An effect will be asked to fill in some region of this infinite plane. The section it is being asked to fill in is called the "Region of Interest" (RoI).
Before an effect has been asked to process a given RoI, it will be asked to specify the area of each input clip it will need to process that area. For example: a simple colour correction effect only needs as much input as it does output, while a blur will need an area that is larger than the specified RoI by a border of the same width as the blur radius.
The purpose of the
action is for the host to ask an effect what areas it needs from each input clip, to render a specific output region. An effect needs to examine its set of parameters and the region it has been asked to render to determine how much of
input clip it needs.