Glossary of terms

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This glossary of imaging terms is a list of terms and definitions relevant to imaging methods applied to the conservation and study of cultural heritage. It includes physical principles, diagrams and formulae, equipment, and disambiguation of colloquial terms. Conservation imaging is a cross-disciplinary practice bridging natural science and humanities disciplines. It features an extensive vocabulary and a significant amount of jargon.

0–9 • A • B • C • D • E • F • G • H • I • J • K • L • M • N • O • P • Q • R • S • T • U • V • W • X • Y • Z • References

A

albedo

A single number that specifies the reflectivity/reflectance of a surface, specifically the ratio of incoming to outgoing light. Albedo varies between 0 for a perfectly dark surface patch, to 1.0 for a perfectly reflective, white surface. Albedo can also be expressed as a percentage, for example the average Earth’s albedo is 30-35%, meaning that 30-35% of incoming solar radiation is reflected back into space, the rest absorbed.

algorithmic rendering

Computational processing of source image data that results in an image that emphasizes or suppresses certain aspects or features. In the same way that Adobe Photoshop filters apply systematic transformations to color data, in algorithmic rendering an algorithm can apply transformations to both color and surface normal data in a Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) processed data set, to produce the digital equivalent of technical illustration or photographic manipulation.

B

blend map

See also highlight map.

A composite image produced by CHI’s RTIBuilder when using the Highlight Method of RTI. The image shows all of the highlights from every image in the Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) image set, positioned on a single sphere. The blend map allows one to see the spread of light achieved for a particular Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) image set.

bidirectional reflectance distribution function (BRDF)

Describes how light is reflected from an opaque surface. A scalar function (that is, a function that returns a single number) of four variables (the inputs to the function). The inputs are two angles that specify the incoming light direction, and two angles that specify the outgoing light direction coming off the surface. Depending on the algorithm used to process the Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) Source Image Set BRDF information can be derived. Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) depend only on the variation in the first two angles; the second two are fixed, determined by the viewing direction.

bidirectional scattering-surface reflectance distribution function (BSSRDF)

Characterizes how light interacts with a reflective surface. Unlike the bidirectional reflectance distribution function (BRDF), it characterizes subsurface scattering by introducing two more sets of independent variables (beyond the four used in bidirectional reflectance distribution function (BRDF)). Subsurface scattering is common in translucent materials such as marble, where light is internally scattered by the material. The two extra pairs of variables correspond to the incoming light location, in addition to the outgoing light location on the surface.

bidirectional texture function (BTF)

Characterizes the spatial variation of reflectance for material that is not uniform, by adding two independent variables to the bidirectional reflectance distribution function (BRDF), for a scalar function of six variables. Can also be considered to have another independent variable, the wavelength of light used in the measurement.

C

camera settings

Basic digital camera settings include the exposure triangle (aperture, shutter speed, and ISO), which determine the exposure. Other common settings that affect image quality include white balance, focus mode, and output file format.

capture client

Software on a computer that aids the Image Capture to obtain a Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) Source Image Set. In that case the digital camera is controlled by that computer software, instead of the camera itself. This manner of working allows one to capture single or sequences of images without touching the camera and to visualize and assess the results directly on a computer screen. This approach can be used for captures with the Single Mobile Light, Highlight and Dome methods. A Light Array typically has a capture client.

compression

A method of reducing the size of a digital image file in order to decrease the needed storage capacity for that specific data set or batch of data sets. Compression technologies are distinguished from each other by whether they remove or simplify detail and color from the image. Lossless compression technologies compress image data without removing detail (TIFF LWZ, Jpg2000), while "lossy" technologies compress images by removing some detail (JPEG). Most Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) processing algorithms produce compressed files and use compressed images (for example JPEGs) as the input Source Image set.

computational photography

The computational extraction of information from a sequence of digital photographs. Extracted information is integrated into new digital representations to yield rich data not found in the original, individual photographs. The Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) methods are one branch of many forms of computational photography.

continuous light

Continuous lights–as opposed to flash or strobe lighting–can be aimed quickly and accurately at the center of a subject, permit longer exposures if more light is required, and do not need to be synchronized to the camera shutter. Continuous light is easy to work with indoors (outside the sun can act as continuous lighting) and have appropriate power for Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) applications. It does not work well in location settings where there is not access to a reliable power source such as electricity, a generator, or batteries.

