Properties meaningful for

*projective*geometry are respected by this new idea of transformation, which is more radical in its effects than can be expressed by a transformation matrix and translations (the affine transformations). The first issue for geometers is what kind of geometry is adequate for a novel situation. It is not possible to refer to angles in*projective*geometry as it is in Euclidean geometry, because angle is an example of a concept not invariant with respect to*projective*transformations, as is seen in*perspective*drawing. One source for*projective*geometry was indeed the theory of*perspective*.