Matte

From Odwiki

There are several meanings of matte in computer graphics.

[edit] Hold-out or Cut-out

The SurfaceShader in mantra is responsible for assigning three attributes (Cf, Of and Af). The Cf variable is the RGB color of the surface. The Of variable is the opacity. This opacity determines which colors are visible from surfaces behind the surface being shaded. For example, if the opacity were {.5, .5, .5}, half of the color of all the surfaces behind the surface would be visible. During the compositing process, the AlphaChannel is a single channel which acts very much like the Of variable in a SurfaceShader.

The Af variable allows a shader writer to specify what value should appear in the AlphaChannel of the resulting image, independent of the opacity of the surface. This allows shader writers to create a cut-out surface by applying what's called a matte shader. Consider the shader:

  surface matte()
  {
     Cf = 0;
     Of = 1;
     Af = 0;
  }

When this shader is applied, the surface color will be black. The surface will also be opaque (not allowing any colors from occluded surfaces to contribute to the final color), but the alpha of the final image will be 0.

If the resulting image is composited over a background image, the background image will show through this surface. However, all geometry in the scene will have been occluded.

[edit] Matte vs. Glossy

Another common meaning for matte is a "flat finish". This is the same term used in photographic processing or paint chip selection. You can have glossy or matte finish.