D

Department of Electrical Engineering (ESAT)

ESAT (Department of Electrical Engineering) is one of the largest research departments of KU Leuven. As one of its divisions, PSI (Processing Speech and Images) performs demand-driven research in the field of audio and image processing. Relevant research topics are situated in the fields of Computer Vision, Artificial Intelligence, and Computational Photography and include 3D modeling and visualisation, photorealistic rendering, object recognition/classification, image compression. In collaboration with various Departments in the Humanities, it developed the MLR products: Portable Light Dome system, the pixel+ viewer and the SCML file format.

discrete modal decomposition (DMD)

Discrete Modal Decomposition (DMD) is an approach to reflectance modelling (like PTM or HSH) which is based on an Eigen basis derived from a structural dynamic problem. DMD is efficient in that it allows for a more accurate modeling of angular reflectance when light-matter interaction is complex (such as in the presence of shadows, specularities, and inter-reflections) than other approaches.

diffraction and image resolution

As aperture decreases, sharpness as a function of the diffraction of light through the lens decreases. While an aperture of f-22 gives more depth of field,sharpness and the ability to resolve definition (that is, line pairs per millimeter) is lost. For this reason, stopping down to gain depth of field must be weighed against image clarity. The impact of diffraction on digital images will depend on a few factors, but will depend largely on the pixel pitch (density of pixels on the sensor). With increasing numbers of pixels the sensors have gotten more dense, making the point at which diffraction comes in earlier (lower MP sensors may have been fine at f/11, but some sensors today may notice diffraction at f/8). When that same number of pixels is distributed over a larger sensor (i.e. half frame, full frame) the pixel density decreases and diffraction might again only occur above f/11. For Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI), avoid the diffraction phenomenon and keep the aperture sufficiently low.

diffuse gain

An enhancement technique that helps to see surface detail due to shape. Keeps the surface Normal for each pixel at the value estimated mathematically from the input images, but allows the user to arbitrarily control the second derivative (curvature) of the reflectance function interactively. This transformation makes the surface more sensitive to variations in lighting direction.

diffuse reflectance

Any reflectance of light off a surface that appreciably scatters light in many directions is considered diffuse. An ideal diffuse reflector is called Lambertian, and emits light in all directions uniformly, regardless of the incoming light direction. An example of an approximately diffuse surface is that of paper. This is opposed to a specular reflectance, which returns a highlight in a specific direction, producing a specular highlight. Many materials have both a diffuse and specular component.

directory structure for RTI

The CHI RTIBuilder software requires a specific folder structure and file-naming convention for the image files it uses to create an RTI file. In order to process the image sequence resulting from your Image Capture, create a project folder. Convert the original Camera Raw Files to Digital Negative (DNG) Format, and store the DNG files in a subfolder named original-capture/. Export JPEG versions of those images, and store them in a subfolder named jpeg-exports/.

Digital Negative (DNG) format

DNG is an open, lossless, Camera Raw File Format for digital images, created by Adobe, based on the TIFF standard. Use of this format preserves initial image state, and provides XMP (Extensible Metadata Platform) records of all transformations, beginning with development from a Camera Raw File. One recommended workflow for producing a Source Image Set from original captured images is to use Adobe Digital Negative Converter to convert in batch Camera Raw Files to the standardized DNG format, and make copies in the JPEG format.

dome

A dome-shaped Image Capture apparatus that has a set of lights at fixed positions. Control software flashes each light in a fixed sequence in order to produce the image sequence needed to produce a Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) file. A Capture Client is often used and knows the position of each light (i.e. a fixed and precalibrated Lighting Position File) and thus, the software uses a Light Map to associate each image with the lighting angle used for that particular image. See also Light Array.

