Distributed ray tracing
Encyclopedia
Distributed ray tracing, also called distribution ray tracing and stochastic ray tracing, is a refinement of ray tracing that allows for the rendering
of "soft" phenomena.
Conventional ray tracing uses single rays to sample many different domains. For example, when the color
of an object is calculated, ray tracing might send a single ray to each light source in the scene. This leads to sharp shadows, since there is no way for a light source to be partially occluded (another way of saying this is that all lights are point sources and have zero area). Conventional ray tracing also typically spawns one reflection ray and one transmission ray per intersection. As a result, reflected and transmitted images are perfectly (and unrealistically) sharp.
Distributed ray tracing removes these restrictions by averaging multiple rays distributed over an interval. For example, soft shadows can be rendered by distributing shadow rays over the light source area. Blurry reflections and transmissions can be rendered by distributing reflection and transmission rays over a solid angle
about the "true" reflection or transmission direction. Adding "soft" phenomena to ray-traced images in this way can improve realism immensely, since the sharp phenomena rendered by conventional ray tracing are almost never seen in reality.
More advanced effects are also possible using the same framework. For instance, depth of field
can be achieved by distributing ray origins over the lens area. In an animated scene, motion blur
can be simulated by distributing rays in time. Distributing rays in the spectrum allows for the rendering of dispersion
effects, such as rainbow
s and prisms
.
Mathematically, in order to evaluate the rendering equation
, one must evaluate several integral
s. Conventional ray tracing estimates these integrals by sampling the value of the integrand at a single point in the domain, which is clearly a very bad approximation. Distributed ray tracing samples the integrand at many randomly chosen points and averages the results to obtain a better approximation. It is essentially an application of the Monte Carlo method
to 3D computer graphics
, and for this reason is also called stochastic
ray tracing.
The term distributed ray tracing also sometimes refers to the application of distributed computing
techniques to ray tracing, but because of ambiguity this is more properly called parallel ray tracing (in reference to parallel computing
).
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...
of "soft" phenomena.
Conventional ray tracing uses single rays to sample many different domains. For example, when the color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
of an object is calculated, ray tracing might send a single ray to each light source in the scene. This leads to sharp shadows, since there is no way for a light source to be partially occluded (another way of saying this is that all lights are point sources and have zero area). Conventional ray tracing also typically spawns one reflection ray and one transmission ray per intersection. As a result, reflected and transmitted images are perfectly (and unrealistically) sharp.
Distributed ray tracing removes these restrictions by averaging multiple rays distributed over an interval. For example, soft shadows can be rendered by distributing shadow rays over the light source area. Blurry reflections and transmissions can be rendered by distributing reflection and transmission rays over a solid angle
Solid angle
The solid angle, Ω, is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large that object appears to an observer looking from that point...
about the "true" reflection or transmission direction. Adding "soft" phenomena to ray-traced images in this way can improve realism immensely, since the sharp phenomena rendered by conventional ray tracing are almost never seen in reality.
More advanced effects are also possible using the same framework. For instance, depth of field
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...
can be achieved by distributing ray origins over the lens area. In an animated scene, motion blur
Motion blur
Motion blur is the apparent streaking of rapidly moving objects in a still image or a sequence of images such as a movie or animation. It results when the image being recorded changes during the recording of a single frame, either due to rapid movement or long exposure.- Photography :When a camera...
can be simulated by distributing rays in time. Distributing rays in the spectrum allows for the rendering of dispersion
Dispersion (optics)
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
effects, such as rainbow
Rainbow
A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...
s and prisms
Prism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
.
Mathematically, in order to evaluate 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...
, one must evaluate several integral
Integral
Integration is an important concept in mathematics and, together with its inverse, differentiation, is one of the two main operations in calculus...
s. Conventional ray tracing estimates these integrals by sampling the value of the integrand at a single point in the domain, which is clearly a very bad approximation. Distributed ray tracing samples the integrand at many randomly chosen points and averages the results to obtain a better approximation. It is essentially an application of 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 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...
, and for this reason is also called stochastic
Stochastic
Stochastic refers to systems whose behaviour is intrinsically non-deterministic. A stochastic process is one whose behavior is non-deterministic, in that a system's subsequent state is determined both by the process's predictable actions and by a random element. However, according to M. Kac and E...
ray tracing.
The term distributed ray tracing also sometimes refers to the application of distributed computing
Distributed computing
Distributed computing is a field of computer science that studies distributed systems. A distributed system consists of multiple autonomous computers that communicate through a computer network. The computers interact with each other in order to achieve a common goal...
techniques to ray tracing, but because of ambiguity this is more properly called parallel ray tracing (in reference to parallel computing
Parallel computing
Parallel computing is a form of computation in which many calculations are carried out simultaneously, operating on the principle that large problems can often be divided into smaller ones, which are then solved concurrently . There are several different forms of parallel computing: bit-level,...
).
See also
- Global illuminationGlobal illuminationGlobal 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...
- Monte Carlo methodMonte Carlo methodMonte 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...
- Ray tracing