The bulk of the music documentary “This Much I Know to Be True” features singer-songwriter Nick Cave and his frequent collaborator, musician Warren Ellis, performing songs from the studio albums “Ghosteen” (2019, by Cave and his band the Bad Seeds) and “Carnage” (2021, credited to Cave and Ellis). There’s no audience here; the footage was shot in the spring of 2021, in the middle of the pandemic. But the performances — lit theatrically, in old, empty spaces in London and Brighton — feature backup singers, a string quartet and an appearance by Marianne Faithfull, reciting the May Sarton poem “Prayer Before Work.” Supplementing these scenes — which have the feel of rehearsals or studio sessions, not quite finished but not entirely unpolished — are occasional interviews with Cave, talking about his long-standing creative partnership with Ellis or reading from his emotionally vulnerable, ask-me-anything-style online forum, the Red Hand Files. The film contains no explicit mention of the 2015 death of Cave’s 15-year-old son, Arthur, but “Ghosteen” was written in the aftermath of that tragedy, and an air of still-raw mourning hangs over the film. (In a terrible footnote, Cave’s eldest son, 31-year-old Jethro Lazenby, also died this spring, after the film was in the can.) There aren’t many talking-head interviews here. Mostly, “True” is a gift for fans of Cave’s musical storytelling and an opportunity for them to bathe in lushly gloomy atmospherics. It’s stirring, dirgelike and richly imagistic stuff. As a bonus, in a brief prologue, filmmaker Andrew Dominik (“The Assassination of Jesse James by the Coward Robert Ford”) takes a peek into Cave’s new ceramics studio, where he unveils an in-progress series of statuettes depicting the life of the devil on Earth. Unrated. Available on Mubi. Contains brief strong language. 105 minutes.