E

Exchangeable Image File (EXIF)

Common Metadata format for technical information relevant to digital images, such as camera, lens, exposure information, date/time, and so on. Typically, the camera automatically collects the data and embeds it in the image file. Processing software and digital asset and preservation management systems can automatically extract information from these metadata fields in the header of the image file.

F

file format

The way an image is saved to a digital camera’s memory or to a computer. The most common file formats for digital cameras are RAW (DNG or other proprietary file formats), TIFF, and JPEG. Each Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) processing software will work with one or several permissible file formats.

fitting algorithm

The mathematical process of finding a low-complexity function that best represents a set of measured values. Term used in particular for PTM and RTI processing algorithms. Typically the order of the function being fit (6 for PTMs), needs to be less than or equal to the number of measured values, or it is referred to as under-constrained and cannot be solved without introducing additional assumptions. The RTIBuilder includes the Polynomial Texture Map (PTM) algorithm (ptmfitter), and the Hemispherical Harmonics (HSH) algorithm (hshfitter). In that case, a PTM or HSH RTI included the results of one fitting algorithm. The SCML file format (can) include(s) the results of PTM, HSH RTI, RBF RTI fitting algorithms and other processing algorithms, such as those based on Photometric Stereo.

flash lighting

Flash lights illuminate the subject for a short amount of time with intense light energy. They permit short exposure times and a more narrow aperture (more depth of field), the imaging needs to be synchronized to the camera shutter. Flash lighting is opposed to Continuous Lighting. Flash lights, with the same settings, always give the same amount of energy, guaranteeing consistency throughout the capture sequence of a Source Image Set. When battery-operated it can be used in remote outdoor conditions with lesser risk of running out of light, when a power source is unreliable or absent. Often called Strobe Lighting in a studio context.

H

highlight map

A composite image produced by the RTIBuilder when using the Highlight Method. The image shows all of the highlights from every image in the Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) Source Image Set, composed onto a single sphere. This Blend Map allows you to see the spread of light achieved for one particular source image set.

highlight method

An image-capture technique for Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI), in which a Source Image Set is captured with at least two Reflective Spheres in each view. In this method, the light may be moved with less than perfect precision; the reflection of the light source on the spheres enables the processing software to calculate the lighting angle for that image and stores that information into a Light Position File. This information is used during the processing source image set and/or when Algorithmic Renderings are applied on the processed results. Sometimes also called Highlight-Reflectance Transformation Imaging (H-RTI). The highlight method can be the light position determining approach both for the Single Mobile Light and Dome methods. For an in-depth discussion, see the RTI Guide to Highlight Image Capture or Setups on this page. Histogram

A visual representation of the exposure values of a digital image, typically a graph that shows the image's shadows, midtones, and highlights as vertical peaks and valleys along a horizontal plane (Figures 4-6). Shadows are represented on the left side, highlights on the right side, and midtones in the central portion of the graph. The histogram allows quick checking of the quality of the exposure. If most of the tones are on the extreme left, the image is probably underexposed. On the contrary, if most of the tones are on the extreme right, the image is probably overexposed. Try to avoid under- and overexposure as much as possible.

hemispherical harmonics (HSH)

A model of distribution across a hemisphere of directions (as opposed to spherical harmonics, which model a distribution across an entire sphere of possible directions). This is a natural representation in the study of reflectance off an opaque surface, which only occurs in a hemisphere. This model is used for the processing Fitting Algorithm of HSH RTI.

I

image capture

The process of creating the initial set of digital images from which to create an Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) file by taking a sequence of images of the subject with a specific set of lighting angles. The Highlight Method or a Light Array can be used to produce the image sequence.

