Radiance (software)
Encyclopedia
Radiance is a suite of tools for performing lighting simulation originally written by Greg Ward. It includes a renderer
as well as many other tools for measuring the simulated light levels. It uses ray tracing to perform all lighting calculations, accelerated by the use of an octree
data structure. It pioneered the concept of high dynamic range imaging
, where light levels are (theoretically) open-ended values instead of a decimal proportion of a maximum (e.g. 0.0 to 1.0) or integer fraction of a maximum (0 to 255 / 255). It also implements global illumination
using the Monte Carlo method
to sample light falling on a point.
Greg Ward started developing Radiance in 1985 while at Lawrence Berkeley National Laboratory
. The source code was distributed under a license forbidding further redistribution. In January 2002 Radiance 3.4 was relicensed under a less restrictive license.
One study found Radiance to be the most generally useful software package for architectural lighting simulation. The study also noted that Radiance often serves as the underlying simulation engine for many other packages.
. Since it was the first (and for a long time the only) HDR image format, this format is supported by many other software packages.
The file starts with the signature '#?RADIANCE' and then several lines listing the commands used to generate the image. This information allows the renderer rpict to continue a partially completed render (either manually, or using the rad front-end). There are also key=value declarations, including the line 'FORMAT=32-bit_rle_rgbe'.
After this is a blank line signifying the end of the header. A single line describes the resolution and pixel order. As produced by the Radiance tools this always takes the form of '-Y height +X width. After this line follows the binary pixel data.
Radiance calculates light values as floating point
triplets, one each for red, green and blue. But storing a full double precision float for each channel (8 bytes × 3 = 24 bytes) is a burden even for modern systems. Two stages are used to compress the image data. The first scales the three floating point values to share a common 8-bit exponent, taken from the brightest of the three. Each value is then truncated to an 8-bit mantissa (fractional part). The result is four bytes, 32 bits, for each pixel. This results in a 6:1 compression, at the expense of reduced colour fidelity.
The second stage performs run length encoding on the 32-bit pixel values. This has a limited impact on the size of most rendered images, but it is fast and simple.
When specifying geometry the first line is
modifier type name
The following three lines contain parameters starting with an integer specifying the number of parameters. The parameters need not be on the same line, they can be continued on multiple lines to aid in readability.
Modifiers create materials and can be chained together, one modifying the next.
For example:
myball.rad
chrome sphere ball
0
0
4 0 0 10
10
This can then be arrayed in another file using the xform program (described later):
scene.rad
void metal chrome
0
0
5 0.8 0.8 0.8
0.9 0.0
!xform -a 5 -t 20 0 0 myball.rad
This creates a chrome
material and five chrome spheres spaced 20 units apart along the X-axis.
Before a scene can be used, it must be compiled into an octree file ('.oct') using the oconv tool. Most of the rendering tools (see below) use an octree file as input.
and Unix-like systems. Many of the tools act as filters, taking input on standard input and sending the processed result to standard output. These can be used on the Unix command line and piped to a new file, or included in Radiance scene files ('.rad') themselves, as shown above.
Rendering (computer graphics)
Rendering is the process of generating an image from a model , by means of computer programs. A scene file contains objects in a strictly defined language or data structure; it would contain geometry, viewpoint, texture, lighting, and shading information as a description of the virtual scene...
as well as many other tools for measuring the simulated light levels. It uses ray tracing to perform all lighting calculations, accelerated by the use of an octree
Octree
An octree is a tree data structure in which each internal node has exactly eight children. Octrees are most often used to partition a three dimensional space by recursively subdividing it into eight octants. Octrees are the three-dimensional analog of quadtrees. The name is formed from oct + tree,...
data structure. It pioneered the concept of high dynamic range imaging
High dynamic range imaging
In image processing, computer graphics, and photography, high dynamic range imaging is a set of techniques that allows a greater dynamic range between the lightest and darkest areas of an image than current standard digital imaging techniques or photographic methods...
