Global illumination
Encyclopedia
Global illumination is a general name for a group of algorithm
s used in 3D computer graphics
that are meant to add more realistic lighting to 3D scenes. Such algorithms take into account not only the light which comes directly from a light source (direct illumination), but also subsequent cases in which light rays from the same source are reflected by other surfaces in the scene, whether reflective or not (indirect illumination).
Theoretically reflections, refractions, and shadows are all examples of global illumination, because when simulating them, one object affects the rendering of another object (as opposed to an object being affected only by a direct light). In practice, however, only the simulation of diffuse inter-reflection or caustics
is called global illumination.
Images rendered using global illumination algorithms often appear more photorealistic than images rendered using only direct illumination algorithms. However, such images are computationally more expensive and consequently much slower to generate. One common approach is to compute the global illumination of a scene and store that information with the geometry, i.e., radiosity. That stored data can then be used to generate images from different viewpoints for generating walkthroughs of a scene without having to go through expensive lighting calculations repeatedly.
Radiosity, ray tracing, beam tracing
, cone tracing
, path tracing
, Metropolis light transport
, ambient occlusion
, photon mapping
, and image based lighting
are examples of algorithms used in global illumination, some of which may be used together to yield results that are not fast, but accurate.
These algorithms model diffuse inter-reflection which is a very important part of global illumination; however most of these (excluding radiosity) also model specular reflection
, which makes them more accurate algorithms to solve the lighting equation and provide a more realistically illuminated scene.
The algorithms used to calculate the distribution of light energy between surfaces of a scene are closely related to heat transfer
simulations performed using finite-element methods in engineering design.
In real-time 3D graphics, the diffuse inter-reflection component of global illumination is sometimes approximated by an "ambient" term in the lighting equation, which is also called "ambient lighting" or "ambient color" in 3D software packages. Though this method of approximation (also known as a "cheat" because it's not really a global illumination method) is easy to perform computationally, when used alone it does not provide an adequately realistic effect. Ambient lighting is known to "flatten" shadows in 3D scenes, making the overall visual effect more bland. However, used properly, ambient lighting can be an efficient way to make up for a lack of processing power.
or photon mapping
, under certain conditions, including radiosity
. These are always methods to try to solve the rendering equation
.
The following approaches can be distinguished here:
In Light path notation global lighting the paths of the type L (D | S) corresponds * E.
(HDRIs), also known as environment maps, which encircle the scene, and they illuminate. This process is known as image-based lighting.
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
s used in 3D computer graphics
3D computer graphics
3D computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering 2D images...
that are meant to add more realistic lighting to 3D scenes. Such algorithms take into account not only the light which comes directly from a light source (direct illumination), but also subsequent cases in which light rays from the same source are reflected by other surfaces in the scene, whether reflective or not (indirect illumination).
Theoretically reflections, refractions, and shadows are all examples of global illumination, because when simulating them, one object affects the rendering of another object (as opposed to an object being affected only by a direct light). In practice, however, only the simulation of diffuse inter-reflection or caustics
Caustic (optics)
In optics, a caustic or caustic network is the envelope of light rays reflected or refracted by a curved surface or object, or the projection of that envelope of rays on another surface. The caustic is a curve or surface to which each of the light rays is tangent, defining a boundary of an...
is called global illumination.
Images rendered using global illumination algorithms often appear more photorealistic than images rendered using only direct illumination algorithms. However, such images are computationally more expensive and consequently much slower to generate. One common approach is to compute the global illumination of a scene and store that information with the geometry, i.e., radiosity. That stored data can then be used to generate images from different viewpoints for generating walkthroughs of a scene without having to go through expensive lighting calculations repeatedly.
Radiosity, ray tracing, beam tracing
Beam tracing
Beam tracing is an algorithm to simulate wave propagation.It was developed in the context of computer graphics to render 3D scenes,but it has been also used in other similar areas such as acoustics andelectromagnetism simulations....
, cone tracing
Cone tracing
Cone tracing is a derivative of the ray tracing algorithm that replaces rays, which have no thickness, with cones. Cone tracing is related to beam tracing, but uses circular rather than polygonal cross sections....
, path tracing
Path Tracing
Path tracing is a computer graphics rendering technique that attempts to simulate the physical behaviour of light as closely as possible. It is a generalisation of conventional ray tracing, tracing rays from the virtual camera through several bounces on or through objects...
, Metropolis light transport
Metropolis light transport
The Metropolis light transport is a SIGGRAPH 1997 paper by Eric Veach and Leonidas J. Guibas, describing an application of a variant of the Monte Carlo method called the Metropolis-Hastings algorithm to the rendering equation for generating images from detailed physical descriptions of three...
, ambient occlusion
Ambient occlusion
Ambient occlusion is a shading method used in 3D computer graphics which helps add realism to local reflection models by taking into account attenuation of light due to occlusion...
