Rendering equation

Last updated
The rendering equation describes the amount of light leaving a point x along a particular viewing direction, given functions for incoming light and emitted light, and a BRDF. Rendering eq.png
The rendering equation describes the amount of light leaving a point x along a particular viewing direction, given functions for incoming light and emitted light, and a BRDF.

In computer graphics, the rendering equation is an integral equation that expresses the amount of light leaving a point on a surface as the sum of emitted light and reflected light. It was independently introduced into computer graphics by David Immel et al. [1] and James Kajiya [2] in 1986. The equation is important in the theory of physically based rendering, describing the relationships between the bidirectional reflectance distribution function (BRDF) and the radiometric quantities used in rendering.

Contents

The rendering equation is defined at every point on every surface in the scene being rendered, including points hidden from the camera. The incoming light quantities on the right side of the equation usually come from the left (outgoing) side at other points in the scene (ray casting can be used to find these other points). The radiosity rendering method solves a discrete approximation of this system of equations. [1] In distributed ray tracing, the integral on the right side of the equation may be evaluated using Monte Carlo integration by randomly sampling possible incoming light directions. Path tracing improves and simplifies this method. [2]

The rendering equation can be extended to handle effects such as fluorescence (in which some absorbed energy is re-emitted at different wavelengths) and can support transparent and translucent materials by using a bidirectional scattering distribution function (BSDF) in place of a BRDF. [3] The theory of path tracing sometimes uses a path integral (integral over possible paths from a light source to a point) instead of the integral over possible incoming directions. [4]

Equation form

The rendering equation may be written in the form

where

Two noteworthy features are: its linearity—it is composed only of multiplications and additions, and its spatial homogeneity—it is the same in all positions and orientations. These mean a wide range of factorings and rearrangements of the equation are possible. It is a Fredholm integral equation of the second kind, similar to those that arise in quantum field theory. [5]

Note this equation's spectral and time dependence — may be sampled at or integrated over sections of the visible spectrum to obtain, for example, a trichromatic color sample. A pixel value for a single frame in an animation may be obtained by fixing motion blur can be produced by averaging over some given time interval (by integrating over the time interval and dividing by the length of the interval). [6]

Note that a solution to the rendering equation is the function . The function is related to via a ray-tracing operation: The incoming radiance from some direction at one point is the outgoing radiance at some other point in the opposite direction.

Applications

Solving the rendering equation for any given scene is the primary challenge in realistic rendering. One approach to solving the equation is based on finite element methods, leading to the radiosity algorithm. Another approach using Monte Carlo methods has led to many different algorithms including path tracing, photon mapping, and Metropolis light transport, among others.

Limitations

Although the equation is very general, it does not capture every aspect of light reflection. Some missing aspects include the following:

For scenes that are either not composed of simple surfaces in a vacuum or for which the travel time for light is an important factor, researchers have generalized the rendering equation to produce a volume rendering equation [7] suitable for volume rendering and a transient rendering equation [8] for use with data from a time-of-flight camera.

References

  1. 1 2 Immel, David S.; Cohen, Michael F.; Greenberg, Donald P. (1986). "A radiosity method for non-diffuse environments" (PDF). In David C. Evans; RussellJ. Athay (eds.). SIGGRAPH '86. Proceedings of the 13th annual conference on Computer graphics and interactive techniques. pp. 133–142. doi:10.1145/15922.15901. ISBN   978-0-89791-196-2. S2CID   7384510.
  2. 1 2 Kajiya, James T. (1986). "The rendering equation" (PDF). In David C. Evans; RussellJ. Athay (eds.). SIGGRAPH '86. Proceedings of the 13th annual conference on Computer graphics and interactive techniques. pp. 143–150. doi:10.1145/15922.15902. ISBN   978-0-89791-196-2. S2CID   9226468.
  3. Glassner, Andrew S. (2011) [1995]. Principles of digital image synthesis (PDF). 1.0.1. Morgan Kaufmann Publishers, Inc. p. 722. ISBN   978-1-55860-276-2 . Retrieved 2025-05-15.
  4. Veach, Eric (1997). Robust Monte Carlo methods for light transport simulation (PDF) (PhD thesis). Stanford University. pp. 219–220. Retrieved 2025-05-15.
  5. Watt, Alan; Watt, Mark (1992). "12.2.1 The path tracing solution to the rendering equation". Advanced Animation and Rendering Techniques: Theory and Practice . Addison-Wesley Professional. p.  293. ISBN   978-0-201-54412-1.
  6. Owen, Scott (September 5, 1999). "Reflection: Theory and Mathematical Formulation" . Retrieved 2008-06-22.
  7. Kajiya, James T.; Von Herzen, Brian P. (1984), "Ray tracing volume densities", ACM SIGGRAPH Computer Graphics, 18 (3): 165–174, CiteSeerX   10.1.1.128.3394 , doi:10.1145/964965.808594
  8. Smith, Adam M.; Skorupski, James; Davis, James (2008). Transient Rendering (PDF) (Technical report). UC Santa Cruz. UCSC-SOE-08-26.