J

Java

The computer language in which the CHI RTIBuilder program is written.

JPEG format

A “lossy”' Compression format, capable of reducing digital image files to about 5% of their normal size. JPEG stands for Joint Photographic Experts Group. Decompression of JPEG files can cause visual artifacts ("blockiness," "jaggies," or “banding”) in digital images. The greater the compression levels, the more artifacts are visible. For RTI (the Source Image Set for the RTIBuilder only works with JPEGs) and for all other MLR Source Image Sets always use the least compression possible. Artifacts are most visible in smooth surfaces with little detail or in zones with high/sufficient contrast. The better the quality of the input images, the potential to obtain high-quality processed results.

L

light array

An apparatus (typically dome-shaped, but an arm or panel are equally possible) that has a set of lights at fixed positions. Control software flashes each light in a fixed sequence in order to capture the image sequence needed to process Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) files. Because each light is at a known position, the software can use a pre-calibrated Light Map and/or Lighting Position File to associate each image with the lighting angle used for it. See also Dome.

light direction extrapolation

An enhancement technique that allows lighting to be at a lower angle than is physically realizable. Once the reflectance functions are modeled based on the input images, lighting direction component values can be used outside the range of (-1 to +1) that are physically unrealizable, producing an extrapolation of the captured reflectance function. With the light direction extrapolation function additional virtual relighting angles can be achieved in the viewers which allow consultation of Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) processed results.

light map

A computer file that maps the angle of a light in a Light Array or Dome to the image taken with that light. Based on a light map a Lighting Position File can be computed.

lighting position file

Lighting positions are needed for all Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) computations to create relightable images. It can be created via the Highlight Method detection or if the lights are in fixed, predetermined positions (Light Array or Dome). A lighting position file (lp) can be generated mathematically and, when the lights are in fixed, predetermined positions, reused without needing to detect lighting angles for every Source Image set.

log file

Processing software often creates log files. For example, RTIBuilder to create an RTI File, the software records its processing steps and writes them to a log file named <project>.xml in the top-level project folder. Thus, that file contains information on the conducted processing, mostly only readable for the specific processing software which created it.

M

metadata

Information tags/fields that are attached to some form of digital data, which describes or gives information about that data. Metadata can be either technical, contextual, machine or human made. Common metadata formats for image data include EXIF (Exchangeable Image File), XMP (Extensible Metadata Platform), or IPTC (International Press and Telecommunications Council).

micro-Reflectance Transformation Imaging

Refers generally to RTI captured under magnification. See also Monkey Brain.

Multi-Light Reflectance (MLR)

Computational photographic imaging method based on the capturing of a Source Image Set on which for each image the subject is lit from multiple lighting positions. The detected reflectance on each of the captured images in such a set is used to estimate the overall Albedo and/or surface orientation for each pixel. A processed result enables the interactive re-lighting of the subject from any direction. The method is also named Single Camera Multi-Light and Reflectance Transformation Imaging.

monkey brain

The Monkey Brain is a dome-shaped light array for capturing Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) Source Image Sets images through the microscope. It was developed by Paul and Andrew Messier as part of the MoMA Thomas Walther collaboration; measuring drawings, user’s manual, and a parts list can be found here. See also Micro Reflectance Transformation Imaging.

N

normal

The mathematical term for the directional vector that is perpendicular to the surface of an object at a specific point. Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) processing software calculate the Surface Normal at each point of an object, using information derived from the lighting angles at each pixel in each of a series of images. Normal information, in the form of surface shape, is included along with color information for each pixel in the resulting Relightable Image. This enables viewer software to show the surface shape of the subject in great detail and more realistic.

