When we see an object in our world, we are seeing the light that has come from its surface. When incoming light strikes and bounces off the object and is captured by an eye or a camera, this light is called reflectance. Reflectance carries the visual information about the object we experience.

The features on an object’s surface people can see and cameras can photograph varies according to how the surface is illuminated. For example, and outdoor rock art inscription may only be visi­ble early in the morning when the sun is low in the eastern sky. At noon when the sun is high, the same inscription may be invisible. When light strikes an object from different directions, it reveals different subsets of the object’s total surface information. This relectance information can tell us much about our world’s the shape and material characteristics

Reflectance Transformation Imaging (RTI) captures this information with digital photography. RTI’s documentary usefulness has been demonstrated in many natural science and cultural heri­tage subject areas and offers significant advantages suggesting widespread future adoption.

RTI was invented by Tom Malzbender at Hewlett Packard Labs. The landmark paper describing these first tools and methods, named Polynomial Texture Mapping (PTM), was published in 2001.

RTI research, application development, and evaluation in practical use environments remains a very active. Since 2001, Many new RTI tools, methods, and uses have emerged. A major international research collaboration is building an new generation RTI tool suite.

RTI enables robust ‘virtual’ examination and interpretation of ‘real world’ subjects possessing surface relief features. RTI information may also be mathematically enhanced. RTI enhancement has been shown to disclose surface features that are impossible to discern under direct physical examination. There is a growing family of enhancement functions that use RTI color and 3D shape data to aid the examination, analysis, communication and interpretation of scholarly material.

For many documentary purposes, RTI also offers cost and precision advantages over other 3D scanning methods. Reflectance information can be captured with widely available and relatively inexpensive digital photographic equipment. RTIs can be captured over a large size range, from several square meters to millimeters and can acquire a sample density and precision that most 3D scanners are unable to reach. RTIs can capture the surface features of a wide variety of material types, including highly specularly reflective material such as jade or gold.

What are RTIs?

RTIs are made 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 different highlights and shadows. After the light has been projected from a representative sample of direc­tions, all the lighting information from the images is mathematically synthesized to generate a viewpoint-specific, per-pixel reflectance function enabling a user to interactively re-light the RTI representation of the subject’s surface.

For each RTI acquired viewpoint of the subject, the resulting synthesized file resembles a single 2D photographic image. Unlike a typical photograph, reflectance information derived from the 3D shape of the image's subject, encoded in the reflectance function, ‘knows’ how light will reflect off the image. When the image 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 is used by the human perceptual system to discloses fine details of the subject's 3D surface form. Each 2D pixel, representing an area on the subject's 3D surface, 'knows' the 3D direction that is perpendicular to that location on the surface. Mathematically, the direction that is perpendicular to the surface at any given location is repre­sented by a vector called a 'normal'. This ability to efficiently document color and true 3D shape information is the source of RTI's documentary power.

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



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.

On the right is a false color display of PTM normal information. Normals facing in similar directions are similarly colored. This information can be displayed in the PTM Viewer by right clicking in the image display window and selecting EFFECTS ---> NORMAL. To display the full PTM Click here.