  Grassmann tensors arise from classical problems of scene reconstruction in
computer vision. Trifocal Grassmann tensors, related to three projections from
a projective space of dimension k onto view-spaces of varying dimensions are
studied in this work. A canonical form for the combined projection matrices is
obtained. When the centers of projections satisfy a natural generality
assumption, such canonical form gives a closed formula for the rank of the
trifocal Grassmann tensors. The same approach is also applied to the case of
two projections, confirming a previous result obtained with different methods
in [6]. The rank of sequences of tensors converging to tensors associated with
degenerate configurations of projection centers is also considered, giving
concrete examples of a wide spectrum of phenomena that can happen.
