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### Object Position

Object position is a description of the orientation and location of complete objects with respect to other objects in the scene. This description is achieved in two ways: firstly in terms of the object axes, and secondly in terms of features that are near each other.

Relative location of two objects is expressed by reference to the intersections of major and minor axes on each object. The distance between these two points is measured in terms of axis lengths, choosing whichever of the main axes of each object is an appropriate measure. The second component of location is the angle relative to each object at which the other intersection is found. This is expressed by dividing the space around the object into four ``quadrants'' (which may or may not meet at right angles) divided by the major and minor axes. The angular location of the other object is then described by the quadrant in which its axis intersection falls.

Relative orientation of objects is described in the same way as relative orientation of features on a single object; where axes of the two objects are parallel, aligned, or perpendicular, this is noted. No effort is made to express exact angular orientation at any other angle, but qualitative orientation can be ascertained by comparing the relative angular location from the point of view of each object.

The relative orientation of features on different objects is described in two different ways. The most functionally important one specifies where objects are in contact: the names of the two features that are in contact are recorded on a global list of all contacts in the scene. The other records sets of parallel, aligned, or perpendicular straight edges. These sets are maintained in the same way as for features on the same object, and for aligned object axes.

Next: Axially Specified Subparts and Up: Describing 2D Shape with Previous: Object Relative Position
Alan Blackwell
2000-11-17