Glossary

Kajiya Rendering equation

When solved, it gives 'perfect' lighting.  So, you can basically see all of computer graphics as attempts to render this equation in a reasonable amount of time. Bidirectional path tracing is the brute force solution to this equation.

Note: The Kajiya rendering equation needs to be extended to cover non-particle effects. For example, it can't simulate a diffraction grating which causes the effects you see on a CD surface) Also, it looks in many ways similar to Feynman's "sum over histories" solution to a quantum mechanics problem (not surprisingly given both are related to particle paths), so if quantum computers ever appear we might just evaluate the equation directly.

Ray tracing

For every pixel in the image you want to produce, you fire rays into the scene (primary rays), and when they hit a surface, trace from this point to the light source (secondary rays). Additional secondary rays allow refractive and reflective surfaces very easily.

Ray casting

Ray tracing using primary rays only. Since there are no secondary rays, no lighting is done. Ray casting is about finding the things closest to the source of the ray.

Primary Rays

Any ray that originates from the image you want to produce.

Secondary Rays

Rays that are sent into the environment purely to acquire lighting data, and are spawned as the result of, and at the point of, the primary rays intersecting objects in a classical ray tracer.

Distributed ray tracing

To send many rays out per sample and integrate the results to anti-alias the results.

Stochastic sampling

Pseudo-random sample positions that are used to remove repeating patterns of aliasing better than fixed sample position approaches.

Local Illumination Model

No data outside the surface properties of the object, the viewer and the lights are used to calculate the lighting for this model.

Global Illumination Model

On top of the factors used in the Local Illumination Model, things such as other objects in scene are additional accounted for when calculating the lighting term.

Direct Lighting

Light that has transported directly from the light to the surface (travelling in a straight line from light to surface).

Indirect Lighting

The transport of light that has bounced off other surfaces rather than directly getting to the surface from the light source.  Light can be bounced, which is a crucial consideration.

Whitted ray tracer

Turner Whitted's seminal work providing the first global illumination renderer and the proper name for what most people mean when they say "ray tracing".

Infinitesimal

Most people have a concept of infinity but the opposite of it is less well known. Simply put (and please, mathematicians, don't crucify me for this explanation!), it's something that is smaller than anything else, and what you get from 1/infinity.

Ray

Infinitely thin rod. The ray is cast and travels in a straight line until it hits something along that path.

Intersection Test

Computation needed to determine if a ray has hit an object in the scene.

Monte-Carlo simulation

If you can't solve something analytically, you can use a Monte-Carlo simulation which basically means doing the thing lots of times with random small changes and averaging the results.

Radiance Transfer Function

A term that relates the incoming energy (radiance) to outgoing radiance. Essentially something that maps how much an object gets illuminated by light hitting it.