Pointer images are used as the primary representation for various types of feature recovered directly from the image (such as edges, regions and corner points). For generic manipulation (functions designed to work on various data types, eg. display functions) the type of feature can be indicated using the imtype field. The absence of a features at a particular image location is indicated by a nil pointer.
Iconic representations are often not the most convenient or efficient form of image access, hence it is often useful to use the property list of the imrect to store one or more alternative indexing schemes more suited to the job in hand.