Melvin Van Peebles's no-budget indie Sweet Sweetback's Baadasssss Song (1971) struck a mighty blow for African-American cinema and inaugurated the blaxploitation boom with its tale of a righteous stud (Van Peebles) on the run from racist cops. It's a terrible movie, clumsy and self-indulgent, but like lava cooling into obsidian, its bracing anger has hardened into myth. This colorful dramatic feature by Mario Van Peebles, the director's son, chronicles the shoestring production of Sweetback from Melvin's tortured screenwriting sessions to the movie's explosive premiere in Detroit, and its mix of personal reminiscence (Mario made his screen debut playing Sweetback as a boy) and cultural history is fascinating. This engages in a fair amount of mythmaking itself, but its lesson in self-empowerment is both vivid and sincere. R, 108 min. Reviewed this week in Section One. Chatham 14, Pipers Alley.