Specular highlight

A specular highlight is the bright spot of light that appears on shiny objects when illuminated (for example, see image on right).

Specular highlights are important in 3D computer graphics, as they provide a strong visual cue for the shape of an object and its location with respect to light sources in the scene.

Specular reflection is visible only where the surface normal is oriented precisely halfway between the direction of incoming light and the direction of the viewer; this is called the half-angle direction because it bisects (divides into halves) the angle between the incoming light and the viewer.

However, many shiny objects show blurred specular highlights.

At points on the object where the smooth normal is close to the half-angle direction, many of the microfacets point in the half-angle direction and so the specular highlight is bright.

As one moves away from the center of the highlight, the smooth normal and the half-angle direction get farther apart; the number of microfacets oriented in the half-angle direction falls, and so the intensity of the highlight falls off to zero.

For example, plastic is made up of tiny beads of color suspended in a clear polymer and human skin often has a thin layer of oil or sweat above the pigmented cells.

Such materials will show specular highlights in which all parts of the color spectrum are reflected equally.

A number of different models exist to predict the distribution of microfacets.

In the Phong reflection model, the intensity of the specular highlight is calculated as: Where R is the mirror reflection of the light vector off the surface, and V is the viewpoint vector.

The number n is called the Phong exponent, and is a user-chosen value that controls the apparent smoothness of the surface.

), or approximately Pearson type II distribution, of the corresponding angle.

[1] While this is a useful heuristic and produces believable results, it is not a physically based model.

[citation needed] The usual function calculates specular highlight intensity as: where m is a constant between 0 and 1 that controls the apparent smoothness of the surface.

[4] Compared to the empirical models above, this function "gives the absolute magnitude of the reflectance without introducing arbitrary constants; the disadvantage is that it requires more computation".

It can be used to model surfaces that have small parallel grooves or fibers, such as brushed metal, satin, and hair.

Note the fact that normal of fiber depends on light position.

To avoid their calculation original formula can be rewritten in next form: T can be observed as bump normal and after that it is possible to apply other BRDF than Phong.