, where light levels are (theoretically) open-ended values instead of a decimal proportion of a maximum (e.g. 0.0 to 1.0) or integer fraction of a maximum (0 to 255 / 255). It also implements global illumination
Global illumination
Global illumination is a general name for a group of algorithms used in 3D computer graphics that are meant to add more realistic lighting to 3D scenes...
using the Monte Carlo method
Monte Carlo method
Monte Carlo methods are a class of computational algorithms that rely on repeated random sampling to compute their results. Monte Carlo methods are often used in computer simulations of physical and mathematical systems...
to sample light falling on a point.
Greg Ward started developing Radiance in 1985 while at Lawrence Berkeley National Laboratory
Lawrence Berkeley National Laboratory
The Lawrence Berkeley National Laboratory , is a U.S. Department of Energy national laboratory conducting unclassified scientific research. It is located on the grounds of the University of California, Berkeley, in the Berkeley Hills above the central campus...
. The source code was distributed under a license forbidding further redistribution. In January 2002 Radiance 3.4 was relicensed under a less restrictive license.
One study found Radiance to be the most generally useful software package for architectural lighting simulation. The study also noted that Radiance often serves as the underlying simulation engine for many other packages.
HDR image format
Radiance defined an image format for storing HDR images, now described as RGBE image formatRGBE image format
RGBE is an image format invented by Gregory Ward Larson. It stores pixels as one byte each for RGB values with a one byte shared exponent. Thus it stores four bytes per pixel....
. Since it was the first (and for a long time the only) HDR image format, this format is supported by many other software packages.
The file starts with the signature '#?RADIANCE' and then several lines listing the commands used to generate the image. This information allows the renderer rpict to continue a partially completed render (either manually, or using the rad front-end). There are also key=value declarations, including the line 'FORMAT=32-bit_rle_rgbe'.
After this is a blank line signifying the end of the header. A single line describes the resolution and pixel order. As produced by the Radiance tools this always takes the form of '-Y height +X width. After this line follows the binary pixel data.
Radiance calculates light values as floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
triplets, one each for red, green and blue. But storing a full double precision float for each channel (8 bytes × 3 = 24 bytes) is a burden even for modern systems. Two stages are used to compress the image data. The first scales the three floating point values to share a common 8-bit exponent, taken from the brightest of the three. Each value is then truncated to an 8-bit mantissa (fractional part). The result is four bytes, 32 bits, for each pixel. This results in a 6:1 compression, at the expense of reduced colour fidelity.
The second stage performs run length encoding on the 32-bit pixel values. This has a limited impact on the size of most rendered images, but it is fast and simple.
Scene description format
A radiance scene is made from one or more object files. The .rad format is a simple text file. It can specify individual geometric objects, as well as call programs by starting a line with an exclamation point '!'.When specifying geometry the first line is
modifier type name
The following three lines contain parameters starting with an integer specifying the number of parameters. The parameters need not be on the same line, they can be continued on multiple lines to aid in readability.
Modifiers create materials and can be chained together, one modifying the next.
For example:
myball.rad
chrome sphere ball
0
0
4 0 0 10
10
This can then be arrayed in another file using the xform program (described later):
scene.rad
void metal chrome
0
0
5 0.8 0.8 0.8
0.9 0.0
!xform -a 5 -t 20 0 0 myball.rad
This creates a chrome
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
material and five chrome spheres spaced 20 units apart along the X-axis.
Before a scene can be used, it must be compiled into an octree file ('.oct') using the oconv tool. Most of the rendering tools (see below) use an octree file as input.
Tools
The Radiance suite includes over 50 tools. They were designed for use on UnixUnix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
and Unix-like systems. Many of the tools act as filters, taking input on standard input and sending the processed result to standard output. These can be used on the Unix command line and piped to a new file, or included in Radiance scene files ('.rad') themselves, as shown above.
Geometry manipulation
Several radiance programs manipulate Radiance scene data by reading from either a specified file or their standard input, and writing to standard output.- xform allows an arbitrary number of transformations to be performed on a '.rad' file. The transformations include translation, rotation (around any of the three axes), and scaling. It also can perform multi-dimensional arraying.
- replmarks replaces certain triangles in a scene with objects from another file. Used for simplifying a scene when modelling in a 3D modeller.
