created to study squeezes (paper molds of Arabic script, Middle Persian, and Cuneiform inscriptions) from ancient archaeological sites in the Near East. More RTI examples…

RTI Training

CHI offers 4-day training classes regularly for groups of up to 15 people. Learn how to apply RTI to capture and study the minute surfaces of objects, and become familiar with related computational photography techniques. More…

Need Some Help?

CHI offers customized consulting services to both institutional and private clients, with special rates for nonprofit organizations. Projects of all sizes can be accommodated. Learn more…

RTI Question? Ask on the CHI Forum

CHI offers a free online forum site where you can share your questions, insights, and issues to gain a more complete understanding of RTI and its practical applications. How are museum conservators, computer scientists, natural scientists, photographers, and other related professional groups using RTI? What problems are they solving and what challenges are they facing? Join the conversation!

Download RTI User Guides and Software

Using CHI's documentation and collaboratively developed software, learn how to use RTI and associated techniques to capture real-world subjects so their surface relief features can be examined in minute detail. Learn more…


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Related Publications

Many of our featured publications include detailed information about RTI projects and RTI research. A few publications are of special interest:



Reflectance Transformation Imaging (RTI)

Contents:  What is it?  How does it work?  Examples 

What is it?

RTI is a computational photographic method that captures a subject’s surface shape and color and enables the interactive re-lighting of the subject from any direction. RTI also permits the mathematical enhancement of the subject’s surface shape and color attributes. The enhancement functions of RTI reveal surface information that is not disclosed under direct empirical examination of the physical object. Today’s RTI software and related methodologies were constructed by a team of international developers.

The Sennedjem Lintel from the Phoebe A. Hearst Museum of Anthropology at the University of California, Berkeley.
RTI representation showing color information (bottom portion) and “specular enhancement” mode showing surface shape and enhanced reflectance (top portion).

RTI images are created from information derived from multiple digital photographs of a subject shot from a stationary camera position. In each photograph, light is projected from a different known, or knowable, direction. This process produces a series of images of the same subject with varying highlights and shadows. Lighting information from the images is mathematically synthesized to generate a mathematical model of the surface, enabling a user to re-light the RTI image interactively and examine its surface on a screen.

Each RTI resembles a single, two-dimensional (2D) photographic image. Unlike a typical photograph, reflectance information is derived from the three-dimensional (3D) shape of the image subject and encoded in the image per pixel, so that the synthesized RTI image “knows” how light will reflect off the subject. When the RTI is opened in RTI viewing software, each constituent pixel is able to reflect the software's interactive “virtual” light from any position selected by the user. This changing interplay of light and shadow in the image discloses fine details of the subject's 3D surface form.

RTI was invented by Tom Malzbender and Dan Gelb, research scientists at Hewlett-Packard Labs. A landmark paper describing these first tools and methods, named Polynomial Texture Mapping (PTM), was published in 2001. Learn more about PTM and the origin of RTI on the Hewlett-Packard Labs web site.

Since then, RTI research, application development, and evaluation in practical-use environments remains very active. Many new RTI tools, methods, and uses have emerged. Go to our Downloads area to access the most recent versions of the software and user guides. Participate in the CHIForums to learn about new projects and new research.

How does it work?

Mathematically, the direction that is perpendicular to the surface at any given location is represented by a vector (direction) called a normal (Figure 1).

surface normals diagram 1

Figure 1

RTI software calculates surface normals per pixel in the image set. The surface normal is the vector that is perpendicular to that location on the surface.

The mathematical description of the normal is saved per pixel, along with the RGB (red-green-blue) color information of a regular photograph. This ability to record efficiently the color and true 3D shape information is the source of RTI's documentary power.

In 3D virtual reality representations, normals are used by lighting models to calculate how light rays will reflect off the surface of virtual 3D geometry.

rti diagram 2

Figure 2

The normal information present in RTI images (in an earlier phase of the technology, they were called Polynomial Texture Maps or PTMs) means they can be analyzed using similar 3D lighting techniques. Figure 3 shows the reflection information captured in the PTM.

rti diagram 3

Figure 3

Examples

There are three video examples embedded below, each showing a different example of how RTI can be applied in the study of cultural heritage objects.

See also our collection of more examples of RTI, where you can read about the Smithsonian Institution's Squeeze Project online, with nearly 400 interactive examples of RTI (in an earlier form of RTI called Polynomial Texture Mapping, PTM).

Video: “RTI Example: Papyrus Fragment”

Ancient papyrus fragment from the Bancroft Library (UC Berkeley).

Video: “RTI Example: Marble Stele”

Inscribed and recarved marble stele from the Tauric Preserve of Chersonesos, Ukraine. Imaged in July 2008.

Video: “RTI Example: Illuminated Manuscript”

An illuminated manuscript page study that demonstrates how RTI can be used to reveal hidden artifacts, in this case a letter that was erased on the page. Courtesy of and in collaboration with the Bancroft Library, UC Berkeley, California.