transforms a dim[a..., b] grid into a dim[a..., b-1] grid. there is a projection plane perpendicular to the last axis and whose position is given by the creation argument. each vector's length is adjusted so that it lies onto that plane. then the last dimension of each vector is dropped transforms a Dim[A..., B] grid into a Dim[A..., B-1] grid. There is a projection plane perpendicular to the last axis and whose position is given by the "depth" parameter. Each vector's length is adjusted so that it lies onto that plane. Then the last dimension of each vector is dropped. useful for converting from 3-D geometry to 2-D geometry. Also useful for converting homogeneous 3-D into regular 3-D, as homogeneous 3-D is really just regular 4-D...(!) depth