Suppose we have a space with points represented by homogeneous coordinate vectors --- that is, {F - 0} factored out, where F is some continuous ground field, in practice \R or \C. How do we tell whether two points are (approximately) the same? If the metric is Euclidean (e.g. real projective), a slightly clumsy solution is to rescale both vectors X,Y to length unity, and check for either |X-Y| and |X+Y| small; unless |X| or |Y| is small to start with, when it can be snapped back to zero (representing no point). But when the metric is more general (e.g. complex projective, conformal inversive), nonzero vectors may possibly be isotropic --- |X| = 0 with X /= 0. Ignoring the local metric, performing the comparison using instead a Euclidean metric, offends against my aesthetic scruples; in particular failing to be invariant under local isometries. This apparently trivial algorithmic conundrum has irritated me for years. Has anybody got a better idea? WFL