Improv, in theory, can be as rewarding as good jazz, since it follows similar rules of composition. The players come onstage with a rough sense of how a piece should be structured, then riff on a germ idea until something interesting happens. Depending on the night, the audience, the players, the chemistry, and a dozen other factors, it might be good or bad or brilliant. Most improv companies stick to the comfortable shore of brief sketches, but Scratch Theater is a rare group that practices "long-form" improv: They spin out three one-act plays ("from scratch"). A typical evening might involve a musical, a '70s crime drama, and a mock-Shakespeare play. Scratch has been together for only a year-and-a-half, so they don't know each other as well as long-standing groups like True Fiction Magazine, but they're composed of players from True Fiction and Bay Area TheaterSports, another veteran group. This weekend Kristina Robbins is rejoining them from L.A.: Robbins won an SF Weekly Black Box Award for her comic solo show last year at the Marsh, called Face by the Door. Scratch performs Friday and Saturday at 8 p.m. (as well as July 17 and 18) at the Noh Space, 2840 Mariposa (at Florida), S.F. Tickets are $10; call 621-7978.