, photon mapping
Photon mapping
In computer graphics, photon mapping is a two-pass global illumination algorithm developed by Henrik Wann Jensen that solves the rendering equation. Rays from the light source and rays from the camera are traced independently until some termination criterion is met, then they are connected in a...
, and image based lighting
Image based lighting
Image-based lighting is a 3D rendering technique which involves plotting an image onto a dome or sphere that contains the primary subject. The lighting characteristics of the surrounding surface are then taken into account when rendering the scene, using the modeling techniques of global...
are examples of algorithms used in global illumination, some of which may be used together to yield results that are not fast, but accurate.
These algorithms model diffuse inter-reflection which is a very important part of global illumination; however most of these (excluding radiosity) also model specular reflection
Specular reflection
Specular reflection is the mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction...
, which makes them more accurate algorithms to solve the lighting equation and provide a more realistically illuminated scene.
The algorithms used to calculate the distribution of light energy between surfaces of a scene are closely related to heat transfer
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
simulations performed using finite-element methods in engineering design.
In real-time 3D graphics, the diffuse inter-reflection component of global illumination is sometimes approximated by an "ambient" term in the lighting equation, which is also called "ambient lighting" or "ambient color" in 3D software packages. Though this method of approximation (also known as a "cheat" because it's not really a global illumination method) is easy to perform computationally, when used alone it does not provide an adequately realistic effect. Ambient lighting is known to "flatten" shadows in 3D scenes, making the overall visual effect more bland. However, used properly, ambient lighting can be an efficient way to make up for a lack of processing power.
Procedure
For the simulation of global illumination are used in 3D programs, more and more specialized algorithms that can effectively simulate the global illumination. These are, for example, path tracingPath Tracing
Path tracing is a computer graphics rendering technique that attempts to simulate the physical behaviour of light as closely as possible. It is a generalisation of conventional ray tracing, tracing rays from the virtual camera through several bounces on or through objects...
or photon mapping
Photon mapping
In computer graphics, photon mapping is a two-pass global illumination algorithm developed by Henrik Wann Jensen that solves the rendering equation. Rays from the light source and rays from the camera are traced independently until some termination criterion is met, then they are connected in a...
, under certain conditions, including radiosity
Radiosity
Radiosity is a global illumination algorithm used in 3D computer graphics rendering. Radiosity is an application of the finite element method to solving the rendering equation for scenes with purely diffuse surfaces...
. These are always methods to try to solve the rendering equation
Rendering equation
In computer graphics, the rendering equation is an integral equation in which the equilibrium radiance leaving a point is given as the sum of emitted plus reflected radiance under a geometric optics approximation. It was simultaneously introduced into computer graphics by David Immel et al. and...
.
The following approaches can be distinguished here:
- Inversion:
- is not applied in practice
- Expansion:
- bi-directional approach: Photon mappingPhoton mappingIn computer graphics, photon mapping is a two-pass global illumination algorithm developed by Henrik Wann Jensen that solves the rendering equation. Rays from the light source and rays from the camera are traced independently until some termination criterion is met, then they are connected in a...
+ Distributed ray tracing, Bi-directional path tracing, Metropolis light transportMetropolis light transportThe Metropolis light transport is a SIGGRAPH 1997 paper by Eric Veach and Leonidas J. Guibas, describing an application of a variant of the Monte Carlo method called the Metropolis-Hastings algorithm to the rendering equation for generating images from detailed physical descriptions of three...
- bi-directional approach: Photon mapping
- Iteration:
- Radiosity
In Light path notation global lighting the paths of the type L (D | S) corresponds * E.
Image-based lighting
Another way to simulate real global illumination, is the use of High dynamic range imagesHigh 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...
(HDRIs), also known as environment maps, which encircle the scene, and they illuminate. This process is known as image-based lighting.
External links
- SSRT – C++ source code for a Monte-carlo pathtracer (supporting GI) - written with ease of understanding in mind.
- Video demonstrating global illumination and the ambient color effect
- Real-time GI demos – survey of practical real-time GI techniques as a list of executable demos
- kuleuven - This page contains the Global Illumination Compendium, an effort to bring together most of the useful formulas and equations for global illumination algorithms in computer graphics.
- GI Tutorial - Video tutorial on faking global illumination within 3D Studio Max3D Studio MaxAutodesk 3ds Max, formerly 3D Studio MAX, is for making 3D animations. It was developed and produced by Autodesk Media and Entertainment. It has modeling capabilities, a flexible plugin architecture and can be used on the Microsoft Windows platform. It's frequently used by video game developers, TV...
by Jason DonatiJason DonatiJason Donati is an animator, educator, and author noted for his animated independent film work, professional career as a 3D visualization artist, and academic authorship including the 2007 book “Exploring Digital cinematography” published by Cengage Learning....