Matte & Fill

From Mographwiki.net

Matte & Fill refer to the two image elements that one uses to construct a composite image. The matte, which is also called an Alpha Channel is generally an 8-Bit greyscale image that determines the edge boundaries as well as transparency values of the image to be composited. Fill refers to the image one wishes to composite.

In compositing there are two different kinds of fill image. The first is called a Straight Fill and is, in essence, rendered with slop that runs outside the bounds of the image. A straight fill, by virtue of having extra image data will produce a perfectly composited image. The second type of fill image is called a Pre-Multiplied Fill. The edge pixels of a pre-multiplied fill are precisely trimmed to the boundaries of the image. This can create, depending on the compositing system, a grey halo effect around the edges of the composited image.

As a rule of thumb, it is always better to create a Straight Fill as it will produce a far better composite result.