“Beijing Spring” tells a story about underground filmmaking, radical art, and censorship that’s dismayingly reminiscent of what’s taking place in Hong Kong today. At Democracy Wall in Beijing 1978, the cheekily named Stars, a group of self-taught artists (including a young Ai Weiwei), challenged propaganda art by displaying new work that championed individuality and free expression, often exposing the inhumanity of the Cultural Revolution. During this brief period of relative freedom, editor and essayist Wei Jingsheng posted his call for democracy on the same wall. The authorities, provoked by the Stars exhibits and protest demonstrations, cracked down and closed Democracy Wall, imprisoning many and slamming the door on this fleeting period of reform. Directed by SLIFF alums Andy Cohen and Gaylen Ross (“Ximei”), “Beijing Spring” features frank, revealing contemporary interviews with the artists — many now scattered around the globe — and remarkable material shot by a daring young filmmaker named Chi Xiaoning, whose long-hidden 16mm film footage is seen here for the first time after 40 years.