P

Phong lighting model

One of the first lighting models used in computer graphics, builds up the brightness of any rendered object as a sum of weighted diffuse and specular reflectances coming from the surface being rendered. Principles of the Phong Lighting Model are used in a number of Visual Styles in the available Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) viewers.

photometric stereo

Photometric Stereo is an alternative technique to PTM/RTI which estimates Surface Normals and Albedo by observing the surface under different lighting conditions. Thus, some of the available MLR methods and processing algorithms compute a relightable image based on these specific principles. Recent work has combined photometric stereo and photogrammetry.

Pixel+ viewer

Open source web-based online viewer for various MLR file formats (i.e. PTM, HSH RTI, RBF RTI, CUN, ZUN and SCML). The viewer also allows comparisons between these various file formats within the same viewing environment. When the processed Relightable File includes multiple registered images or maps (e.g. based on several multi-spectral captures), these can be viewed and consulted superimposed and in interaction with each other (e.g. false colors). Similarly, when present in the relightable file, up to six processed recordings can be presented next to each other (e.g. six viewpoints of an object). The viewer can also handle relightable files computed according to the Deep Zoom Image Format. The pixel+ viewer has been developed by ESAT at Leuven University in Belgium and can be found here.

polynomial texture map (PTM)

The first implementation of Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) imaging was Polynomial Texture Mapping, invented by Tom Malzbender at HP Labs in 2001. A PTM Fitting Algorithm is available for producing a Relightable File.

portable light dome (PLD)

The Dome-shaped Image Capture devices with sets of lights at fixed positions developed at the University of Leuven by ESAT, a first in 2005. The PLD system also includes control software for flashing each light in a fixed sequence in order to obtain the source image set to process a MLR file. This integrated system has devices equipped with white light LEDs and with 5 different narrow band LEDs (infrared, red, green, blue & ultraviolet); another division is made between PLD domes with 260 LEDs (minidomes) and with 228 LEDs (microdomes).The PLD system makes use of the Photometric Stereo Fitting Algorithm. The system also includes an open source viewer interface for its MLR file formats, which offers besides the standard relighting option and various Visual Styles also BRDF functionalities.

R

raking illumination

The use of light position at a low angle in relation to the plane of an object, to create shadows that emphasize elevations or depressions which deviate from that same normal plane. This lighting technique helps to record the topography and texture of an object. Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) source images are captured with a range of degrees and positions of raking light.

RAW

A proprietary raw file format produced as an output option in digital cameras, also commonly called a Digital Negative. It contains all the pixel information captured by the camera's sensors without compression or processing applied. Raw files should be used to capture and keep most of the registered information by the light sensor in the camera, and to have total control over the future processing. Raw files are processed through specific software where you can change settings such as white balance, tonal curve, …., all post capture. There are both open source (RawTherapee, DarkTable, …) and commercial packages (Adobe Camera Raw, Capture One, …) available. However, because they are proprietary, raw files should not be used for true archiving. In that case the raw image data can be converted to an archival format such as Digital Negative (DNG) format or a Tag Image File Format (TIFF). When capturing a Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) Source Image Set, established best practice is to capture in a raw file format because it allows retention of the least processed information for archival purposes even if derivative images are created for further processing.

reflective spheres

Two or more shiny black spheres included in Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) source images to allow Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) software to detect the reflection highlights from the light source in each image, and use that data to calculate the exact angle of the light. At least two spheres are required for Algorithmic Rendering. This is the basic technology for the Highlight method of image capture that allows you to produce Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) images without a fixed Light Array. See also Spheres.

reflectance field

Equivalent to the BSSRDF (bidirectional scattering-surface reflectance distribution function), an 8-dimensional quantity that maps incoming lighting direction and position to reflected lighting direction and location, taking into account subsurface scattering.

reflectance function

The amount of light reflected from a given surface point, as a function of the two angles of lighting direction from a directional light source. It is the measurement represented in an RTI or PTM for each surface point. Viewpoint is assumed to be fixed for acquiring the reflectance function.

reflectance transformation imaging (RTI)

