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Light transport theory deals with the mathematics behind calculating the energy transfers between media that affect visibility. This article is currently specific to light transport in rendering processes such as global illumination and high dynamic range imaging (HDRI).
The amount of light transported is measured by flux density, or luminous flux per unit area on the point of the surface at which it is measured.
Radiometry is the science of measuring electromagnetic radiation, including visible light. It forms the foundation of light transport theory, which models how light interacts with surfaces, volumes, and media.
Given a surface S, a hemisphere H can be projected onto S to calculate the amount of incoming and outgoing light. If a point P is selected at random on the surface S, the amount of incoming and outgoing light can be calculated by its projection onto the hemisphere.
The hemicube model works similarly to the hemisphere model, except that a hemicube is projected instead of a hemisphere. The similarity is only conceptual; the actual calculation, done through numerical integration, has a different form factor.
Light transport theory incorporates both wave-based and particle-based descriptions of light. While wave-based models rely on the principles of Maxwell's equations, particle models use ray optics and Monte Carlo methods to simulate light paths.
Bidirectional Reflectance Distribution Function (BRDF)
The BRDF models how light is reflected on an opaque surface. It is defined as the ratio of reflected radiance in a given direction to the incident irradiance. BRDFs are crucial in light transport theory for simulating realistic material behavior. [3]
Participating Media
Light transport within volumes (e.g., fog, smoke, or translucent objects) is modeled using the radiative transfer equation (RTE). Participating media are integral to achieving photorealism in scenes involving volumetric light effects. [4]
Rendering converts a model into an image either by simulating a method, such as light transport, to get physically accurate photorealistic images, or by applying some kind of style as non-photorealistic rendering (NPR). The two basic operations in light transport are transport (how much light gets from one place to another) and scattering (how surfaces interact with light).
Global illumination simulates all light interactions in a scene, including indirect lighting. It employs light transport theory to compute effects such as color bleeding and soft shadows.
Unlike photorealistic rendering, non-photorealistic rendering (NPR) uses light transport models to stylize images, often prioritizing artistic intent over physical accuracy. [5]
Rendering is the process of generating a photorealistic or non-photorealistic image from input data such as 3D models. The word "rendering" originally meant the task performed by an artist when depicting a real or imaginary thing. Today, to "render" commonly means to generate an image or video from a precise description using a computer program.
Global illumination (GI), or indirect illumination, is a group of algorithms 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 that comes directly from a light source, but also subsequent cases in which light rays from the same source are reflected by other surfaces in the scene, whether reflective or not.
In 3D computer graphics, radiosity is an application of the finite element method to solving the rendering equation for scenes with surfaces that reflect light diffusely. Unlike rendering methods that use Monte Carlo algorithms, which handle all types of light paths, typical radiosity only account for paths which leave a light source and are reflected diffusely some number of times before hitting the eye. Radiosity is a global illumination algorithm in the sense that the illumination arriving on a surface comes not just directly from the light sources, but also from other surfaces reflecting light. Radiosity is viewpoint independent, which increases the calculations involved, but makes them useful for all viewpoints.
In 3D computer graphics, ray tracing is a technique for modeling light transport for use in a wide variety of rendering algorithms for generating digital images.
Metropolis light transport (MLT) is a global illumination application of a Monte Carlo method called the Metropolis–Hastings algorithm to the rendering equation for generating images from detailed physical descriptions of three-dimensional scenes.
In computer graphics, photon mapping is a two-pass global illumination rendering algorithm developed by Henrik Wann Jensen between 1995 and 2001 that approximately solves the rendering equation for integrating light radiance at a given point in space. 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 second step to produce a radiance value. The algorithm is used to realistically simulate the interaction of light with different types of objects. Specifically, it is capable of simulating the refraction of light through a transparent substance such as glass or water, diffuse interreflection between illuminated objects, the subsurface scattering of light in translucent materials, and some of the effects caused by particulate matter such as smoke or water vapor. Photon mapping can also be extended to more accurate simulations of light, such as spectral rendering. Progressive photon mapping (PPM) starts with ray tracing and then adds more and more photon mapping passes to provide a progressively more accurate render.
Bidirectional texture function (BTF) is a 6-dimensional function depending on planar texture coordinates (x,y) as well as on view and illumination spherical angles. In practice this function is obtained as a set of several thousand color images of material sample taken during different camera and light positions.
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 geometrical optics approximation. It was simultaneously introduced into computer graphics by David Immel et al. and James Kajiya in 1986. The various realistic rendering techniques in computer graphics attempt to solve this equation.
In 3D computer graphics, modeling, and animation, ambient occlusion is a shading and rendering technique used to calculate how exposed each point in a scene is to ambient lighting. For example, the interior of a tube is typically more occluded than the exposed outer surfaces, and becomes darker the deeper inside the tube one goes.
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.
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 and electromagnetism simulations.
The bidirectional reflectance distribution function (BRDF), symbol , is a function of four real variables that defines how light from a source is reflected off an opaque surface. It is employed in the optics of real-world light, in computer graphics algorithms, and in computer vision algorithms. The function takes an incoming light direction, , and outgoing direction, , and returns the ratio of reflected radiance exiting along to the irradiance incident on the surface from direction . Each direction is itself parameterized by azimuth angle and zenith angle , therefore the BRDF as a whole is a function of 4 variables. The BRDF has units sr−1, with steradians (sr) being a unit of solid angle.
Path tracing is a computer graphics Monte Carlo method of rendering images of three-dimensional scenes such that the global illumination is faithful to reality. Fundamentally, the algorithm is integrating over all the illuminance arriving to a single point on the surface of an object. This illuminance is then reduced by a surface reflectance function (BRDF) to determine how much of it will go towards the viewpoint camera. This integration procedure is repeated for every pixel in the output image. When combined with physically accurate models of surfaces, accurate models of real light sources, and optically correct cameras, path tracing can produce still images that are indistinguishable from photographs.
In computer graphics, per-pixel lighting refers to any technique for lighting an image or scene that calculates illumination for each pixel on a rendered image. This is in contrast to other popular methods of lighting such as vertex lighting, which calculates illumination at each vertex of a 3D model and then interpolates the resulting values over the model's faces to calculate the final per-pixel color values.
3D rendering is the 3D computer graphics process of converting 3D models into 2D images on a computer. 3D renders may include photorealistic effects or non-photorealistic styles.
Computer graphics lighting is the collection of techniques used to simulate light in computer graphics scenes. While lighting techniques offer flexibility in the level of detail and functionality available, they also operate at different levels of computational demand and complexity. Graphics artists can choose from a variety of light sources, models, shading techniques, and effects to suit the needs of each application.
In 3D computer graphics rendering, a hemicube is one way to represent a 180° view from a surface or point in space.
Unbiased rendering in computer graphics refers to techniques that avoid systematic errors, or biases, in the radiance approximation of an image. This term specifically relates to statistical bias, not subjective bias. Unbiased rendering aims to replicate real-world lighting and shading as accurately as possible without shortcuts. Path tracing and its derivatives are examples of unbiased techniques, whereas traditional ray tracing methods are typically biased.
Volumetric path tracing is a method for rendering images in computer graphics which was first introduced by Lafortune and Willems. This method enhances the rendering of the lighting in a scene by extending the path tracing method with the effect of light scattering. It is used for photorealistic effects of participating media like fire, explosions, smoke, clouds, fog or soft shadows.
This is a glossary of terms relating to computer graphics.