Reflection and Shading Models

Models of reflection/scattering and shading are used to describe the appearance of a surface. Although these issues may seem like a problems all on their own, they are studied almost exclusively within the context of rendering. Modern 3D computer graphics rely heavily on a simplified reflection model called Phong reflection model (not to be confused with Phong shading). In refraction of light, an important concept is the refractive index. In most 3D programming implementations, the term for this value is "index of refraction," usually abbreviated "IOR." Shading can be broken down into two orthogonal issues, which are often studied independently:

• Reflection/Scattering - How light interacts with the surface at a given point
• Shading - How material properties vary across the surface

Reflection

Reflection or scattering is the relationship between incoming and outgoing illumination at a given point. Descriptions of scattering are usually given in terms of a bidirectional scattering distribution function or BSDF. Popular reflection rendering techniques in 3D computer graphics include:

• Flat shading: A technique that shades each polygon of an object based on the polygon's "normal" and the position and intensity of a light source.
• Gouraud shading: Invented by H. Gouraud in 1971, a fast and resource-conscious vertex shading technique used to simulate smoothly shaded surfaces.
• Texture mapping: A technique for simulating a large amount of surface detail by mapping images (textures) onto polygons.
• Phong shading: Invented by Bui Tuong Phong, used to simulate specular highlights and smooth shaded surfaces.
• Bump mapping: Invented by Jim Blinn, a normal-perturbation technique used to simulate wrinkled surfaces.
• Cel shading: A technique used to imitate the look of hand-drawn animation.

Shading

Shading addresses how different types of scattering are distributed across the surface (i.e., which scattering function applies where). Descriptions of this kind are typically expressed with a program called a shader. (Note that there is some confusion since the word "shader" is sometimes used for programs that describe local geometric variation.) A simple example of shading is texture mapping, which uses an image to specify the diffuse color at each point on a surface, giving it more apparent detail.