A computational photographic method that captures a subject’s surface shape and color by photographing an object from a fixed point of view with varying lighting angles. After processing these captures, it enables the interactive re-lighting of the subject from any direction in the viewer program. 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. The method is also labeled as (see) Single Camera Multi-Light or (see) Multi-Light Reflectance imaging.

relightable images

Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) Source Image Sets processed with particular Fitting Algorithms result in relightable images. Within the cultural heritage field, this term has been used in particular by the Italian research group at the ISTI-CNR throughout their “Relight” initiative; it has since become common terminology within Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI).  

rendering modes

Mathematical transformations (also called signal processing filters) that allows a viewer to show enhanced versions of an Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) that disclose and emphasize certain features, often difficult or impossible to see under direct empirical examination. Viewer Modes and Visual Styles are synonyms.

RGBN

An acronym for the representation of the three color quantities (Red, Green, Blue) along with the Surface Normal (typically a unit length vector representing the surface orientation) for each pixel.

RTIBuilder

An interface to a set of tools that process a source image set to produce an RTI file. Built by CHI, it is written in the Java programming language. The program incorporates a user-selected Fitting Algorithm to transform the Source Image Set into either PTM or HSH RTI format. This is only currently available for 32-bit Windows OS; see Technical Support for a longer discussion.

RTIViewer

A software interface that allows loading and examining processed Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) Source Images Sets (see also Fitting Algorithms & Relightable Images) created with RTIBuilder or similar applications. The original RTIviewer is free and can be downloaded via CHI and GitHub. RTIViewer offers interactive rendering of images, allowing the alteration of the apparent direction of lighting. In addition, it offers a number of enhancement modes (see also Rendering Modes, Viewer Modes, Visual Styles), which apply mathematical transformations to processed image data to enhance or emphasize particular features of the target object. Other open access viewers for RTI Files are WebRTIViewer (site & GitHub) and Pixel+ Viewer (site & GitHub).

RTI File

A computer File Format with the extension .rti or .ptm, produced using Fitting Algorithms of the RTI technology. An RTI file is a Relightable Image. RTI files can be created using the CHI RTIBuilder interface or similar applications, and view them using the/a RTIViewer interface.

S

SCML file format

Container File Format to combine several Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) processed Source Image Sets optimized for web-based consultation. The single SCML File stores the data uncompressed. The general architecture consists of one or more entries (i.e. files or directories) and a central directory placed at the end of the file. Each entry has a local header containing per entry information such as file name, (un)compressed size, a CRC Check, etc. At the end of the file, a central directory contains information about each of the entries and where to find them (relative byte offsets). For high-resolution results the images or maps can also be stored in the Deep Zoom Image format. Existing source image sets or processed Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) files can be converted into a SCML file.  

single camera multi-light (SCML)

Computational photographic imaging method based on the capturing from a single camera position of a Source Image Set on which for each image the subject is lit from multiple lighting positions. The method is also named Multi-Light Reflectance and Reflectance Transformation Imaging.

single mobile light method

Method to obtain a source image set for Reflectance Transformation Imaging, Multi-Light Reflectance or any Single Camera Multi-Light imaging technique. Only one light source is used which is manually repositioned for each image capture. To document the light positions this capture technique is combined with the (see) Highlight Method.

source image set

Set of images acquired during a Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) capture session, and to be used as a set for further processing (see Fitting Algorithms). On each image in this set, the subject is the same and the light settings differ (i.e. another illumination angle towards the center of the imaged subject).

specular enhancement

An image enhancement technique that yields improved perception of surface shape by photographically acquiring the reflectance functions of a surface, extracting per-pixel surface normals from these reflectance functions, and then rendering the resultant surface with added specular highlights computed from the surface normals using a Phong Lighting Model.

specular reflectance

The mirror-like reflection of light, shiny highlight, or spot reflection. The direction of reflectance is equivalent to the direction of incidence relative to the surface normal at the point of reflectance. See also Diffuse Reflectance.