Generators
Generators simplify the task of modelling a scene, they create certain types of geometry from supplied parameters.- genbox creates a box.
- genrprism extrudes a given 2D polygon along the Z-axis.
- genrev creates a surface of revolutionSurface of revolutionA surface of revolution is a surface in Euclidean space created by rotating a curve around a straight line in its plane ....
from a given function. - genworm creates a worm given four functions - the (x, y, z) coordinates of the path, and the radius of the worm.
- gensurf creates a tesselated surface from a given function.
- gensky creates a description for a CIEInternational Commission on IlluminationThe International Commission on Illumination is the international authority on light, illumination, color, and color spaces...
standard sky distribution.
Geometry converters
Radiance includes a number of programs for converting scene geometry from other formats. These include:- nff2rad converts NFF objects to Radiance geometry.
- obj2rad convert Wavefront .obj files to Radiance geometry.
- obj2mesh convert Wavefront .obj files to a Radiance compiled mesh. This can then be included in a scene using the recently added mesh primitive. More efficient than using obj2rad and includes texture coordinates.
Rendering
- rpict is the renderer, producing a Radiance image on its standard output.
- rvu is an interactive renderer, opening an X11X Window SystemThe X window system is a computer software system and network protocol that provides a basis for graphical user interfaces and rich input device capability for networked computers...
window to show the render in progress, and allowing the view to be altered. - rtrace is a tool for tracing specific rays into a scene. It reads the parameters for these rays on its standard input and returns the light value on standard output. rtrace is used by other tools, and can even be used to render images on its own by using the vwray program to generate view rays to be piped to it.
- dayfact is an interactive script to compute luminance values and daylight factorDaylight factorA daylight factor is the ratio of internal light level to external light level and is defined as follows:where,Ei = illumiance due to daylight at a point on the indoors working plane,...
s on a grid. - findglare takes an image or scene and finds bright sources that would cause discomforting glareGlare (vision)Glare is difficulty seeing in the presence of bright light such as direct or reflected sunlight or artificial light such as car headlamps at night. Because of this, some cars include mirrors with automatic anti-glare functions....
in human eyes. - mkillum takes a surface (e.g. a window or lamp shade) and computes the lighting contribution going through it. This data is then used by the illum material modifier to make lighting from these secondary sources more accurate and efficient to compute.
Image manipulation and analysis
- pfilt filters an image. The common technique to achieve anti-aliasedAnti-aliasingIn digital signal processing, spatial anti-aliasing is the technique of minimizing the distortion artifacts known as aliasing when representing a high-resolution image at a lower resolution...
images is to render several times larger than the desired size, and then filter the image down using pfilt. - pcompos composites images, either with anchor coordinates or by adding several images on top of another.
- pcond conditions images. Can simulate a number of effects of the human visual response e.g. defocusing dark areas, veiling due to glare, and colour loss due to mesopicMesopic visionMesopic vision is a combination of photopic vision and scotopic vision in low but not quite dark lighting situations. Mesopic light levels range from luminances of approximately 0.001 to 3 cd m-2. Most night-time outdoor and traffic lighting scenarios are in the mesopic range.Humans see...
or scotopic visionScotopic visionScotopic vision is the vision of the eye under low light conditions. The term comes from Greek skotos meaning darkness and -opia meaning a condition of sight...
in low light. - pinterp interpolates between two images provided they both have z buffers. Uses rtrace to fill in gaps. Is used to speed up the rendering speed of simple animations.
- ximage is an image viewer for viewing HDR Radiance images. It can adjust the simulated exposure and apply some of the human visual effects of pcond.
Integration
- rad is a front-end which reads a '.rif' file describing a scene and multiple camera views. Previously, make and a makefile were used in a similar role. rad coordinates oconv, mkillum, rpict/rview and other programs to render an image (or preview) from the source scene file(s).
- trad is a GUI front-end to rad using Tcl/Tk.
- ranimate is a front-end which coordinates many programs to generate virtual walk-through animations i.e. the camera moves but the scene is static.
Sources
- Greg Ward Larson and Rob Shakespeare, Rendering with Radiance, Morgan Kaufmann, 1998. ISBN 1-55860-499-5