A model of scene composed of 2D discrete objects is presented. A generalization of 2 1 2 D positioning features usually implemented in 2 1 2 D environments is given, treating object superimposition. The positioning representation lays on an explicit topological and geometrical encoding of object boundaries. This approach allows structured manipulations of objects (intersection, combination, : : :) as it is Euclidean 2D objects and eecient redisplay of scene, such that every modiied pixel is drawn exactly once. Moreover, this model is consistent from a geometrical and topological point of view and allows the construction of a scene from a digital image.