Often he bases a whole movie on a single visual idea, methodically and meticulously worked out. It's a credit to his artistry that the results -- in his best work -- are as pleasing to the eye and emotions as they are to the mind.In a recent one-man show at the Millennium film workshop, Gehr introduced a couple of past efforts followed by three (untitled) premieres. The program opened with ''Table,'' in which a piece of furniture is seen from two rapidly alternating perspectives; and ''Shift,'' in which traffic on a city street forms painterly and frequently amusing patterns on the screen.