spheres

Two or more shiny (black) spheres included in each capture of an Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) Source Image Set to allow Algorithmic Rendering software packages to detect the reflection highlights from the light source in each image, and use that data to calculate the exact angle of the light directed to the center of the images subject. Two spheres are required for correct Algorithmic Rendering. This is the basic technological approach for the Highlight method of image capture that allows the production of Relightable Images. Reflective spheres are also needed for some Dome and Light Array automated Image Capture devices.

string

When using the Single Mobile Light Method (see also Highlight Method) for Image Capture, the distance of the light source from the subject should remain nearly constant; this is best achieved by using a string, or an equivalent measuring device, of a specific length to place the light for each image in the Source Image set.

strobe lighting

See flash lighting.

surface normal

See normal.

sync cable

A cable that connects the light source to the camera so that the flash occurs when the shutter is open. The cable can connect the light directly to the camera, or can connect the light to a wireless trigger device; see Wireless Light Control.

T

tethered capture

Tethering–which connects the camera to a computer–allows remote control of camera settings and the ability to fire the camera from the computer. Images are transferred immediately to the computer and can be instantly viewed. Most camera vendors have their own tether software solution but commercial (e.g. Lightroom, Capture One) and open source tools (e.g. digiCamControl) are available.

Tagged Image File Format (TIFF)

A format (identified by the .tif file extension) for flexible bitmap image files. Supported by virtually all image editing and page-layout applications, and produced by virtually all sensor-based image capture devices. The format supports CMYK, RGB, LAB, grayscale files with alpha channels, and bitmap files without alpha channels. TIFF also supports LZW Compression, a lossless Compression format. A TIFF file is suitable for digital preservation.

Two-and-a-half-dimensional Image

Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) and some of the other related computational imaging techniques can be referred to as 2 ½ dimensional, because they create pseudo-3D information extracted from 2D image sets. While the digital images used for Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) builds do not contain information about depth, the coordination of data from the image set demonstrating the effect of known or implied light angles allows for the calculation of surface normals, which define the 3-dimensional surface being examined. Thus Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI), while not a true 3-dimensional imaging technique, is more descriptive of surface topology than 2D images alone. Interactive viewing in the RTIViewer or web viewers also enables this richer understanding of the surfaces.

V

viewer modes

Mathematical transformations (i.e. signal processing filters) that allow Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) viewers to show enhanced versions of a Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) that disclose and emphasize certain features, often difficult or impossible to see under direct empirical examination. Rendering Modes and Visual Styles are synonyms.  

visual styles

A synonym for Viewer Modes or Rendering Modes.

W

wireless light control

A wireless transmitter/receiver can be used to receive signals from the camera so that the flash is triggered when the shutter opens. This minimizes the number of cables in use during a Multi-Light Reflectance (MLR) / Reflectance Transformation Imaging (RTI) Image capture session. Examples are the Speedlite Transmitter ST-E2, a camera-mounted infrared controller for Canon's flashes, and the PocketWizard, which is attached to a light by a Sync Cable.

X

Extensible Markup Language (XML)

A text-based markup language that can be used for tagging information, making it easier for a computer to scan and to automate various processes. In the RTI workflow, CHI RTIBuilder uses XML format for the log file it produces.

Extensible Metadata Platform (XMP)

A specific XML (Extensible Markup Language) schema used to store Metadata in image files. A unique advantage of XMP is that it allows creation of custom metadata, as well as supporting certain standards such as IPTC and EXIF (Exchangeable Image File). See XML (Extensible Markup Language).

Z

zeroed out settings

A recommended custom preset for Adobe Camera Raw, which allows the user to make their own adjustments to white balance and exposure (if needed) in captured source images, before using them to create an RTI file. A zeroed-out preset ensures that data is not being processed, interpreted, or stylized to fit consumer tastes. To create it, edit any image in Adobe Camera Raw, set all possible options to 0, save the current settings as a named preset, then make that preset the default.