There are certain problems that can occur when the rules of premultiplication are ignored. These can range from problems with edges to raised global levels. Additionally, when the premultiplied state of an image is ignored, there may also be unwanted fringing around a masked object or the unwanted side effects that may occur when a node affects parts of an image that it should not.
A possible result of compositing with a premultiplied image would be slight lightening or darkening of the composite areas where the foreground matte is semitransparent.
Another common problem is when color correction has been applied to a foreground premultiplied image. This could result in the blacks in the scene being elevated above the value of 0, which means that even though the matte channel may still specify that the surrounding field is in black, the RGB channels may have some small, often visually undetectable value.
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