## Glossary

specular reflection

A mirror-like reflection of light from a surface. Specular reflections reflect light; thus, the color the viewer sees is strongly based on the view angle relative to the light. Specular reflections often do not affect the color of the incoming light.

specular highlights

Mirror-like reflections directly from light sources. Since light sources are brighter than light reflected by other objects, modelling only specular highlights can provide useful realism without having to model reflections from light produced by other objects in the scene.

angle of view, viewing angle

The angle between the surface normal and the direction to the viewer/camera.

microfacet model

Describes a surface as a number of flat planes called microfacets. Each microfacet reflects light using a simple lighting model. The light from a portion of the surface is simply the aggregate of the light from all of the microfacets of the surface. The statistical distribution of microfacet directions on a surface becomes an integral part of the lighting equation. The normal of a surface at a point is the average normal of the microfacets of that part of the surface.

The microfacet model can be used to model the reflectance characteristics of rough surfaces.

Phong specular model

A simple model for creating specular highlights. It uses a power function to determine the distribution of microfacets of the surface. The base of the power function is the cosine of the angle between the view direction and the direction of perfect reflection along the surface normal. The exponent is an arbitrary value on the range (0, ∞); large values describe increasingly shiny surfaces, while small values are for rough surfaces.

half-angle vector

The vector halfway between the direction towards the light and the view direction. When the half-angle vector is oriented exactly with the surface normal, then the view direction is oriented along the reflection direction. For a given light and view direction, it is the direction that the surface normal would need to be facing for a direct light reflection to go from the light source to the viewer.

Blinn-Phong specular model

A simple model for creating specular highlights. Like standard Phong, it uses a power function to model the distribution of microfacets. The base of the power function is the cosine of the angle between the half-angle vector and the surface normal. The exponent is an arbitrary value on the range (0, ∞); large values describe increasingly shiny surfaces, while small values are for rough surfaces.

Gaussian distribution, normal distribution

A common statistical distribution. It defines the familiar bell-shaped curve, with the average value at the highest point of the distribution.

Gaussian specular model

A model for creating specular highlights. It uses the Gaussian distribution to model the distribution of microfacets on a surface. It uses a value to control the distribution; this value ranges on (0, 1], where small numbers are smooth surfaces and large numbers are rough surfaces.

inverse cosine, arc cosine

Performs the opposite of the cosine function. The cosine function takes angles and returns a value on the range [-1, 1]. The inverse cosine takes values on the range [-1, 1] and returns an angle in radians.